Crackers notes that writing the same value to all 3 playfield registers
looks cool, but he wants to scroll it. He asks:
> One could really easily create a symetrical horizontally scrolling playfield
> by using ROL and ROR the only problem is the Carry Bit. How can I easily
> remove the problem of the Carry Bit and just have Bit 7 rotate into
> bit 0 and vice versa?
Assume the data's in A.
To rotate left is the easy one:
asl a
adc #0 ; move old bit 7 into bit 0
To rotate right takes a little more effort:
lsr a
bcc .no_carry
ora #$80
.no_carry:
Or time-invariant (may be important in display routines) but slower:
sta temp ; not needed if temp (or wherever) already has a copy of
; the same value that's in A
lsr a ; move bit 0 to carry
lda temp ; get original data back
ror a ; move old bit 0 into bit 7
Or even slower and takes fewer bytes and is still time invariant if
your display loop has time to kill:
pha
lsr a
pla
ror a
HTH - Jim
> CRACKERS
> (Tickled pink from hell!!!!)
Is the virtual pet going to be "Tickle Me Crackers?" Hee hee! Stop!
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