Question

Still new to bitwise operations in 6502 assembly. I would like to have a byte that
has 8 flags. This will store the status of my meta-sprite.

I want to be able to set specific flags without altering the others.

I know how to use ORA to set them to one:

  lda status
  ora #%00000001 ; set bit 0 to true
  sta status

I know how to use EOR to toggle them:

  lda status
  eor #%00000001 ; if bit 0 = true, then bit 0 = false and vise versa
  sta status

Lastly, I know how to check if a bit is true:

  lda status
  and #%00000001 ; if bit 0 = true then set overflow flag to true

But how to set a specific flag to 0, without altering any others? Even if I used
AND, how would I force it to set the desired bit to 0?

Thanks, I’m sure I’m missing something simple.

Answer 1

How about

lda status
and #%11111110 ; set bit 0 to false
sta status

Note that all other bits are not touched, since 1 AND X is always X for every bit. Only bit 0 changes since 0 AND X is always 0.