CS/A65 PETCPU
=============

This board implements a 6502 CPU board for the CS/A65 bus, with access to
64k memory (instead of the 1M available on the bus). The board includes
a socket for a 32k RAM chip, as well as a socket for up to 512k ROM
(where up to a maximum of 32k can be used at any one time).

The memory is mapped into the 64k CPU address space (RAM at the bottom,
ROM at the top). IC2 and IC9 define the size of the respective ROM and
RAM areas. The place "in the middle", between the RAM and the ROM is
selected in the memory area of the CS/A65 bus. So for example the
RAM can be set to zero, ROM to 20k and thus the lower 44k are provided
by the bus, e.g. by the video card. 

In addition to the CPU and memory, it includes an RS232 interface implemented
by a UART chip.

The name of this board comes from the fact that it can be used to 
implement a PET compatible computer. IC3 selects a PET compatible I/O
area (selectable from $E8xx to $E800-$EFFF), and IC5B and IC5C are used
to invert the memory address lines so the values for the video address
stored in the video chip match with the addresses used from the PETCPU
board.