197606 Byte Magazine Vol 00 10 The Game of LIFE in Color

ISSUE NUMBER 10 JUNE 1976 $1.50 PRINTED IN USA the small systems journal i = r Mn NM NAR My — = nnn ie Pg ecm wf 1 at q r LA Wi HH oO, WN, Ll an The Game of LIFE Played in Color COMPARE Feature Suulr? —6800 Theirs—Y our Choice Processor— The best ‘Motorola MC6800", Two accumulators, automatic Some are almost as good in one respect or another. vectoring, seven addressing modes and complete set of branch None can offer all of the features of a real MC6800. instruction. The more powerful instruction set and memory orientated architecture makes programming very straight for- ward and easy to learn. Operates from a single +5 Volt supply, Memory— Static 2102-1 type memories. Fast enough to allow the pro- Various types available. Often not included in the cessor to run at full speed at al! times. No refresh cycles, no basic kit, and must be purchased as an extra cost problems with glitches and flakey dynamic memories, option, (this is an option?) Power Supply— 10 Amp. Capacity. More than enough to power a fully ex- | Some expand more than others with the supply Panded system. Power supply uses a rugged 25 amp bridge provided, Check carefully. rectifier and a 91,000 mfd computer grade filter. Regulators on th dividual plug-in cards. Expansion— Seven slots for processor and memory boards. Eight 1/0 slots. | Varies from “0” to 16, or more. 1/0's are programmable type. All decoding and clocking pro- vided from mother board making additional interfaces very inexpensive. Baud rates may be independently selected for each interface card. Start Up— Automatic start and reset provided by “Motorola” Mikbug® Anything from switch and status light to automatic ROM. No fiddling with switches and status lights. Just push ROM loading. tf ROM is not a standard part, the the button and go. Use of standard Motorola firmware makes software may be unique to that machine. software 100% compatibie with Motorola evaluation module programs, Clock— Crystal controlled master clock oscillator with high power Anything from cheap dual monostable systems to clock drivers, Insures reliable, consistant operation with no crystal control. Crystal oscillators are best. Dual noise problems, Baud rate divider operating from the master one-shots can develop phase overlap problems and clock Oscillator provides the various baud rates for the 1/0 are more susceptible to noise problems. devices with crystal accuracy. No adjustments necessary to lock everything in at the proper frequency, Buffering— Tristate bi-directional buffers on ali data lines, address lines | Various—from full buffering to almost no buffering. and clock lines on ALL boards. Insures trouble free noise Lack of full buffering can lead to noise problems, immune operation. Documentation— | very complete. Our own notebook, plus the ‘Motorola Programming Manual and Applications Manual. Detailed instructions along with sample programs help you to under- stand programming. You will be ready and able to write your own programs after mastering these instructions. Price— $395.00 For the whole thing. You get the case, power sup- | As we said—COMPARE. Bly,processor2K word memory and serial interface. No extras to buy, Just connect a terminal and start operating. © Registered Trademark Motorola COMING ATTRACTIONS CASSETTE INTERFACE—Our new AC-30 will make it possible to load and dump programs to cassette tape using ordinary inexpensive recorders, Uses the standard “Kansas City” recording format for compatibility. LINE PRINTER—At last—hard copy at a reasonable price. Our new PR-40 printer will print program listings, or any other material you might want to keep in printed form, A dot matrix printer at a price you are not going to believe. GRAPHICS TERMINAL—A universal—works with any computer—graphics terminal. With this connected to your computer you can play games in style. May be used with our CT-1024 to put both graphics and alpha- numerics on the screen simultaneously. PLEASE—Don't call or write. We will have details on these projects in our next ‘ad. Government regulations prevent us from giving prices, or taking orders npuTeR S9stem yet. SOFTWARE—The flood is near. Editor and assembler now available. BASIC and more games right away. Yours for the cost of copying. WE DON'T SELL SOFTWARE—WE GIVE IT TO YOU. ENJOY IT, COPY IT, WE WON'T COMPLAIN. . . Southwest Technical Products Corp., Box 32040, San Antonio, Texas 78284 Tu In PLAYING cnun LER “he tS Now your color TV can be your computer display terminal Cromemco’s new computer/tv interface circuit lets you have a full-color computer display terminal for little more than a black-and-white terminal. The Cromemco interface also lets you do vastly more with your color terminal than you can do with ordinary black-and-whites. We call our interface the TV Dazzler®. It consists of two circuit boards that plug directly into your Altair 8800 or IMSAI 8080 computer. Alphanumerics plus action, and graphics The Dazzier® maps your computer memory content onto your color tv screen in full color. That doesn’t mean just that you see alphanumerics in color. You can display any information in memory, And do so in color. LIFE in color You can display computer games or animated shows (rocket ships). What's more, you can display business or technical graphics — multi-colored charts, graphs, histograms, educational material — all from computer memory. -Even light shows. Not TV DAZZLER SOFTWARE CONTEST Write for details even the biggest computer manufacturers offer all this in color, Needs only 2K memory, Technically, the Dazzler® scans your computer memory using direct-memory access (DMA). It formats each memory bit into a point on the ty screen to give a 128 x 128-element picture. Only a 2K-byte computer memory is required (only 512 bytes for a 32 x 32 picture). The quality of the pictures is evident in the photos. The Dazzler® output is a video signal that goes directly to the tv video amp or to the antenna terminal through an inexpensive commercially-available device. Inexpensive — and so much better You can see from the list below that the Dazzler® is little if any more in price than an ordinary b/w interface or tv typewriter, But it does so much more. Order now By mail or at your computer store If you're into computers (or want to be), FORRES qanary sizer ONZZLE-URITER RUNNING UITH ek preroriit alphanumeric display generated with Dazzle-Writer software (see below) to invent these beautiful displays or games, or to plot colorful if you want material inexpensively at home or in business, the Dazzler® is for you. Not only is it reasonable, but it’s sold at computer stores from coast to coast. Or order directly by mail on your bank card. TV DAZZLER® (complete kit)... . . $215, TV DAZZLER® (fully assembled ee $350 SOFTWARE (punched paper tape with documentation) RARE MT ROE ee esi exe ace $16 KALEIDOSCOPE in full color ..... $15 DAZZLE-WRITER (for alphanumeric displays in color)... . $15 Aga MATION (for computer- ited animated displays... . . . $15 TIGTACTOE (you pley the computer) 2... 2. $15 Shipped prepaid if fully paid with order. California users add 6% sales tax, Mastercharge and BankAmericard accepted with signed order. Delivery: from stock for immediate shipment. LIFE ¢ DAZZLEMATION * DAZZLE-WRITER * TIC-TAC-TOE * KALEIDOSCOPE SOME SEQUENCES FROM CROMEMCO SOFTWARE Cromemco Specialists in computer peripherals One First St., Los Altos, CA 94022 » (415) 941-2967 enecoercnt. 8° MSRM. "C ISH2—_ 78123456789 * Tae. | CABCOUEF CHE KL NMOPORSTUUMKYZEN3+— | E°SSXE. °C FEL—_ /B12S456789 «5 <=! CHE. NF E"8S26."¢ 384,~_ 78123456789 * 5¢) @ABCOEFCHI IKE MNOPORS' vz! E“ S828" (384. ~ _/O1 23456789" 5 @ASCOUEF CHEM 0 3 eva) What does it take to make your microcomputer keep track of that small spare time “moonlight” busi- ness? For small businesses, one of the biggest problems is all the paperwork required of the entrepeneur. John A Lehman provides an introduction to the subject of automated accounting procedures in his description of A Small Business Accounting System. Find out how to Build a Television Display which can be interfaced to your computer’s memory address space by consulting CW Gantt Jr’s article on a 15 IC controller for a 32 by 16 display. Programming to a large extent is organizing your ideas about what the computer should do. Ronald T Her- man gives some basic pointers on Programming for the Beginner, con- cerning the practice of structuring Program designs into well defined verbal descriptions, before you gen- erate a single line of code. This prac- tice makes programming easier and less subject to nasty errors which interfere with the goal of a working application. page 16 !s your high school’s computing budget crimped? Use Christopher Lett’s experience with A High School Computing System as a way to show that a small budget does not neces- sarily rule out getting a system. A Systems Approach to a Personal Microprocessor is the inaugural article in a new series of detailed design articles which is being prepared for BYTE by Dr Robert Suding. In this first instalment, you’ll find some of Dr Suding’s views on the philosophy of system design, to lay the groundwork. Build a computer? Sure. Bob Abbott shows you the circuitry and some photos of a wire wrapped M6800 system in his article on Building an M6800 Microcomputer. In This BUTE One of the principal uses of a computer is data processing. Phillip L Hansford describes one simple data processing application which he imple- mented with minimal equipment: How to Strike a MATCH between penpals for his penpal club. Last month, Bob Nelson introduced “Chip” Off the Olde PDP 8/E. In Part 2, he discusses the interrupt structure, control panel features and support chips of the }*.tersil 1M6100 design. One key component of system soft- ware for any computer is a monitor program. You can find out how to program and Interact with an ELM (Eloquent Little Monitor) by reading G H Gable’s article on his design of a handy piece of system software. Techniques of developing the ef- fective address for operands in mem- ory vary with the choice of CPU. In An Introduction to Addressing Methods. John Zarrella discusses some of the classical ways computers use to calculate the address of operands in memory. You can use this background information in forming your own opinion about the instruction sets of the various microcomputers presently available. What's in a language? Nat Wads- worth and Mark Arnold present some information on their interpretive lan- guage system, SCELBAL, in a product description article. BUTE +10 JUNE 1976 * > Foreground 16 BUILD A TELEVISION DISPLAY PUBLISHERS Hardware — Gantt Virginia Pesch ke 40 BUILDING AN M6800 MICROCOMPUTER Manfred Peschke Hardware — Abbott EDITOR 48 STRIKE A MATCH Carl T Helmers, Jr Applications Software — Hansford GENERAL MANAGER 66 INTERACT WITH AN ELM Manfred Peschke Software — Gable PRODUCTION MANAGER Judith Havey Background PRODUCTION ASSISTANT Elizabeth Alpaugh 8 A SMALL BUSINESS ACCOUNTING SYSTEM CIRCULATION Applications — Lehman Deborah R Luhrs 22 PROGRAMMING FOR THE BEGINNER PUBLISHERS ASSISTANTS Software — Herman prey Hi 28 A HIGH SCHOOL COMPUTER SYSTEM bang 2ecly Review: Lets ADVERTISING 32 SYSTEMS APPROACH TO A PERSONAL MICROPROCESSOR Elizabeth alpaugh System Design — Suding Virginia Peschke 58 “CHIP” OFF THE OLDE PDP 8/E—PART 2 TYPOGRAPHY Hardware — Nelson Custom Marketing Resources, Inc 76 AN INTRODUCTION TO ADDRESSING METHODS Goodway Graphics Hardware — Zarrella sll ed 3 82 SCELBAL aim) war = ay PHOTOGRAPHY Product Description — Wadsworth -Arnold fed Crebitree Custom Marketing Resources, Inc ART Nucleus Bill Morello 2 In This BYTE PRINTING | 4 Toward a Parallel Interface Standard Custom Marketing Resouroes; Ing ‘The George Banta Company 5 Call for Papers ASSOCIATES 6 About the Cover Bob Baker 14, 73, Dan Fyistra 81, 95, 96, Don Lancaster 98,100,110 What's New? Harold A Mauch 36 The Albuquerque Happenings Chris Ryland 52 Letters 64 Components and Parts (Puzzle) BYTESiiaeheIne le pubuanea 87 Classified Ads monthly by BYTE Publica- 88 Book Reviews boreude New darnpsnire 90 Clubs, Newsletters 03458. Subscription rates are 94 Programming Quickies Two yeare, $32. Three years, 104 Software Bug of the Month 830, Second oes poste paid 1 fe 4 104 BYTE’s Bugs Sfive 03488 and at adaitional a a ae 106 SystemsiofNots 