summary.doc
Wed Sep
3 00:52:27 1986
1
Yacc (Yet Another Color Computer) Functional Description
The Yacc is a 68010 based single board computer.
It's main components
are the 68010, a megabyte of RAM, up to 1/2 megabyte of ROM, a Zilog sec
for serial communicatJLon, an Apple IWM for controlling Sony microfloppies,
3 6522 VIA's, a byte wide port to a Priam Datatower (85 MB), and a byte
wide port to an Apple ProFile or a 3COM Ethernet box.
The translation
buffer portion of a paged MMU is implemented. The following is a simple
description of each of the main components.
Memory Management Unit
The MMU consists of 2 - 2Kx8 static RAMs, comparators, multiplexors,
bus drivers, and a controlling PAL. There are two translation buffers that
are 1024 entries in lE~ngth, with each entry having 16 bits. Which of the
two buffers are selected are determined by the Supervisor/User output from
the 68010, and both buffers are accessible to the CPU when in Supervisor
state. The pages that are mapped by the MMU are 1024 bytes (lK) in size;
this allows the entirE~ megabyte of the machine to be mapped at one time.
The 16 bits of each entry are used as follows:
Bit
Bit
Bit
Bit
Bit
US
#14
#13
U2:03
#02:00
-
RefE~renced Bit.
Set to '1' if the page is ever referenced.
Modified Bit. Set to '1' is the page is ever written to.
Valid Bit. User controlled, set to '1' is page is mapped.
Physical Address. These are physical addresses 19:10.
Tag Field. These map 8 logical megabytes to 1 physical meg.
The 16 megabyte address space of the 68010 is divided into two spaces,
Logical RAM and I/O space. The I/O space is selected by via address line
#23 being equal to 1; there is no protection feature that disables a user
from addressing the I/O space.
Because the haLrdware only supports the translation of addresses and
the referenced and modified bits, the entire overhead of providing a
virtual system must be done in software. The hardware will cause a bus
error if the page is not mapped; the software must maintain it's own
page tables and other such data structures.
Video Frame Buffer
The Yacc has an eight plane deep frame buffer with a 256 x 16 bit
color lookup table.
~~he video access to memory is interleaved with CPU
access; this interleave is transparent to the software. The physical
memory layout to support the 8 planes needs support by MMU mapping
to allow the 8 planes to appear contiguous. The memory can be viewed
as divided into 8 128Kb partitions. Each video plane resides in one
of these 8 partitions. The physical starting address is the same for
each of the planes, and can be viewed as an offset from the start of
partition. This starting address loaded from a 16 bit wide register.
Interrupt Structure
There are five lines into a 8 to 3 line priority encoder. They
consist of three interrupt request lines from the 3 VIA's, one interrupt
request line from the SCC, and a non-maskable interrupt. Each VIA has
2 timers that can inte:rrupt. The VIA #1 can interrupt because of
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