Re: Recreate 6502 Bug II

["Mark Cummings" <NOTfigjams@primus.com.au>]

"Bryan Parkoff" <spam> wrote in message
uRudndM_6oeJqGSjXTWcqw@texas.net">news:uRudndM_6oeJqGSjXTWcqw@texas.net...
>     After you have read my post, "Recreate 6502 Bug", I would like to
> provide additional information.  It is what I thought that Z flag was
placed
> into N flag that showed invalid flag.
>     I have studied 65536 BCD records in my database.  I have discovered
that
> it has nothing to switch between N flag and Z flag since V flag and C flag
> are always correct.  $60 and $66 show enabled Z(1) flag while $00 shows
> disabled Z(0) flag.  It is the inversion or one complement for Z flag.
>     It is the same for N flag.  Between $00 and $5F show enabled N(1) flag
> while between $80 and $DF show disabled N(0) flag.  It is also the
inversion
> or one complement for N flag.
>     How do 6502 programmers survive with 6502 bug using BCD?  If they try
> this example below.
>
>     CLC
>     SED
>     LDA    #$20
>     ADC    #$FA
>     CLD
>     EOR    #$80
>     BMI    +$03
>     STA    2000
>     RTS
>
> --and--
>
>     CLC
>     SED
>     LDA    #$20
>     ADC    #$FA
>     CLD
>     EOR    #$80
>     BPL    +$03
>     STA    2000
>     RTS
>
>     None of both BMI and BPL are working if we want to inverse N flag.
The
> result is $80 that will show cleared N flag.  If we replace from #$80 to
> #$00 in EOR, both BMI and BPL are working, but N flag is still wrong.
>     I am curious how you can experience with 6502 bug.  Tell me how you
can
> survive using EOR #$80 to inverse N flag and Z flag.
>
> Bryan Parkoff

hi Bryan,

I am certainly no expert with BCD, but I have used it on at least one
occasion on a Z80 based countdown timer I wrote for my Sprinkler Controller.
Why would you be adding an illegal BCD value of $FA to a valid BCD value of
$20. I don't think anything over $0-$9 should be used for each digit.

Mark