803-324-7217. Entire contents 112 BOMB copyright 1976 BYTE Publica~ 112 Reader's Service tions, Inc, Peterborough NH 03458. Address editorial cor respondence to Editor, BYTE, 70 Main St, Peterborough NH 03458, Toward Articles Policy BYTE is continually seek- ing quality manuscripts writ- ten by individuals who are applying personal systems, or who have knowledge which will prove useful to our read- ers, Manuscripts should have double spaced _ typewritten texts with wide margins, Num- bering sequences should be maintained separately for fig- ures, tables, photos and list- ings, Figures and tables should be provided on separate sheets of paper. Photos of technical subjects should be taken with uniform lighting, sharp focus and should be supplied in the form of clear glossy black and white prints (if you do not have access to quality photog- raphy, items to be photo- graphed can be shipped to us in many cases). Computer list- ings should be supplied using the darkest ribbons possible on new (not recycled) blank white computer forms or bond paper. Where possible, we would like authors to supply a short statement about their background and experience. Articles which are accepted are typically acknowledged with a binder check 4 to 8 weeks after receipt, Honorari- ums for articles are based upon the technical quality and suitability for BYTE’s reader- ship and are typically $15 to $30 per typeset magazine Page. We recommend that au- |- thors record their name and address information redun- dantly on materials submitted, and that a return envelope with postage be supplied in the event the article is not accepted. . PERSONAL COMPUTING ’76 Consumer Trade Fair Atlantic City NJ Personal Computing '76 will be the first large industry wide show for the personal computing field. It is being sponsored by the Southern Counties Amateur Radio Associa- tion of New Jersey, and will be held August 28 and 29 1976 at the Shelburne Hotel and convention center on the Boardwalk at Atlantic City. For information on exhibit space and the show in general, contact: John H Dilks III, chairman Personal Computing '76 503 W New Jersey Av Somers Point NJ 08244 Phone: (609) 927-6950. © For reservations, contact: Shelburne Hotel/Motel Boardwalk & Michigan Av PO Box 1138 Atlantic City NJ 08404 @ Manufacturers who provide products for the personal computing market- place are invited to attend. © Aseries of seminars on subjects related to small computer systems is being arranged. @ The show will be almost entirely devoted to computers and related products. @ Admission to the general public will be $7.50 at the door, $5 if ordered in advance by individuals. Additional dis- counts are possible for group pur- chases by clubs, companies and other organizations. a Parallel Interface Standard The idea of a parallel interface standard which encourages interesting combinations of peripherals and processors from different manufacturers is one which in my opinion should be pursued to help foster the growth of the personal computing marketplace, a growth which will provide a wider range of options for both users and suppliers of products. The User’s Eye View The need of a parallel interface standard from the user’s point of view is readily perceived. The parallel interface standard will be the personal computing equivalent of what exists in the audio equipment industry: a widely manufactured, readily available physical interconnection with logically and electrically compatible signal definitions. The plug of choice in the audio field is the RCA style phono plug universally used to interconnect low level audio signals via shielded cables. This enables the purchaser of brand X turntable to plug it into a brand Y receiver using shielded cables of brand Z. The ideal for the parallel interface defini- tion in personal computing is similar. What is needed is a definition which will allow the owner of brand X processor to plug his system physically, logically and electronical- ly into a brand Y music synthesizer or brand Editorial By Carl Helmers Q graphics display, using brand Z cable assemblies. The interface definition to be created will at a minimum guarantee hard- ware compatibility. The applications soft- ware compatibility can be provided by the peripherals manufacturer in the form of simple relocatable routines with common functional documentation and detail code generation for the various microprocessor instruction sets. The user will see a much more highly desirable product if it contains provision for the standard interface, since he or she will then be able to interface a wide variety of specialized applications and systems oriented peripherals without the necessity of per- forming the systems engineering equivalent of reinventing the wheel at the interface level. The Manufacturer’s Eye View The manufacturer of a product for the personal computing field has as a goal the maximization of sales, and hopefully as a result, the maximization of profits. This isa simplistic economic analysis which ignores the existence of specialized counter exam- ples, but it is generally valid in most cases. Continued on page 112 Call for Papers PERSONAL COMPUTING '76 — Technical Session on Standards As part of the Personal Computing '76 convention August 28—29, Atlantic City NJ, BYTE magazine will coordinate a technical session on areas of standardization for the personal computing field. The purpose of defining standards is to provide a consistent and workable set of specifications for inter- faces between different equipment which permit combined operation as a system. Standards are in the interest of manufactur- ers who will find wider markets as a result, and standards are in the interest of users who will benefit from a wider choice of compatible options. The following technical areas are identi- fied at this point in time: Audio Cassette Standard A provisional interchange standard for audio media was defined by a working meeting sponsored by BYTE magazine in November 1975. This standard will be re- viewed, and the idea of a higher performance audio media standard should also be consid- ered. Parallel Interface Standard The proposal here is to define a parallel interface standard for the connection of peripherals to central processors. The stan- dard should specify logical interface, electri- cal interface and possibly a set of preferred physical plug and receptacle designs. Software Interface Standard The proposal here is to define several byte sequential media independent record formats for data involved in interchange between systems. This standard will prove useful to the development of software mar- kets, and could be incorporated into the firmware of operating systems and loaders supplied with processors. Arrangements Individuals and firms wishing to partici- pate in the discussions should send written position papers on their area of interest to: Carl Helmers Editor, BYTE 70 Main St Peterborough NH 03458 Attention: Technical Standards Papers should be neatly typed camera ready copy which will be used as is to create a proceedings booklet to be distributed at the technical session. It is suggested that draw- ings be done using ink or felt tip pen, and that film ribbon be used for typewritten copy. The standardization categories of audio media, parallel interface definitions, and software data formats should not be viewed as exclusive. Identification of additional areas where standards are appropriate will also help the growth of the personal comput- ing field. It is expected that positions and propos- als presented at the Personal Computing '76 show will be used as a starting point for the second annual BYTE magazine symposium on standards to be held later in the fall. Questions regarding standards activities and coordination of this session should be direct- ed to Carl Helmers at BYTE at the above address, or phone (603) 924-7217.8 Photo 1: Here is a black and white reproduction of a single frame of a wine pouring animation — se- quence created by Steve Dompier using his Dazzle- mation program, The colors of the original re- produce as shades of gray- ness in this black and white picture. About the Cover [This short account is based upon materials supplied by Harry Garland of Crom- memco, ... CH] Imagine being able to look inside your computer memory, actually being able to see the individual bits. With this sort of X ray vision your computer memory could also serve as your computer display. Messages could be spelled out by lighting some bits and darkening others. Games could be played with clusters of bits forming game pieces and markers. Space War might be played with miniature rocket ship patterns zooming in, out and around the visible region of memory address space. The key element of hardware required to actually achieve this imagined result is a memory module which has provisions to map its contents onto a television screen. This is precisely what Cromemco has done in creating its TV Dazzler product, the results of which were used to create this month’s cover, The TV Dazzler hardware features two modes of operation providing high resolu- tion and low resolution generation of a television picture. Through software selec- tion the TV Dazzler can be programmed either as a 128 x 128 point black and white display, or as a 64 x 64 point colored display. The points of the display grid are tiny square regions on the screen which map into segments of the 2K byte memory of the TV Dazzler module. In the high resolution “bit mapped’” mode, TV Dazzler uses its 2 K byte memory as a means of storing 2l4 = 16,382 bits required to generate a unique “on” or “off” value for each location of a 128 x 128 grid. This high resolution black and white mode is very effective for alphanumeric displays and detailed computer controlled images. 6 In the low resolution “‘nybble mapped” mode, TV Dazzler uses its 2 K byte memory as a means of storing 212 = 4096 four bit nybbles of data needed to generate a color display on a 64 x 64 grid. Each nybble determines the color and intensity of the corresponding picture element on the grid. The most significant bit sets either high or low intensity, and the next three bits inde- pendently select the blue, green and red channels of the color TV signal. Like a metaphorical beachball, (see Janu- ary 1976 BYTE editorial), the Dazzler pro- vides the hardware for an incredible variety of applications. This variety is realized through the software for games and other purposes developed by people who buy and use this type of peripheral. One particular application of the peripheral is a program called Dazzlemation which was written by Steve Dompier. The purpose of Dazzlema- tion is to record an animated sequence of TV frames in color, then play these back. In order to make such a sequence, Dazzlema- tion is used to color in the appropriate regions of single frames which are stored in memory. Steve’s standard demonstration se- quence shows a carafe of red wine being poured into a wine glass. One frame of the carafe sequence is illustrated by photo 1. This is just one of an endless variety of computer generated animated displays which is made possible by programs like Dazzlemation. A second application of the Dazzlemation hardware was used to generate the pattern which forms the main portion of the cover, This is a program called Dazzler-LIFE which was written by Ed Hall. John Conway’s fascinating game of LIFE gains a new dimen- sion when it is displayed in color. Watching the patterns evolve can be intoxicating in black and white, but becomes truly addictive when color is used to illustrate the game board. In the Dazzler-LIFE program, the game begins in a drawing mode which allows the user to draw an initial colony of cells on the screen using controls from the ASCII keyboard. Then the evolution process is initiated with each succeeding generation being displayed on the screen with colors marking the health of each cell. Cells that are too crowded, or too remote, turn a flaming red color, then wither away. New- born cells first appear in green, then grow up to a mature blue color. The kaleidoscopic result is fascinating to watch. One frame of a colorful LIFE history was photographed for the cover. . Still another application of the Dazzler is as a hardware game board for sophisticated computer automated games. One example of such an application is the Tic Tac Toe software written by George Tate. Dazzler Tic Tac Toe is written in BASIC, and demonstrates how very well suited the MITS BASIC is for creating colorful creations. George’s program is one of a class of “man versus computer” game applications, and is reputed to be extremely competent at Tic Tac Toe. A sample of the output is repro- duced here in black and white as photo 2. A useful utility program for the Dazzler, which demonstrates the bit mapped mode of operation is the Dazzlewriter software created by Ed Hall. This program turns your ASCII keyboard/computer/Dazzler combina- tion into a TV typewriter by generating the 5 x 7 dot matrix display for each keyboard character. A sample of Dazzlewriter activity is shown in photo 3. Since the main memory of the computer is used to store the charac- ter generation information, there is no need for any additional hardware beyond the memory requirements of Dazzlewriter. Another delightful application of the display is an “idling” program you’ll pro- bably want to leave in the computer system when you’re not using it for another pur- pose. This program is Li-Chen Wang’s color- ful Kaleidoscope program. The program is surprisingly short, just 127 bytes long, yet it generates an unending sequence of captiva- ting patterns. These programs were created by some of the first individuals who had access to the Dazzler hardware. They are written for the 8080 instruction set (except George Tate’s BASIC Tic Tac Toe) and are available in paper tape form from Cromemco at $15 each.