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91c154ea7b
dos33fsprogs/pt3_player
History
Vince Weaver 91c154ea7b pt3: fix an issue with signed 8-bit comparisons 
signed 8-bit comparisons are a huge pain on 6502
2019-05-13 12:33:52 -04:00
..
EA.PT3
empty.dsk
gr_fast_clear.s pt3: start implementing things 2019-05-07 00:28:42 -04:00
HELLO pt3: start implementing things 2019-05-07 00:28:42 -04:00
hello.bas pt3: add pt3 dumper 2019-05-09 10:40:25 -04:00
interrupt_handler.s pt3: copy more code in 2019-05-07 15:11:52 -04:00
Makefile pt3: temporarily bump up to $3000 as getting too big 2019-05-12 22:17:03 -04:00
pt3_dumper.s pt3: temporarily bump up to $3000 as getting too big 2019-05-12 22:17:03 -04:00
pt3_lib.s pt3: fix an issue with signed 8-bit comparisons 2019-05-13 12:33:52 -04:00
pt3_player.dsk pt3: start implementing things 2019-05-07 00:28:42 -04:00
pt3_player.s pt3: temporarily bump up to $3000 as getting too big 2019-05-12 22:17:03 -04:00
qkumba_rts.s pt3: start implementing things 2019-05-07 00:28:42 -04:00
README.pt3 pt3: fix an issue with signed 8-bit comparisons 2019-05-13 12:33:52 -04:00
song_list.inc pt3: start implementing things 2019-05-07 00:28:42 -04:00
text_print.s
TODO pt3: start implementing things 2019-05-07 00:28:42 -04:00
zp.inc pt3: sort of have ornament/sample loading going 2019-05-12 00:14:09 -04:00

README.pt3 

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