===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 FF to be output and an invalid PSG register to be selected:
ld bc,&f400
ld bc,&f792
out (c),c
;; At this point FF appears in PPI emulated 8255 port A. This selects an invalid PSG register'&ff', when read &FF is returned when attempting to read this register. This is one source of keyboard reading bugs.
Therefore use this:
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.
===Automatic feeding of sound generator===