™ Photo 2; Here is the game board of George Tate’s Tic Tac Toe application, written in MITS Altair BASIC with the TV Dazzler as its display peripheral. PHEGUSSRy RNS eapeeye T7239? MEMORY SIZE? TERMINAL WIDTH? DAZZLE-WRITER RUNNING WITH 8K BASIC! ! ff Photo 3: Here is a sampling of outputs generated using Ed Hall's Dazzlewriter program to turn the TV Dazzler/computer[/keyboard combina- tion into the logical equivalent of a TV typewriter style display. A Small Business Accounting System The least sophisticated form of bookkeeping is single entry accounting; it is not, however, generally suitable for preparing fi- nancial statements for banks, investing brothers- in-law, and so forth. Double entry bookkeeping has the advantage of incor- porating redundancy and error checking techniques. It is the most common form of business accounting. Or, How Your Microcomputer Can Take the Worry Out of Tax Time John A Lehman 716 Hutchins #2 Ann Arbor MI 48103 Here's an outline of an accounting system suitable for small business use on a micro- computer. It is designed for a small, inexpen- sive system having a central processor, Tele- type IO, one or preferably two cassette tapes for storage, and a high level language facility such as BASIC. It could probably be written in assembly language, but at a price of inconvenience. The system is designed to be used by an individual proprietorship (one man business) or a small partnership. While perhaps suitable as a bookkeeping system for a small corporation, it is not intended to produce the sort of reports which various regulatory agencies may require of one. It is designed to keep books, produce tax returns (either Form 1040 schedule C for pro- prietorships or Form 1065 for partnerships), produce balance sheets which may be re- quired either for management information or for the information of banks and other outside investors, and to be adaptable for check reconciliation, cash budgets, pro forma balance sheets and the like. Its use requires about the same amount of time and effort as keeping a journal would normally, with the added advantage that the entries are pretty much self checking. All other reports are produced by the programs which would be used. I'll try to describe the system in enough detail so that anyone who is skilled in BASIC and knows a little about account- ing could write a program to do all of the above. First, however, it might be a good idea to take a quick look at accounting systems and what they’re used for. Of the various systems available, the 8 simplest is the single entry system. A check book is a good example; each time money goes in or out, a notation is made of the date, the amount, and any comments on sources, uses, etc. This sort of system is obviously very simple to keep, and has the additional advantage of being accepted by the IRS for preparing tax returns. However, it has a number of disadvantages. The first is that it is not self checking, as anyone who has ever tried to balance a checkbook can testify. Also, while capable of producing an “income statement” (the generic term for what a tax return amounts to), it is not suitable for the preparation of other finan- cial statements that may be required by banks, investing brothers-in-law and so forth. These disadvantages make a single entry accounting system unsuitable for the system under discussion here. Double Entry Accounting The other major accounting system is the double entry system. It was invented about 600 years ago, and came into widespread use because it was self checking. It is also quite a bit more complicated than a single entry system. The basic idea behind the double entry system is that each transaction has two parts: where money comes from and where it goes. So each transaction is entered twice, each time in a different account. The mech- anism behind this is the idea of debits (DR) and credits (CR). Very briefly, a debit represents an addition to something which you have (an asset) or to an expense. A credit represents a subtraction from one of these. On the other hand, a debit represents a subtraction from something which you owe or from a revenue, while a credit represents an addition to one of these. All of which can be very confusing. As a quick example, suppose you pay $100 on your BankAmericard and receive $150 for some service which you performed, You would debit accounts payable (sub- tracting from what you owe) for $100, and credit cash (subtracting from something you have) for $100. Then you would debit cash (adding to something you have) for $150, and credit income or revenue (adding to revenue) for $150. The self checking feature is provided by the fact that debits must always equal credits. It would probably be a good idea to look through a beginning accounting book to get more examples to help explain accounting techniques. I've listed some at the end of the article. Besides being self checking, a double entry system has the advantage of being able to churn out all sorts of reports on what is going on in the business in question. The IRS approves of it; and in fact, large companies have no choice — they have to use it. Now that we’ve described the major accounting systems, let’s get on to what they do and how they can be used in a computer- ized system. The purpose of any accounting system is to provide information (another purpose is to provide employment for accountants, of course). This information is of use to various people. The owner of a business uses it to see how well he’s doing, and more impor- tant, where he’s not doing so well. Another important user is your friendly local IRS agent; anyone in business is required to produce accounting reports to the Internal Revenue Service’s specifications. Banks and other investors also are likely to be quite interested in this sort of information, espe- cially when their services are requested for loans rather than for deposits. Corporations are also required to provide statements to various government agencies, but we’re not going to be concerned with that here. The basic statements and reports were mentioned earlier. The first is the balance sheet such as the one shown in figure 1. This represents the financial state of the company at a particular time. The left hand side (in the US at least) represents assets, or what the firm has. The right hand side represents liabilities and equities. (Liabilities and Equities is accounting terminology for where the stuff on the left came from.) Liabilities are amounts owed; equities are amounts contributed or earned by the own- er(s). The second statement is the income statement. As was mentioned above, a tax return is a species of income statement. This shows what happened over a period of time. Other statements, such as the cash budget and the pro forma balance sheet, show what may happen in the future. These are the ASSETS: LIABILITIES: Cash 1000 Payables 2000 Receivables 2000 Notes from bank 1000 Equipment 4000 — Equity: Total Proprietor 4000 Total 7000 Figure 1: The Balance Sheet. This document shows the current financial state of a business operation, It is used by businesses large and small, and is one of the end products of the automated accounting system. EXPENSES (Debit to add, Credit to ASSETS (Debit to add, Credit to subtract) subtract) Return & Allowances RTN Cash cSH Depreciation* DEP Receivables RBL Business Taxes* TAX Inventory INV Rent RNT Prepaid expenses PPD Repairs* RPR Supplies suP Salaries & Wages SAL Equipment EQT Insurance INS Investments vs Professional fees PRE Misc, ETC Commissions com Amortization* AMT LIABILITIES & EQUITY (Credit to add, Pension/Profit sharing PEN debit to subtract) Interest INT Bad Debts BDB Payables PBL Depletion ODPL Notes’ NOT Other (specify if common, eg: MIS Long Term Payables LTP Fuel FUL Proprietor PRP Electricity PWR Drawing ORW Telephone FON Cost of Goods Sold which ccs REVENUES (Credit to add, Debit to includes subtract) Purchases PUR Materials/supplies MAT Gross Receipts RCP Other costs OTR Other Revenue REV Labor (used for or directly related to Production — does not include money paid to you) LAB “Items for which the IRS requires supplementary schedules or statements Figure 2: Account Files Example. When the double entry accounting system is designed, one of the first steps is to create a list of accounts and their corresponding mnemonic codes. The mnemonic codes are used internally by the computer in order to save memory space. If you are lavish with memory, texts of the long names could be looked up in a table when you generate reports. statements which our system is going to be able to churn out. Now, having got an overview of what we’re trying to do, let’s take a look at our data base requirements. The first thing we are going to need here is a set of names for our accounts. This is a “chart of accounts” to use the jargon of the accounting trade. A small system such as ours will need about 35 of these, selected for the most part to make our output match 9 ACTIVITY INPUT FROM DAILY a JOURNAL, IT e0! PROGRAM JOURNAL TAPE SUMMARY EoIT fi PROGRAM LEDGER EoIT PROGRAM i DETAILED LEOGER REPORT Figure 3: An Accounting System Flow Chart. This is a software system flow chart which identifies the major program com- ponents of an accounting system, The representation assumes that tape files will be used to maintain mass storage on your personal system; floppy disks with sequential access file orga- nizations could be used as well, FORECAST PROGRAM CASH BUDGET PRO FORMA BALANCE SHEET what the IRS requires. In order to save memory space in the programs, each of these accounts is also given a three letter mne- monic code. Two letters would be possible, but some ease of use would be sacrificed. Figure 2 gives a sample list of accounts and mnemonics, broken down by classification. A brief explanation of some of the accounts might be in order. Returns and Allowances is for goods which are returned for one reason or another. Its purpose is to reduce the amount in gross receipts while keeping the amount of returns separate. The category SAL includes only those wages paid which are not included in cost of goods sold (CGS). This would involve such things as clerical 10 help. INT is interest paid, not received. BDB (bad debts) is used if we want to use the specific charge off method of accounting for such unfortunate happenings. The IRS also allows use of another method, called the reserve method, but it is more complicated. DPL (depletion) is used for things like oil wells and mines. DEP (depreciation) is used for equipment, machines and the like, while AMT (amortization) is used to charge part of the cost of such things as organization expense, capitalized research and develop- ment and so forth. Some of these things can be listed as assets when the money is first spent, and the cost spread