At 5:41 PM +0000 2/15/04, KirkIsrael@xxxxxxxxxxxxx wrote:
> Is there a better way of dividing a signed 1-byte integer
> by 2 than this? (which says, if var is positive, just do LSR,
> otherwise LSR and OR w/ -128 i.e. #%10000000)[?]
I'll leave that answer to someone more qualified...
> It just seems odd to have to branch to preserve the sign bit
It wouldn't be if there was a DIV command of some sort, but
since there isn't, you need some trickery.
> but I wasn't clever enough to come up with a better way...
>
> LDA var
> BPL posval
>negval
> LSR
> ORA #-128
> JMP done
>posval
> LSR
>done
> STA var
I think another way (but arguably not any better or worse?) is:
LDA var
ASL ;sign (Bit7) goes into C, 0 into Bit0
BCC adjust
ORA #1 ;1 into Bit0
adjust
ROR ;C (was Bit7 from ASL) into Bit7, Bit0 into C
ROR ;C into Bit7 (#1 from ORA if negative, else 0)
STA var
It plays with the accumulator more, but has only one branch, so I
think it comes out a little faster (less cycles) for negative numbers
as a result. It's fewer bytes too? Of course, for positive numbers,
I think your method is faster -- less monkeying around. Do you need
speed or consistency?
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