Adapter Commodore 64 <-> CS/A bus
---------------------------------

whatsit : Adapter to use CS/A cards in a C64
I/O     : window to CS/A bus at $de00-$de7f and VIA 6522 at $de80-$deff
Mem     : --


This is an adapter to make most of the CS/A bus cards available to the 
Commodore 64 home computer. It also serves as an example on how to 
use the CS/A bus on other 6502-based computers.

The C64 has two available I/O areas on the expansion port, $DE** and $DF**.
Preferably $DE** is used.

IC1 buffers the data bus to the card. IC2 buffers the available address 
bus lines A0-A7. A part of IC3 buffers the R/W line.

IC7 (together with another part of IC3) delays the rising transition
of the system clock Phi2 by approx. 100ns. This makes the active part
of Phi2 (high) shorter, but provides that the address lines are already
stable when Phi2 goes high (during Phi2 low the Video chip has the bus
and switching is too slow). This is needed for chips like the 6522 that
samples the address bus at the rising flank of Phi2.

The CS/A bus has a line 2Phi2 that can be used as /RAS signal for dynamic
RAM. It is produced with IC4 (should be a slow version, no 'LS or faster)
and IC5.

As only 8 address lines are available, we need to produce more 
address lines for the full CS/A bus. Therefore the address line
A7 is used to select a 6522 that produces the bus address lines
A7-A19. Also it switches between I/O space and Memory space on the
CS/A bus. This is implemented with IC6.
Thus we have a window of 128 byte to the CS/A bus address space,
where the window can be moved with VIA PA and PB.