Description for PET CS/A board adapter, version 1.1
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whatsit : Adapter to use CS/A65 boards in a Commodore PET
I/O     : CS/A bus address lines $E808, Control port $E809
Mem     : 4k window into CS/A bus at $9*** or $A*** (jumperable)

The board goes between the PET 6502 and the main PET board.
It uses the PET "/NOROM" line on pin 5 of the CPU socket, that allows
to disable the ROMs on the main PET board. This pin is unused by
the CPU, so this is a feature Commodore explicitely enabled for
such an expansion board like the one described here - this feature
is for example also used by the SuperPET expansion boards.

IC1 selects the I/O area $E80* for the control ports, and IC8
generates the select lines with the correct timing for IC3
(address line buffer) and IC4 (control port). Those two ports
can only be written to.

IC2 selects the memory window, jumperable to $9*** or $A*** with
JP1. IC7A, IC7B and IC5A then generate the /NOROM signal when the
mapping is enabled.

IC10A generates the CS/A65 bus select lines /MEMSEL and /IOSEL
using the control bits from the control port IC4.

Memory Map:

$E808 (59400): 
	CS/A bus address lines A12-A19
$E809 (59401):
	bit 0: 1= enable the bus extension
	bit 1: 0= select CS/A bus memory
		  i.e. map the CS/A bus memory selected by $E808 into
		  PET address space
	       1= select CS/A bus I/O area
		  i.e. map the CS/A bus I/O area into PET address space