6502 Shorts - Toggle carry. By Lee Davison
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Carry bit the wronge sense? Want a 0 when it's 1? Read on....

Recently in some otherwise elegant code the result of a compare left the carry bit in exactly the wrong sense for the following add. The usual way to fix that is to do something like ..

BCC WasClear ; if clear go set it CLC ; was set so clear it BCC AllDone ; continue WasClear SEC ; was clear so set it AllDone

While this works there had to be a more elegant way of doing this. It turns out there is, you do this ..

ROL A ; Cb into D0 EOR #$01 ; toggle bit ROR A ; D0 into Cb

The ROL puts the carry into bit 0 in the accumulator.
The EOR toggles the state of that bit without affecting any other bits.
The ROR puts the bit back into the carry, restores the accumulator and sets the N and Z flags.

There are a couple of advantages of doing things this way instead of the branch and set (or clear) given earlier ..

It only takes four bytes instead of six.

It always takes six cycles, not five or seven depending on the state of the carry.

There is one possible disadvantage in that it sets the N and Z flags from what is in A at the time. But, most of the time, if you're interested in the carry state the N and Z states are unimportant.

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