ADC mis-simulation (carry bug)

[migry]

I have now run a 6502 instruction test suite on my Acorn Atom simulator which uses code from this repo. It reported a problem with the ADC instruction, and manual analysis suggested that it was the carry bit which was mis-calculated.

I modified the code to re-calculate the carry bit only. The testsuite then ran until I stopped it. Also the Acorn Atom simulator now behaved 100% normally (as much as simple testing can show).