over several periods. For details see the IRS books listed at the end of the article. Cost of goods sold (CGS) is the total of the costs incurred to get something ready for sale; the breakdown is listed below it. Cash (CSH) is mostly checking account balances. Receivables (RBL) are what customers owe you on account. Payables (PBL) are what you owe on account. Proprietor (PRP) is what you put into the business and what it has earned so far. Drawing (DRW) is the account you use to take money out of the business for personal uses. Notes (NOT) is money bor- rowed from banks and other lenders. The rest should be pretty much self explanatory. These 35 or so accounts are the data files which we're going to be working from; all of the information we put into the system goes into them and all of the output uses them as building blocks. Now, having taken care of all of the groundwork, we are ready to start running information through the system. Reference to the system flow chart of figure 3 shows that the journal is the first thing we produce. It’s shown being produced on tape, since that way we can use it to produce all of the other reports without having to type in any more material, at least until we come to the forecasts. Also, by writing our journal entries onto tape as soon as they’re checked by the editing program, we save much memory space, since we need keep only a little bit of data in memory at any given time. So, in this automated sys- tem, the journal is the only file we really have to manipulate on a day by day basis. To use it, first we enter the date. Then we enter each transaction through the checking program which makes sure we have two entries for each amount and that the num- bers we give the machine match. A sample of a possible format is given as figure 4. We debit the power expense account (re- member, we debit an expense when we want to add to it) for $58, and enter the comment that this is for the month of March. Then we credit cash (to decrease it), but reverse the numbers. The program sees that the debits do not equal the credits, and fires off an error message, prompting us to enter a correction. Note here that we include the check number; this is very important when it comes time to reconcile our records with what the bank statement says. Also, the editing program should provide the ability to debit and credit unequal numbers of ac- counts so long as the totals are equal. If this would be too much of a demand on memo- ry, amounts can be split up before entry. Going on, the OK indicates that the entries check, and at this point they should be written onto the tape, Entries for the journal can come from cash register tapes, bills, etc. Up through this point our system is about as much work as a manual system, but from here on in things get much easier. The next item on the system flow chart is the ledger. This is a set of files which puts all of the journal entries for each account together. In our system, there are two types: summary and detailed. In a more advanced system, all of the ledgers would be detailed, but this would require much more memory than most small systems would have avail- able. Basically, what we do at this point is have the program read the journal entries one by one and keep a running count of the amount for each of the different-accounts in use. Beginning balances may be read in either via the Teletype or via a separate ledger tape. The ending balances should be printed on the Teletype if the user wishes to see what they are, but they should also be saved on tape for use in preparing the rest of the statements. Detailed ledgers will require a separate run for each one desired; they might be run on a weekly or monthly basis. The most important one is the cash ledger, since this will provide a record of every check written and every deposit made to the checking account by date and number. This should make balancing one’s checkbook a fairly simple task. The one thing to be careful of in this program is to be sure that the rules for addition and subtraction of debits and credits are carefully written into the program. Otherwise all that will come out is garbage. Once we have the ledger, it’s fairly easy to see how the balance sheet is generated. A look back at figure 1 will show that there are only about a dozen of the ledger accounts which have to be put together. All of the asset accounts are added together, and the sum is listed at the bottom of the column as total. Subtracting the sum of the liabilities from the stm of the assets leaves what is left for the owner. If the amount in the drawing account is set beforehand, that leaves only the Proprietor (PRP) account to be “plugged,” which is to say, given whatever value is necessary to make the two columns come out equal. So, if the assets total $7000, the liabilities total $3000 and there are $500 in the drawing account, that leaves 7000—3000-—500=3500 for PRP. The only other detail is that the program should either write the date at the top, or it should be filled in by hand. A balance sheet may be prepared at any time; it will often be required for getting a loan from a bank. Besides being run on paper, it should be run onto tape for use in preparing forecasts. Probably the most important report which our system will prepare is the income statement. This is a report which shows what has happened over a period; usually a year, but often prepared on a quarterly or a monthly basis. Its importance arises not so much from the fact that people like to see how much money they’ve made as from the fact that the government is quite interested in this information — so they can take their cut, of course. The system being illustrated produces an income statement patterned The balance sheet is a snapshot of the current status of the business. A mass storage file comes in handy for business ac- counting, since much of the work involved is ac- complished by reviewing the same data with dif- ferent criteria to produce reports. Figure 4: An Example of the Interactive Dialog with the Journal Edit Program. The purpose of this program is to filter your own manual inputs looking for certain known discrepancies which can be detected by the double entry bookkeeping method. In this example, upper case letters are the computer output to a Teletype (or video terminal) and the lower case letters indicate manual keyboard inputs taken from daily activity records such as receipts, checks written, etc. OK Interactive program for journal entries might read: ENTER NAME OF ACCOUNT DEBITED,AMOUNT,AND COMMENTS SEPARATED BY COMMAS. Pwr,58,march ENTER ACCOUNT CREDITED,AMOUNT,AND COMMENTS SEPARATED BY COMMAS: csh,85,check 346 DEBITS DO NOT EQUAL CREDITS—ENTER IF DR OR CR TO BE CHANGED: cr ENTER ACCOUNT CREDITED,AMOUNT,AND COMMENTS SEPARATED BY COMMAS: csh,58,check 346 ENTER NAME OF ACCOUNT DEBITED,AMOUNT,AND COMMENTS SEPARATED BY COMMAS: iam done OK GOODBYE W Figure 5: The object of much of this program ac- tivity is filling out IRS Schedule C for your small business. after Form 1040 Schedule C (figure 5), but could produce Form 1065 for partnerships with minor changes. As is fairly obvious to those who can wade their way through the governmentese, what we have to do here is state all income and then subtract expenses. The accounts which we have been working with will do this on what is called an accrual basis, which is to say future expenses and revenues are included if they are certain and we know how much money is involved. For example, if we have charge customers, we include what they are scheduled to pay us in revenues. For a small business it is often better to file a tax return on the cash basis in which only cash in is considered revenue and cash out is considered expense. This system can prepare cash basis returns too; one must eliminate receivables, payables, prepaid ex- penses and materials and supplies not yet part of cost of goods sold. The effect of all of these should be taken out of the revenue and expense accounts too, That’s the basic system. Using this system alone would be a pretty respectable account- ing setup for a smal! business. But as long as we’re using a personal microcomputer, we might think of adding a few bells and whistles. These would pretty much depend on individual wants. We could have the computer automatically calculate FICA de- ductions when payroll expense is debited. We might also have the machine figure our depreciation and amortization schedules for GLOSSARY Accrual: Including payments and receipts in the future. Check reconciliation: Accounting buzzword for balancing a checkbook. Credit (CR): An addition to the righthand side of the balance sheet or to income. Debit (DR): An addition to the lefthand side of the balance sheet or to an expense. Journal: The accounting equivalent of a check register. Ledger: Book or file which contains the totals from the journal broken down by categories. Payables: Amounts which will have to be paid in the future. Pro forma: Buzzword used to describe reports which show how things might be or might have been rather than what they are. Proprietorship: A one man business; one owner. Receivables: Amounts which are not yet on hand in cash but which will definitely be coming in in the near future. us. For this we would need (for each item or class of items) initial value, estimated life and age. For tax purposes we would want to get our annual depreciation by taking two divided by the life of the object and multi- plying the total times the remaining value. In more symbolic fori (2/total life)*(initial — depreciation). This would give us the depreciation to date and the amount for this year, both of which are needed for the flip side of the tax form. We could also do forecasting with the system. For this we would want an inter- active program which would ask for esti- mated expenses and receipts in afl the different categories for x number of months. Then we would prepare a (pro forma) balance sheet for the end of the period if our predictions were correct, so that we could see where things would stand if the predic- tions came true. It could also prepare a month by month schedule to show whether the firm would have enough on hand to meet projected outflows, This is called a cash budget, and is quite a handy thing to have since it enables you to forecast cash shortages far enough in advance to do something about them, and also to compare the results of different courses of action. And there’s the system. While not very fancy from either an accountant’s or a system designer’s point of view, it ought to be enough to handle much of the record- keeping for those firms on the other end of the spectrum from GM, IBM and ITT. It might be too that the availability of a few business oriented systems like this will help increase the sales of microcomputers and bring the prices down even more through mass production.™ REFERENCES 1. Accounting Essentials, Margolis, Wiley and Sons 1972. 2. Elementary Accounting, College Outline Series 3. Management Accounting, Anthony and Reece, Irwin, Inc, 1975 (Note: this is a college accounting textbook — heavy reading). 4. Recordkeeping for a Small Business, |RS #583, 1976. 5. Tax Guide for Small Business, IRS #334, 1976 (complete handbook). 