Delete comment from: Ken Shirriff's blog
Professor Sockpuppet said...
@robus You would set the carry flag before calling SBC in that case, as it should be the complement of borrow. You're starting a new subtraction, therefore you've got no borrow to start with, which means carry should be set (to 1). A good example would be subtracting the same number from itself. If you add the one's complement of the same number, the result is all 1s (0xff). As this was a new subtraction, borrow should be 0, so carry should be set to 1 before the SBC. The result of the ADC with the one's complement results in a 1 in carry, meaning borrow = 0, which is correct when you're subtracting N - N. I know this seems unintuitive, I'm not sure how to explain it better :)
Jan 8, 2023, 6:57:19 AM