dos33fsprogs/pt3_player/README.pt3_player
2019-05-15 11:55:43 -04:00

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PT3_player
ASR = CMP #$80 / ROR
Notes:
signed 8-bit comparison
see http://6502.org/tutorials/compare_beyond.html#2.2
SEC ; prepare carry for SBC
SBC NUM ; A-NUM
BVC LABEL ; if V is 0, N eor V = N, otherwise N eor V = N eor 1
EOR #$80 ; A = A eor $80, and N = N eor 1
LABEL
If the N flag is 1, then A (signed) < NUM (signed) and BMI will branch
If the N flag is 0, then A (signed) >= NUM (signed) and BPL will branch