Posted By
Blacky Stardust
on 2013-01-06
14:20:45
|  Re: $00-$01 behavior help needed for accurate emulation
Hoi Attila,
Wat een toeval om iemand 'tegen te komen' die nederlands 'praat' 
Thank you both for testing this, and yes, it seems the 7501/8501 is the exception where unused bits drop-off is concerned. Now the question is, can the unused bit in $00 be set/reset, can the unused bit in $01 be set/reset in output mode, and can the unused bit in $01 be set/reset in input mode.
To find that out another test is required:
sei
ldx $00
ldy $01
lda #$20
sta $00
lda $00
sta $0c00
lda #$00
sta $00
lda $00
sta $0c01
lda #$20
sta $00
sta $01
lda $01
sta $0c02
lda #$00
sta $01
lda $01
sta $0c03
lda #$00
sta $00
lda #$20
sta $01
lda $01
sta $0c04
lda #$00
sta $01
lda $01
sta $0c05
stx $00
sty $01
cli
rts
The values in $0c00-$0c05 are the ones that show what $00/$01 can do.
Thanks in advance.
Signed, Marco van den Heuvel.
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