Changes
/* Reading of write-only I/O registers */
Because of timescale pressures, the data separator design in the ASIC has been deleted rather than improved . Thus all models with a disk drive use an external SED9420 data separator.
===8255===
* When switching port A of ASICs emulated 8255 to input, FF is present on the emulated 8255's port A outputs.
This will cause an invalid PSG register to be selected:
ld bc,&f400
out (c),c
ld bc,&f6c0
out (c),c
ld bc,&f792
out (c),c
;; At this point FF appears in emulated 8255 port A. This selects an invalid PSG register '&ff', when read &FF is returned. This is one source of keyboard reading bugs.
Therefore use this:
ld bc,&f400
out (c),c
ld bc,&f6c0
out (c),c
ld bc,&f600 ;;; << use inactive
out (c),c
ld bc,&f792
out (c),c
* When switching input/output of port A, on a normal 8255, the outputs are all cleared to 0. This doesn't happen on the emulated 8255. This is another source of keyboard reading bugs.
===Reading of write-only I/O registers===
