YACC Description

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 vi…