Re: [stella] 6502 question

Subject: Re: [stella] 6502 question
From: Nick S Bensema <nickb@xxxxxxxxxxxx>
Date: Fri, 4 Apr 1997 21:53:52 -0700 (MST)
>>The general rule is SBC does the exact same thing as ADC but the operand
>>is exclusive-or'ed with $FF first.  That is why you always see people SET
>>the carry bit before subtracting: suppose for some dumb reason you wanted
>Aha.. I figured out by trial and error that SBC did A-value-(NOT carry)
>instead of A-value-carry as my reference said and as 80x86 processors do,
>but thinking of it this way makes things clearer.

That confused me when I was doing software sprites on an atari 8-bit demo
I was working on.  Some weird bug caused me to abandon it.

>>CMP works like SEC+SBC, without changing any registers other than P.
>>Therefore, if everything I said above isn't a total load of bull...
>>      LDA #5
>>      CMP X
>Are those comparisons signed or unsigned?  As in, if X is #$F6 = -10, the
>result would be 5+9+1=15, with the carry clear, so the BCC would happen and
>the comparison is unsigned. 


>Yeah, I checked my code and DASM was turning it into an Absolute,Y for me
>without telling me.  Thanks again for all the help.  One (well, two) more
>questions: what does ADC do to the overflow flag, and how does BIT affect
>the flags?  (I need a more complete reference..)

Duh, I dunno.  I think the overflow flag is like the carry bit, but it goes
off if the addition surpasses 0x80 (128), not 0x00.  But I never use it.

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