From 91c154ea7b79a270d4a3bbef275f091b06d99811 Mon Sep 17 00:00:00 2001
From: Vince Weaver <vincent.weaver@maine.edu>
Date: Mon, 13 May 2019 12:33:52 -0400
Subject: [PATCH] pt3: fix an issue with signed 8-bit comparisons

signed 8-bit comparisons are a huge pain on 6502
---
 pt3_player/README.pt3 | 16 ++++++++++++++++
 pt3_player/pt3_lib.s  | 28 +++++++++++++++++++++-------
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/pt3_player/README.pt3 b/pt3_player/README.pt3
index 1d81c1fa..f7bf843b 100644
--- a/pt3_player/README.pt3
+++ b/pt3_player/README.pt3
@@ -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 
diff --git a/pt3_player/pt3_lib.s b/pt3_player/pt3_lib.s
index cd873b3c..2e21ba89 100644
--- a/pt3_player/pt3_lib.s
+++ b/pt3_player/pt3_lib.s
@@ -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 {