6. Tax Information on Accounting Periods and Methods, \RS #538, 1975. The last three are available free from any IRS office. MEET THE DIGITAL GROUP If you're taking time to read this ad, we already know a thing or two about you. We know you're interested in devouring everything you can get your hands on that deals with microcomputer systems, and we also figure you're ‘out there shopping around . . . looking for the most sensible pro- ducts to fit your needs and your budget. That's why we think you should get to know us. We're the Digital Group, a relatively small, 18-month-old organ- ization obstinately dedicated to providing quality in every product we offer. You may have already heard a little something about us from a friend — our reputation does seem to be getting around quickly, even though we've never advertised before. We think it’s due to a number of important factors: state-of-the art designs, a really complete systems philosophy, unexcetled quality, reasonable software, three-week delivery, and no pre-announcements until we're ready to deliver. Our products are not just a gleam in our designers’ eyes; they are currently being delivered . . . fast! The Advantages Here are a few specific advantages of our product line: @ We offer CPU's from different manufacturers which are inter- changeable at the CPU card level. That way, your system won't be- come instantly obsolete with each new design breakthrough. The major portion of your investment in memory and I/O is protected. @ Digital Group systems are complete and fully featured, so there's no need to purchase bits and pieces from different manufacturers. We have everything you need, but almost any other equipment can be easily supported, too, thanks to the universal nature of our systems. @ Our systems are specifically designed to be easy to use. With our combination of TV, keyboard, and cassette recorder, you have a sys- tem that is quick, quiet, and inexpensive. To get going merely power on, load cassette and go! @ Design shortcuts have been avoided — ali CPU's run at full maxi- mum rated speed. The Features Digital Group Systems — CPU's currently being delivered: 8080A/9080A 6800 6500 by MOS Technology All are completely interchangeable at the CPU card level. Standard features with all systems: Video-based operating system @ Video/Cassette Interface Card 512 character upper & lower case video interface 100 character/second audio cassette interface @ CPU Card 2K RAM, Direct Memory Access (DMA) Vectored Interrupts 256 byte 1702A bootstrap loader All buffering, CPU dependencies, and housekeeping circuitry @ Input/Output Card Four 8-bit parallel Input ports Four 8-bit parallel Output ports | © Motherboard Prices for standard systems as featured above, start at $425 for 8080 or 6800 and $375 for 6500. More Many options, peripherals, expansion capabilities, and accessories are already available. They include rapid computer-controlled cassette drives for mass storage, color graphics, memory, I/O, monitors, multi- ple power supplies, prototyping cards, and others. Software packages include Tiny BASIC Extended, games, ham radio applications, soft- ware training cassettes, system packages, and more (even biorhythm). Sounds neat — now what? Now that you know a little about who we are and what we're doing, we need to know more about you. In order for us to get more infor- mation to you, please take a few seconds and fill in our mailing list coupon. We think you'll be pleased with what you get back. Ee eT) I THE DIGITAL GROUP, Inc. P.O, Box 6528 4 fl Denver, Colorado 80206 mmm (303) 861-1686 OK, I'd like to get to know you guys better. Send me the whole package! Name Address City/State/Zip SS Photo 1: Here is a pair of the new mN601 microNOVA chips which come in 40 pin dual in line ceramic packages. The performance of the microNOVA processor is quite good as microcomputers go. The 16 bit accumulator to accumulator addition time is quoted as 2.4 ws, and the accumulator load time is quoted as 2.9 ps. This compares very favorably with the original Data General NOVA minicomputer performance of several years ago. 14 16 Bits Doth a General Purpose Minicomputer Make A new option in the high performance 16 bit microcomputer field has now arrived on the scene. This is the Data General micro- NOVA, introduced early in March as BYTE’s June issue went to the press. Here is another instance of a widely available minicomputer architecture being made available in the form of a relatively inexpensive micro- computer, For those who are new to the computer field, the Data General NOVA line of mini- computers is one of the oldest and most widely used products in the conventional minicomputer product area. NOVAs are available with performance which now ranges from the microcomputer chip level to the complete general purpose data pro- cessing systems represented by the top of the line ECLIPSE computer. NOVAs have long been an option used by persons who purchased minicomputers as combination business/pleasure computers, and the new microNOVA product will expand this area of use. Based on the product description information supplied by Data General, here is a summary of the microNOVA product’s semiconductor line: mN601: 40 pin package NMOS central processor. The full NOVA instruction archi- tecture is implemented in a single chip design, which also includes several additional features not generally found in NOVA computers: © hardware stack and frame pointer @ SAVE and RETURN instructions for efficient subroutine linkages © 16 bit hardware multiply and divide © real time clock © memory management logic: memory Photo 2: One of the simplest systems products above the chip set level is the microNOVA computer on a board product, Model 8563. This version of the microNOVA one board computer includes 4K words of semiconductor memory and the ability to run software accumulated during Data General's eight year history with the NOVA computer line. control, timing and hidden refresh logic for dynamic RAMs. © device data channel contro! mN603: 40 pin package, NMOS, IO controller. This is an integrated circuit de- signed to implement the standard NOVA 10. instruction set via DMA operations. System Buffer Elements: A family of several integrated circuits designed to pro- vide system expansion capabilities. Data General provides a fully integrated line of hardware packaging and software support to accompany this microNOVA system. Data General primarily orients its products towards the OEM and systems integration markets, so this product will probably prove most attractive to a fairly sophisticated personal computing user, or to the person who works with OEM computing as part of his or her professional activities. But for those who know how to specify and pur- chase a minicomputer system, a floppy disk based microNOVA system with BASIC or FORTRAN capabilities could prove to be a quite powerful personal computing system useful for both business and pleasure pursuits. Data General Corporation is located in Southboro MA 01772. Sales and service offices are located in major cities.= Photo 3: For the do it yourself systems house, Data General provides a complete line of modular components which can be integrated to form a microNOVA system. One of the most interesting characteristics of the microNOVAs integrated to the minicomputer level is a ‘Programmer's Helper” remote console packaged in a calculator style case at the end of a ribbon cable. The console is shown in its storage position in the front panel assembly at the center of this picture, MODEL CC-7 SPECIFICATIONS: A. Recording Mode: Tape saturation binary. This is not an FSK or Home type recorder. No woice capability. No Modem. (NRZ) B, Two channels (1) Clock, (2) Data. OR, Two data channels providing four (4) tracks on the cassette, Can also be used for Bi-Phase, Manchester codes etc. Inputs: Two (2). Will accept TTY, TTL or RS 232 digital. Outputs: Two (2). Board changeable from RS 232 to TTY or TTL digital. Runs at 2400 baud or less. Synchronous or Asynchronous. Runs at 4800 baud or less, Synchronous or Asynchronous. Runs at 3.1"'/sec. Speed regulation + 5% 9 9 F. Compatability: Will interface any computer or terminal with a serial 1/0. (Altair, Sphere, M6800, POPS, LSI 11, etc.) G. Other Data: (110-220 V), (50-60 Hz); 3 Watts total; UL listed 955D; three wire line cord; on/off switch; audio, meter and light ‘operation monitors, Remote control of mo- tor optional, Four foot, seven conductor remoting cable provided. Uses high grade audio cassettes, . Warrantee: 90 days. All units tested at 110 and 2400 baud before shipment. Test cas- sette with 8080 software program included, This cassette was recorded and played back during quality control, ALSO AVAILABLE: MODEL CC-7A with vari- able speed motor. Uses electronic speed contro! at 4" sec. or less, Regulation + .2% Runs at 4800 baud Synchronous or Asynchro- nous without external circuitry. Recommended for quantity users who ex- change tapes. Comes with speed adjusting tape to set exact speed, = DIGITAL DATA RECORDER $149.95 FOR COMPUTER or TELETYPE USE, Uses the industry standard tape satura- NOW AVAILABLE RECORD/PLAYBACK AMPLIFIER KIT This expanded version of our Computer Aid board can be used with your own deck {cassette or reel to reel), Go to 9600 baud ‘on reel to reel with suitable heads, Digital in, digital out, serial format. Kit includes all parts, case and power supply. $59.95 COMING SOON — IN KIT FORM Any baud rate up to 4800 * 1/0 Board for use with Computer Aid or other digital recorders, Variable baud rate selectable on externally located unit by ‘one knob. Can load computer or accept dumps without software, thus providing Turnkey Operation, For any 8 bit compu- tion method to beat all FSK systems ten to ‘one. No modems or FSK decoders required. Loads 8K of memory in 17 seconds. This recorder, using high grade audio cassettes, enables you to back up your computer by toading and dumping programs and data fast as you go, thus enabling you to get by with less memory. Can be software controlled. Master Charge & BankAmericard accepted. On orders for Recorders and Kits please add $2.00 for Shipping & Handling. (N.J. Residents add 5% Sales Tax) NATIONAL multiplex CORPORATION 3474 Rand Avenue, Box 288 South Plainfield, New Jersey 07080 (201) 561-3600 ter, Automatic semi DMA operation of cassettes with automatic return to termi- nal a 3810 board for a complete COS with the Altair or Imsai. $100 Kit. $150.00 wired and tested. * Hexadecimal or Octal Keyboard — Load Programs direct from keyboards’ 20 keys and verifying display. Does not use Com- puter 1/0, Can be wired Octal or Hex. — ‘Your choice, * Interested in these? Send your name and address for brochure when released. Send One dollar for Cassette Operating and Maintenance Manual with Schematics and Software control data for 8080 and 6800. Also applies to Kit above. (Postpaid) 15 Build a Television Display C W Gantt Jr 6 Fieldpoint Rd Aurora IL 60538 As a small system expands and becomes more sophisticated, the limiting factor is often the speed of input and output (10). In addition to being noisy, mechanical, and paper consuming, the slow clacking of a TTY may account for a large percentage of system time. Among the alternatives, the display of characters on a standard TV set is among the simplest and most economical methods, This TV display (TVD) is designed to take data from 512 bytes of memory and convert it into a video signal with 16 lines of 32 characters. This can be used to feed a black and white or color TV. The data in the TVD memory is in a six bit ASCII subset and is updated by the CPU to create the desired display. The processor addresses the TVD memory just as it docs any other portion of memory and can actually execute instructions from the TVD memory if so programmed. Of course, some provisions FIRST LINE, SECOND FRAME FIRST LINE, FIRST FRAME HORIZONTAL RETRACE \ —~ HORIZONTAL sone Starrs vertican ReTRACE | ~ onizon aL, SyanTs vertican Sine SIMPLIFIED INTERLACED RASTER SCAN Figure 1: This shows how the electron beam is moved during an interlaced scan in a television monitor. The dashed lines are quick retrace motions which are normally invisible. The solid lines are periods during which the display presents video information controlling brightness on the tube face. 16 must be made to prevent the CPU and TVD from simultaneously accessing the TVD memory (more about this below). As designed, the TVD is strictly a display device with the central processor of your system doing all housekeeping (entering characters, etc). This approach simplifies the hardware at the expense of extra software, but also allows the user to take advantage of the flexibility offered by software data manipulation and formatting. At present one TVD is up and running in my system, but the memory and central processor interfaces are incomplete. The remainder of this article therefore empha- sizes the TVD design and only offers some basic ideas on interfacing to processors. Although simple items such as the power supply and oscillators have been omitted, the information furnished should be suffi- cient for the more experienced readers to assemble a working version. The straight forward TVD design allows easy modifica- tion to meet individual system requirements, Television Raster Scanning Before going too deeply into operation of the TVD, a review of the basic television scanning system will clarify some terms with which pure digital designers may