On Wed, 25 May 2005, Dennis Debro wrote:
> > jsr getRandomByte ; byte is 0-255
> > lsr a ; now it's 0-127, carry may or may not be set
> > adc #9 ; now it's 9-137, done
> > sta whereever
>
> I like this one. Sure the right side get's cheated but I'm okay with the
> max here.
If this is the only place you ever use getRandomByte, you could change
it to a routine that gets 7 random bits in the first place, and avoid
the LSR... however, your range then becomes 9-136 (or 10-137, depending
how the getRandom* routine leave the carry flag).
It's also possible to write a getRandomByte routine with multiple
entry points, so you can get any number of random bits as needed.
Assuming the existence of a getRandomBit that returns a random bit
in the carry flag:
getRandomByte:
jsr getRandomBit
rol a
; You need to make sure A is zero before calling any of the
; getRandom0to* entry points!
; Use something like:
; lda #0
; jsr getRandom0to7
; This could be hidden behind macros...
getRandom0to127:
jsr getRandomBit
rol a
getRandom0to63:
jsr getRandomBit
rol a
getRandom0to31:
jsr getRandomBit
rol a
getRandom0to15:
jsr getRandomBit
rol a
getRandom0to7:
jsr getRandomBit
rol a
getRandom0to3:
jsr getRandomBit
rol a
getRandom0to1: ; not very useful, really
jsr getRandomBit
rol a
rts
...Of course this wastes a lot of bytes. It might be better to write
a loop version of it, that takes a parameter (number of bits), but
it'd be a bit slower...
Probably no 2600 game would ever need this, anyway. It's just an
exercise..
> BTW, I don't know why my responses haven't been showing up. Then again
> maybe they are and they're just not coming to me.
I'm seeing them just fine...
--
B.
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