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pt3: fix an issue with signed 8-bit comparisons

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signed 8-bit comparisons are a huge pain on 6502
This commit is contained in:
Vince Weaver 2019-05-13 12:33:52 -04:00
parent 4672350134
commit 91c154ea7b
2 changed files with 37 additions and 7 deletions
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16
pt3_player/README.pt3
View File

@ -3,3 +3,19 @@ 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

28
pt3_player/pt3_lib.s
View File

@ -601,17 +601,29 @@ no_tone_sliding:
and #$40
beq amp_slide_down
amp_slide_up:
; if (a->amplitude_sliding < 15) {
; a pain to do signed compares
lda note_a+NOTE_AMPLITUDE_SLIDING,X
cmp #15
bcs done_amp_sliding ; bge
; if (a->amplitude_sliding < 15) {
inc note_a+NOTE_AMPLITUDE_SLIDING,X ; a->amplitude_sliding++;
sec
sbc #15
bvc asu_signed
eor #$80
asu_signed:
bpl done_amp_sliding ; skip if A>=15
inc note_a+NOTE_AMPLITUDE_SLIDING,X ; a->amplitude_sliding++;
jmp done_amp_sliding
amp_slide_down:
; if (a->amplitude_sliding > -15) {
; a pain to do signed compares
lda note_a+NOTE_AMPLITUDE_SLIDING,X
cmp #$f2 ; -14 1111 0010
bcc done_amp_sliding ; if (a->amplitude_sliding > -15) {
dec note_a+NOTE_AMPLITUDE_SLIDING,X ; ; a->amplitude_sliding--;
sec
sbc #$f1 ; -15
bvc asd_signed
eor #$80
asd_signed:
bmi done_amp_sliding ; if A < -15, skip subtract
dec note_a+NOTE_AMPLITUDE_SLIDING,X ; a->amplitude_sliding--;
done_amp_sliding:
@ -710,6 +722,8 @@ last_envelope:
adc pt3_envelope_add
sta pt3_envelope_add ; pt3->envelope_add+=j;
jmp noise_slide_done ; skip else
noise_slide:
; Noise slide
; else {