not be familiar. A television picture is formed by scanning an electron beam across the face of the picture tube, A TV line is one sweep of the electron beam from the left of the picture tube to the right (as viewed from the front of the set) and is initiated by the horizontal sync (see figure 1). The horizontal syne pulse causes termination of a line, horizontal retrace of the electron beam back to the left side of the screen, and the start of a new line. During the time of retrace the beam is blanked so that the retrace will not be seen. The time allotted for each complete line (including retrace) is 63.5 microseconds. Of this about 16% is taken by retrace, leaving 53.5 us of usable line. Video information in the form of a voltage fed to the picture tube controls the brightness of the beam as it is swept across the screen. To trace out a frame, the electron beam is slowly deflected from the top of the screen to the bottom as it rapidly sweeps horizontal fines. This vertical sweep is allotted 16.67 milliseconds (60 Hz) so there are 262' lines in one frame. In a manner similar to the horizontal sync, the vertical sync causes the beam to be returned to the top of the screen to start a new frame. The beam is blanked during vertical retrace which takes about 1250 us. This leaves 242 usable lines in each frame. A complete picture is formed by two consecutive frames that are interlaced with each other. Interlacing means that the hori- zontal lines of one frame fit in between the horizontal lines of the other frame. The result is 30 complete pictures every second of about 484 usable (525 total) lines each. Because of the interlacing, however, the screen is illuminated at a 60 Hz rate. This eliminates an objectionable “flicker” that would be seen if the screen were only scanned at a 30 Hz rate. The TV signal received at the antenna terminals contains the information needed to generate the vertical and horizontal sync, blanking, and video. The TVD simulates a TV signal by supplying a composite wave form containing the same information nor- mally present except sound. The full schematic of this TVD design (except memory) is shown in figure 2. Character Generation The scanning nature of the TV raster requires that the video (or brightness) infor- mation be sent in serial form to control the electron beam as it sweeps lines across the _ screen. Suppose, for example, that the character “‘H” is to be displayed as shown in figure 3. The first line can be represented as 10001, ones signifying light spots (dots) and zeros signifying dark spots. The remaining six lines can similarly be represented as a series of dots and dark spaces. When the seven lines are displayed one above the other, the character “‘H” is seen. The tedious job of deciding where to put the dots (ones versus zeros) to generate a given character is done by the 2513 read only memory, IC14. It has been mask programmed at the factory with the bit patterns required for 64 separate five by seven dot matrix characters. The 2513 sup- plies five bits of parallel output data repre- senting one line of a given character. It requires the six bit ASCII subset code of the character and the three bit line number as inputs. The five bit parallel output of the 2513 is converted to serial data by the 74165 shift register, 1C15. To produce one line of video, five bits are required for each character in the line, plus spacing bits. Note 7 Photo 1; This is a test display pattern generated by connecting the low order outputs of the char- acter and line counters to the character generator ROM'’s 6 input bits. The \result presents every com- bination of the character set, so every character pat- tern is visible on the screen. +8 SYSTEM ADDRESS BUS — +5 36 1 2 +5 TOP OF PAGE TOs : K 6 CINE AS AG AT AB COUNTER 4 wd sf ol sl 25 IcHaracTeR 2 4 vind lad TP6 UP 1c2 car 2 1c3 CARI 74192 rR 74193 A 8 cp : PAGE, ONE SHOT al el 7] ROW Ic13 PIN IS MEM CTL ADDRESS TO RAM ‘MEMORY BUSY ‘O=MEMORY AVAIL, *— 1C13 PINIZ SYSTEM ADDRESS BUS Me ——s c Jo.1U09 pub Auy1n34J9 Uo}JoJauab asnq alUdt} ay] JO Sj/DJaP Sapnjrut wo.iboip sty, ‘Avjdsig AL ay) Jo wivsbojg 2oWAYIS :Z ainbly *p ainbly ul Apjonjdaguos uMoys synd1J9 Kiowaul ayy) JO Buljim pal{oyap SYWIO If ‘uol.iauab Avjdsip uolsjaajay 404 2160p a DOT COUNTER 9 Ice 8 7490 IRccK ica RES 7400], ; 7492 lAcLK 10 MHz CLOCK No ctky [3 a) ls mnz onl +8 1248 MEM. CTL. 16. s AQ AT A2 AS aa +9 15] iL iol oft +5 ABCD OL ja o 1c 74193 pall ra & A Bo 32. ey ad ADORESS TO RAM abe CHARACTER COUNTER 82 83 L=END OF VIDEO LINE 2MHz if ¥%s iCi0o ONE SHOT ¢ 1ob 3 6 7400 5} 7400 y|270 ¥ 7 RAM $s DATA z COMPOSITE SYNC Icl6e TPS = = owxfeeE0 191 orzofzeZo 9191 zoxhse) Eo 207. 0N V2W i cL sHiET [— rar) 74165 x ouwovad sat) ta] 151 16) 1 15T 16) ROW FROM Ic2, 74192 ea RFC T= BLANK COMPOSITE BLANKING ‘O=BLANK VCs. PINIA 220, MODULATED. 5 RF OUT Power Supply Connections for Integrated Circuits Type yar2i 74192 74193 7490 SV 12 No. Ico Ic10 1c 112 1c13 1c14 1c15 tc16 Type 7492 7400 74193, 74193, MM5320 2513 74165 7404 6 16 8 24 16 “4 +8V GND SV -12v oo. 1 nae .oon 8 that the TVD generates two identical inter- laced frames to make a complete picture. The result is that each character is actually 14 lines high. Sync Generator The MM5320 sync generator chip, IC13, uses a single 2.0 MHz input to produce all the syne and blanking signals needed for a 525 line interlaced raster. The same logic could be wired using TTL but would require considerably more hardware and probably cost just as much. (The 5320 runs $4.95 ppd from NEXUS Trading Co, Box 3357, San Leandro CA 94578.) The only disadvantage fund thus far with the 5320 is that it prefers a square wave 2 MHz source. To this end the 100 nanoseconds pulse from the 7490 “D” output is squared using two 7400 sections of 1C10 as a oneshot. Line Generation Horizontal drive (coincident with hori- zontal sync) from the 5320 triggers a 74121 oneshot, IC1, to delay the start of each line and establish the left hand margin on the screen. The output of the oneshot serves three purposes: 1. Triggers the 74192 row counter, 1C2. 2. Resets the 74193 character counters, 1C11 and 1C12. 3, Inhibits the dot counter, 1C9, until the start of the line. When the line oneshot output pulse ends, the dot counter starts counting at S MHz. It resets itself every seventh count to allow for the five dots of the character plus a two dot space between characters. When the dot counter resets, it also loads the next charac- ter into the 74165 shift register, IC15. (The very first character of each line is all zeros since the 74165 is not loaded until the dot counter resets the first time.) The 74165 shifts out the two dot space and the five dot character at a 5 MHz rate. As each character is loaded, the 74193 character counter incre- ments by one to change the address for the RAM to the next character. When the 32nd load pulse occurs, the 5 MHz input to the dot counter is inhibited using the “B” output of the second 74193 character counter. The 74165 continues to shift out the 32nd (last) character and then shifts out a steady zero. When the character counters are reset at the start of the next line, the process repeats itself. Line Counter The 74192, IC2, counts each video line dis- played. It counts to 10 for the seven lines of character information plus a three line space. BIT (DOT) 43 2 I LINE | 10001 (ROW) 2 10001 3 10001 4 mi 5 WSS tooo1 6 10001 7 10001 5 DOT WIDE BY 7 DOT HIGH CHARACTER The A”, “B”’, and “C’’ outputs control the row inputs to the 2513 character generator chip. The first video line is all zeros since the row input to the 2513 is zero. Lines 9 and 10 are blanked using output “D” of the 74192, resulting in a total of three lines blanked. At the end of each complete line of characters, the 74193 line counter, 1C3, increments by one until, at the end of the 16th line, a carry pulse is produced. This carry pulse resets the 7490, ICS, and signi- fies the end of a page. Output “A” of the 7490 is used to inhibit the 7492 dot counter and prevent the first line from being repeated at the bottom of the page. Page Control The 7490, ICS, stays reset until the top of the next page. Output “‘A’’ can be used to tell the memory control circuits that the TVD is not using the memory so that any required updates may be made by the CPU. Output “A” also inhibits the “B Clock” input via the 7410. The “D” output inhibits the line oneshot. When a vertical drive pulse (coincident with vertical sync) triggers the 74121 page oneshot, IC6, the TV set syncs to the top of the next frame. The page oneshot delays the start of the first line to establish the top margin. At the end of the oneshot’s output, IC5, bit ‘A’ is clocked to a one. This tells the memory control that the TVD needs to resume control of the memory address and also enables the ICS “B Clock” input via the 7410, 1C7. The ‘“‘B” section of the IC5 then proceeds to count color burst gate pulses to give the memory time to complete any access already in progress. The color burst gate was used only because it was convenient and occurs at the same rate as the horizontal drive — the horizontal drive could be used at the expense of a buffer since the 5320 can 19 Figure 3: An example of a dot matrix pattern gen- erated by the television display. ADDRESS BUS ——~ S LINES SELECT DECODE TV DISPLAY T_oap oaTA ADDRESS le—— READ / WRITE PATA 0 AVAILABLE mem NeMoRY cpu RAM > READ/WRITE | CTL READY 6 ORB LINES] 9 LINES ane fbi x6 (oR 8) RAM CONTROL f INFERe ACES TRIZSTATE SUINES BUFFER ate gus Figure 4: System Diagram. This figure details how the TV display fits into a central processor’s memory address space. The low order 9 lines of address go directly to the line and character counters of the TVD; the memory array is addressed by the outputs of the counters, which are connected logically to the address bus when the load line demands central processor access. The high order bits of the processor’s address are decoded separately and are used to enable processor access if the TV display portion of address space is referenced, only drive one TTL load per output. When output ‘‘C’’ of the ICS goes high, the line counters are reset. When output ‘‘D’” goes high the ‘'B clock” is inhibited via IC7b, and the line oneshot is enabled. This allows the first line to start. Composite Video Generation The video and sync are independently adjusted and then added to produce com- posite video. This can be piped directly into a set (be sure not to touch a hot chassis!) or used to modulate a low power RF source. A signal generator works fine for tests. (See “Television Interface’ by Don Lancaster, page 20, October 1975 BYTE, for a thorough discussion of the various tricks to improve the interface.) Memory Interface Figure 4 illustrates how the TVD fits into a larger system. It is intended that the address outputs of the 74193 character and line counters (IC3 and C11) be hard wired to the address lines of a 512 or 1 K by 6 static random access memory using 2102s or similar parts. The data outputs of low order 6 bits of this memory are the ASCII charac- ter select inputs to the 2513 character generator, 1C14, and can be gated back to your system’s data bus if you want the CPU to be able to read from the RAM. (Of course, a 512 by 8 memory would be needed if the CPU is to be able to use the RAM for other tasks.) The data inputs of the RAM tie 20 to the data bus to allow the CPU to write into the RAM. To avoid breaking up the picture on the display during access, the memory control logic must use the “A” output of ICS to tell when the CPU can use the RAM and when it must signal a busy to the CPU at the start of a page. There is more than ample time between the “A” output and the line counter reset to finish any access in progress. To use this feature, the memory busy line must be wired to your processor’s ‘‘memory ready” line (possibly through an inverter if the logic of your particular computet i%- quires it). This method will work well for any processor, like the 8008 or 8080, which allows unlimited “memory busy” delays. However, for dynamic processors such as the 6800, the maximum processor delay time of about 5 us dictates use of an alternative approach. One simple approach is to ignore the effect of memory access on the display. The result will be a short glitch in the display corresponding to each computer access. The nature of the glitch will be a resetting of the line and character counters to a new location, causing a scrambling of the display for the remainder of the current frame. A second approach is to wire the memory ready line into a single bit input port which can be tested as a status flag: If the line indicates a retrace, then the memory access software for the display will allow an update to occur. The CPU addresses the RAM through the character and line counters (IC3 and IC11) by tying their data inputs to the system address bus and using the load control of pin 11. The 74193s can also be used as tem- porary storage for the address in a system with a common address and data bus. Note that the TVD does not interfere with CPU access to the remainder of the system’s memory at any time and only delays the CPU by one of the techniques discussed above if it tries to access the TVD RAM while a page is being displayed. The CPU has the entire vertical retrace to make updates at once every 16.67 milliseconds. Lacking a memory for my initial testing, the 2513 data inputs were temporarily tied to the 74193 address outputs (2513 PIN 17 to character counter PIN 3, etc) to display the complete 2513 repertoire every two lines as in photo 1. The 74193 load lines must also be connected to a ‘‘one.”” Modifications and Adjustments 1. There is one known bug so far and no doubt more will show up when the TVD is integrated into a system. The 7490 can, on power up, hang in state with both the “C’’ and “D” outputs a one. This state con- tinuously resets the 74192 row counter and is nonrecoverable. A cure would be to power-on-reset the 7490 “R9” which does produce a recoverable state. 