Delete comment from: Ken Shirriff's blog
Lori said...
This was exceptionally useful! I have used this article a few times to reference overflow stuff. The truth tables are especially useful! A little late here, but worth noting, some of the hex results from your truth tables have the wrong values.
It's also worth pointing out, you can work out C6 with the following:
C6 = S7 xor M7 xor N7
This lets you use C6 xor C7, which works for both ADC and SBC.
In C++ that ends up looking something like:
auto c6 = ((result >> 7) & 1) ^ ((left >> 7) & 1) ^ ((right >> 7) & 1);
auto c7 = c6 ^ ((result >> 8) & 1);
I'm a little confused by what Zuolio said though, as C6 xor C7 seems to pass my 6502 conformance tests without issue. I may be missing something though!
Dec 15, 2021, 4:26:43 PM