2. As mentioned before, the 5320 likes a square wave input, so check the 7400 imitation of 1C10a and b oneshot, or better yet use a 74121, 3. Using a separate oscillator for charac- ter generation (5 MHz) would allow adjust- able character width, but watch out for any interaction with the 2 MHz — it shows up as a torn, garbled display, as will most sync or jitter problems. A crystal is best. (It is possible to use 12 MHz and a 7492 in place of IC8 to get 6 MHz for the characters and 2.MHz for the 5320.) 4, The prototype is wire wrapped on a 4% by 6 inch (11.43 by 15.24 cm) vector board (see photo 2) with room to spare, although a slightly larger board would accommodate more interface goofs. Fulp’s corollary says things like this always get bigger. Also, the 44 pin connector planned for the prototype is not large enough count- ing the additional RAM address and data lines. 5. The modulation levels for the radio frequency modulator are fairly critical and misadjustment of sync or video levels will cause a torn display. Try setting video level control for 1/2 of maximum and sync for 3/4 of maximum. 6. Harmonics from the 10 MHz tend to leak into the TV so pick a higher channel (5 or 6) if herringbone is noticeable on your display. 7. There are many causes for ghosting and smeared characters including VSWR {voltage standing wave ratio) on the cable to the TV, misadjusted fine tuning, or a narrow band width TV. 8. Character and line spacing can be altered by modifying the dot and row counters, respectively, to reset at different counts. Be careful though, or the display will not fit on the screen. 9. The unused bits C and D of the second 74193 character counter, 1C12, may be used for the 512 and 1024 bits if a two or four page RAM is desired. Some method of controlling these bits during display time is needed to select the page. 10. A light pen could strobe the present RAM address into a latch to be read by the CPU via the data bus. 11. The 5320 provides color sync gating so how about color characters? The extra 2 bits available with an 8 bit wide RAM can provide software control of many goodies (like brightness, color, blinking, underlining, black on white, etc). The TVD as described can be used to display one or more pages of ASCII charac- ters and opens up many possibilities of modernizing the IO portion of a small system. My home brew computer will be a complete microcomputer chip based system, designed with the TVD as the main man machine interface.@ 21 Photo 2: Prototype Cir- cuit. The large socket is for the character genera- tor. Test points are the 6 small rectangular objects along the left hand side of the board. A 7812 regula- tor in a plastic package is to the right of the charac- ter generator, and is used to provide the —12 V bias for the ROM. A zener diode with a dropping re- sistor is used to create the —5 V bias required for the ROM. Programming for the Beginner A Structured Start Ronald T Herman Simpson Rd RFD 1 Box 125 Windham NH 03087 A program can be viewed as an edifice built from the bricks of SEQUENCE blocks, and the mortar of IFTHENELSE, DOW- HILE, DOUNTIL and SELECT blocks. For a number of years now the field of computer programming has been moving from the realm of a black art to an organized and systematic process. A number of pro- gramming techniques have evolved during this change. This article will present the basics of a technique known as structured, top down programming. In the process of applying these techniques in my own work, it occurred to me that the basic concepts could be useful to those just learning to program, not to mention the veteran hackers in the crowd. If learned at an early stage, these techniques can lead to more rapid and sound development of one’s programming skills. A structured approach to program devel- opment has among its virtues the following points: e It allows the novice programmer to get acquainted with programming logic without having to be concerned with a specific machine or programming lan- guage. It allows him to grasp the flow of a program without worrying about bits and bytes. e Followed correctly, structuring can lead to a program that is relatively free from logical errors the first time it is coded and relatively easy to debug once it is run on the machine. e Pseudo code, a byproduct of struc- turing, allows a means of exchanging program ideas with others, regardless of the machine with which they might be familiar. 22 e Pseudo code provides a convenient alternative to flow charts that can be incorporated into a program listing as comments for future reference and explanation. This process of getting things done in an organized fashion has its drawbacks. How- ever, most of these seem to be psychological. Properly applied structured technology tends to minimize one of the facets of programming that has attracted many in the past: the chance to see how cleverly and concisely one can write a software routine. This seems to have been replaced by the challenge of trying to write a routine in a straightforward manner and at the same time trying to rigidly follow a set of fairly simple rules. What will be presented in this article are some of the basic building blocks of struc- tured programming and an example illus- trating the design of a simple program using these blocks. The Building Blocks of Structure So much for the sales pitch. What then is structuring? Some number of years ago it was shown that a program could be built from a set of simple building blocks all having the property of one input and one output. While not everyone agrees on what composes this set of building blocks, the one in, one out property is common to all. Presented here are a few of the most common examples that should cover most situations. The SEQUENCE Block Probably the simplest (and most trivial) unit of structure is the SEQUENCE. This is illustrated in figure 1 and is nothing more than one process performed after another. The IFTHENELSE Block One of the powers of a computing ma- chine is to make a decision based ona set of conditions and take a specific action as a result of that decision. This capability is represented as the IFTHENELSE block shown in figure 2. In the figure, “p” is an expression or some set of conditions. In a checking account, for example, one adds deposits and subtracts checks written. An IFTHENELSE statement of this fact would appear as follows: IF (transaction 19 2 deposit) THEN i (add amount of trangaction to balance) ELSE (eubtract anount of transaction from balance) ENDIF Here is our first example of writing a program step in a machine independent “pseudo code.” The format of pseudo code is mostly a matter of taste. The punctuation is optional, but the indentation is necessary for readability where many complex IFTHENELSE decisions are grouped to- gether. Some people use asterisks (*) instead of colons (:) to mark margins and some omit the parentheses around descriptive phrases. The ENDIF helps clarify the limit of opera- tions within a more complex statement. Each statement line represents a process to be performed or a condition to be tested. The statement or condition preferably should not be continued on another line. The DOWHILE Block The decision making capability of com- puters, combined with the ability to change the order in which instructions are executed, provides an even more powerful feature — the ability to repeat a calculation or series of operations many times. This capability is represented in the DOWHILE building block shown in figure 3. The DOWHILE is just a special application of the IFTHENELSE given earlier. In a DOWHILE block, a proc- ess is done as long as a set of conditions “‘p” is true. Note that the condition is tested first before the process is performed. Suppose you have 10 transactions to update into your checking account, some checks written and some deposits. In pseudo code this becomes: (set councer to nusber of teanaact ions) 10 WHILE (count ta non zero) (process the transact ion) (decrement the count) exoD0 Note that the DOWHILE is terminated by an ENDDO. The “(process transaction)” state- ment could be the IFTHENELSE given above. If combined, the result would be as follows: (wet counter to number of transactions) Do MAILE (count is non zero) 1 ELSE (subtract asount of transaction from balance) ENDIF (decrement the count) ENDD0 The DOUNTIL Block The DOUNTIL block is shown in figure 4. It differs from the DOWHILE only because the condition “p”’ is tested after the process is performed. This can simplify the writing of machine code from pseudo code. Suppose one wanted to read characters from a keyboard until a carriage return is en- countered. It could be done with a DOWHILE by saving the last character read as follows: (clear last character read) DO WHILE (ase character not a carriage return) (get a character from the keyboard) 2 (eave character in last character read) eNpD0 SEQUENCE ; PROCESS 8 END IETHENELSE STRUCTURE THEN PROCESS PROCESS A 8 23 Using structured program- ming concepts, many logical errors and bugs can be caught at an early stage in the design process. Figure 1: The SE QUENCE structure is a series of self contained processing steps which are executed one after anoth- er. Flow in this diagram begins at the top and pro- ceeds down the diagram, The number of steps de- fined in a SEQUENCE block is arbitrary; the example here shows two steps, A and B. In this article’s figures, the nota- tion BEGIN and END is used to mark the well de- fined extrance and exit points of the structures depicted. (NOTE: Proces- ses A and B may be more complex combinations of the building blocks in all of these figures.) Figure 2: The IFTHEN- ELSE structure is a con- ditional test and two alter- native SEQUENCE struc- tures. The THEN alter- native is executed if the condition, P, is found to be true. In this illustration, the THEN alternative is shown as a one step SE- QUENCE structure called B. The ELSE alternative is executed if the condition is found to be false. In this itustration, the ELSE al- ternative is shown as a one step SEQUENCE structure called A. DOWHILE STRUCTURE DOUNTIL STRUCTURE PROCESS. A Figure 5: The SELECT structure is a more com- prehensive version of the IFTHENELSE concept; it allows data to be tested for multiple cases. The re- sult is the picking of one of “N”’ cases, In this exam- ple, N is 3, so there are three SEQUENCE struc- tures which might be exe- cuted depending upon the case determination. PROCESS A Figure 3: The DOWHILE structure is a looping form which repeats a specified SE- QUENCE structure over and over again as long as a condition, P, is true. DOWHILE tests the condition prior to executing the SEQUENCE structure for the first time. Thus in this example, the SEQUENCE struc- ture A could be executed 0, 1, 2...N times, depending upon how soon the condi- tion P becomes false as a result of A’s work. Figure 4: The DOUNTIL structure is anoth- er looping form which repeats a specified SEQUENCE structure over and over again until the condition, P, is true. DOUNTIL, in contrast lo DOWHILE, tests the condition after executing the SEQUENCE structure. Thus in this example, the SEQUENCE struc- lure A could be executed 1, 2, 3... N times depending upon how soon the condition P becomes true as a result of A’s work, SELECT STRUCTURE SS 4 | DETERMINE | A case i (se PROCESS a case 2 PROCESS 8 CASE 3 PROCESS c 24 This would require an extra instruction or two when translated into machine code, since the “last character read” must first be initialized to contain something other than a carriage return. [Implemented as a DOUNTIL it is simply: The SELECT Block Sometimes it is necessary to select one of many possible processes based on some quantity that may take on any number of values. Suppose, in addition to updating your checking account balance, you decided to keep a tally of money spent on each of several budget items such as food, medical, car, electric and so forth. This could be done with a string of IFTHENELSEs as follows on the next page. Two possible methods are shown but both are somewhat awkward to follow. IF (check was veitten to super market) THEN (244 anount to food total) IF (check vas veteten to doctor) THEN (add anount to medical total) ELse. IF (check written to auto repair shop) THE (add arount to car total) ELSE TF (check written to electric company) THEN : (add anount to electric total) Alternate method: (check uritten to super market) THEN ‘add amount to food total) IF (check written to doctor) THEN 2 (add amount to medical cotal) ENDIF TF (check written to auto repair shop) THEN (adé amount to car total) ENDIF IP (check written co electric company) THEN {add anount to electric total) ENDIF A more concise and meaningful way to describe this process is with the SELECT block shown in figure 5. Note that although there are many paths through the block, there is only one entrance and only one exit. Our bookkeeping example now becomes: SELECT (based on vho check written to) (CASE (written to super marker) 2 (add amount to food total) CASE (written to doctor) : (add amount to medical total) SE (uritten to auto repair shop) (aad amount to car total) CASE (uritten Co elect ric company) 2: (add amount to electric coral) ENDSELECT These then are the building blocks of a structured program. Others could be in- vented, but these should suffice for most situations. In any case, each should exhibit one entry point and one exit point. It should be noted that none of the building blocks transfer control (jump) into another, never to return. This so called GOTO is a definite “no no" in structured programming. All processes are either done in line or are called as subroutines that are presented elsewhere. Frequent jumping around in a program results in a maze of paths that becomes difficult to follow and even more difficult to deal with in the event that a change in one is necessary. Building From the Top Down Earlier when the subject of structure was intraduced, the term “top down”’ was used. If you wanted to build a computer, you could start by getting the processor, then some memory and |O devices and a power supply. Then you would have to try to determine how to connect all the parts together. On the other hand, you could start by deciding what the specifications for the machine are to be, such as word length and speed, what the IO ports look like and what controls and devices are to be attached. From there the problem is to select or design the components and parts to do the job. So it is with software. In the past the tendency has been to first develop the pieces like Teletype handlers, tape read/write sub- routines and others. Then the pieces would be fitted together into a functioning module, hopefully without having to make any major changes to the pieces already developed. The experience of many people in the profes- sional software field has indicated that this is not an efficient way to design a software module. Instead the approach is to start ata high level of abstraction to describe the basic function to be performed. From there each unit of this description is broken into more detailed modules. Once designed, the pro- gram is coded and debugged a piece at a time starting at the topmost level. Subordinate levels of code are temporarily replaced by dummy “stubs” which do nothing. Then as each level is coded and incorporated into the program, any problems that develop usually can be isolated to the modules just added. As an example of this approach and the use of pseudo code, let us design a simple editor program, This editor reads a. line of text from an input device (paper tape reader or magnetic tape recorder). The line is saved in memory and displayed on a video monitor or typed on a Teletype printer. A limited number of responses from the input key- board allow changes, deletions, and inser- tions to be made. Upon completion, the line is written to the output device (punch or another magnetic tape recorder). The proc- ess continues until the end of tape is reached on the input device. Changes and insertions are made by typing the character on the Teletype directly below the input line. Inserts are indicated by terminating the line with a carriage return (CR) and changes by a line feed (LF). The Teletype carriage or video display cursor is positioned using a “Control P” character (holding the CON- TROL key down while striking the “! key). This is not a sophisticated editor, but should serve as a good example of how to use the techniques described. The topmost abstraction level of the editor program can be described in pseudo code as follows: DO UNTIL (end of input cape) (get Line from Input and eype on printer) (get response Line from keyboard, store and echo (t) IF (only CR or LF entered) THEN 2 _ (do nothing) ELSE : IF (last character ts LF) THEN (do character changes and output 1ine) ELSE (do character inserts and output Line) 2 ENDIF ENDIF ENDDO This then is our editor in its most abstract form. Note that an input line is deleted by entering only a carriage return or line feed. Now let us refine the description by de- scribing each process identified above. Getting a line from the input device requires turning on the input device, reading characters, and storing them until a line feed or carriage return has been recognized. The stored line is terminated with a zero (null) character so that the end of the line is more easily recognized later. (set inpur Line pointer co first address of Line) (urn on input device) DO UNTIL (a LF oF CR is read) (gee characcer from device) (Store character @ input line potncer) (advance input line pointer one posit ton) Gend character to printer) DDO (clear a character at the pointer address) (eurn off Input device) Likewise getting the response from the keyboard is similar except that Control P characters are echoed as spaces on the Teletype printer. (set keyboard Line pointer to first address of Line) D0 UNTIL (LF or CR ts typed) (get character from keyboard) IP (character {8 not a LF or C8) THEN : (Store character @ keyboard Line pointer) (advance keyboard Line pointer) IF (character is aot Control P) THEN (echo the character on printer) ELSE (echo a space) 2 ENDIF ENDIF ENoDo (clear a byte @ keyboard line pointer) Character replaces and inserts are done by using the Control P characters on the key- board to indicate where the changes are to be made. For each Control P character in the response, an input line character is sent to the output. When a character other than Control! P is encountered, it is cither inserted into the output or replaces a character about 25 For a number of years, the field of computer program- ming has been moving from the realm of a black art to an organized and systematic process. “Top down - structured programming” is a verit- able buzzword in the data processing and computer science fields. Structured programming is a systematic way of think- ing about processes, the result of which is a well designed and under- standable program specif- ication, This article concerns or- ganizing and planning a program, which is ex- pressed in a_ structured “pseudo code.” The next step after the plan is created is to translate the pseudo code into the de- tailed machine code of your personal computer. to be outputted depending on the last character from the keyboard (line feed or carriage return). Thus the replace operation becomes: (set Input Line pointer to start of input Line) (set keyboard Line pointer to start of keyboard line) (curn on output device) PO UNTIL (end of keyboard Line) (get keyboard line character ® keyboard Tine polncer) TP (character (8 Control P) TH (get character @ Inout Ite pointer and send to output) 2 _ (echo character on teletype printer) ELSE (send the keyboard character to the output) (echo the keyboard character on printer) ENDIF {advance keyboard Line pointer) £ Cadvance input Line potnter) ENDO {put out _cest of characters In Input Line) (turn off output device) Note that the resulting output is echoed on the Teletype to enable verification of the operation. The insert operation is given below: (set Input Iine pointer to start of input Line) (set keyboard Tae pointer to start of kevboard Lae) (turn on output device) WO UNTIL Cond of keyboard iIne) rd (cranafer character @ faput Line pointer to output) (echo character on teletype printer) rest of input line characters) ff output device) The routine that “puts out the rest of the input line characters” is: PO UNTIL (tnput Line pointer points to a null) (get character @ toput Line pointer) IF (character 1s noe a null) THEN (put character to output device) Z {echo character on printer) (advance Input line pointer) + ENDIF ENDO Finally the routines to get a character from the input device and keyboard in this simple system are identical except for the address of the device referenced. Do UNTIL (input device ready flag 1s on) (get input device ready flag) ENDDO (get character from device dita port) The character output and type routines are likewise the same. DO UNTIL (output device ready (lag Is on) (get output device ready flag) ENDO (Bend character to output device data port) We have now arrived at such a level of detail that the code could be written with- out much difficulty from the pseudo code on an almost one for one basis. Each module except for the top level description could and probably would be written as a separate 26 subroutine. Note that each module can be read starting on the first line and ending on the last. No transfers are made out of any module to another without returning to the line following. Modules should be kept short (no more than a page) so that they can be read without constantly flipping pages back and forth. Conclusion What has been presented in this article is a description of a systematic approach to program design and a means of describing it so that almost any individual should be able to understand it. The resulting program when coded will have been well thought out and may even have been reviewed and partially debugged by other individuals not intimately familiar with the machine upon which it will ultimately be executed. Much discussion has occurred about standards for data exchange between various computer hobbyists. On a higher level, the pscudo code approach makes possible a standard way to exchange program ideas. In fact, higher level languages have been de- veloped that, at least in part, resemble the pseudo code language used here. Using this approach, programs might be written to convert pseudo code into machine instruc- tions for the 8080, 6800, 6502 or other CPUs as they become available. All hobby- ists could then share programs in a higher level language, each doing the necessary conversion on his own machine. There are a number of references on the subject of structured programming. The idea has been discussed extensively in computer science circles in recent years, to the point that “structured programming” has become a buzz word in the business. This writer is familiar with the two texts given in the bibliography. The IBM te