From b6fcd7efa88fc67ed8d1a26f117b419b6186614a Mon Sep 17 00:00:00 2001
From: Vince Weaver <vincent.weaver@maine.edu>
Date: Thu, 14 Apr 2022 16:12:21 -0400
Subject: [PATCH] lemm: more work on iic vblank

---
 games/lemm/hardware.inc  | 21 ++++++++++++---------
 games/lemm/intro_level.s | 36 +++++++++++++++++++++++++++++-------
 2 files changed, 41 insertions(+), 16 deletions(-)

diff --git a/games/lemm/hardware.inc b/games/lemm/hardware.inc
index db595cdb..122551dd 100644
--- a/games/lemm/hardware.inc
+++ b/games/lemm/hardware.inc
@@ -8,11 +8,14 @@ CLR80COL	=	$C000	; PAGE0/PAGE1 normal
 SET80COL	=	$C001	; PAGE0/PAGE1 switches PAGE0 in Aux instead
 EIGHTYCOLOFF	=	$C00C
 EIGHTYCOLON	=	$C00D
-VBLANK		=	$C019	; IIe *not* RDVBL (VBL signal low)
+VBLANK		=	$C019	; IIe *not* RDVBL (VBL low), IIgs opposite 
+RSTVBL		=	$C019	; reset VBLANK interrupt (iic)
+RDVBLBAR	=	$C019
 TBCOLOR		=	$C022	; IIgs text foreground / background colors
 NEWVIDEO	=	$C029	; IIgs graphics modes
 SPEAKER		=	$C030
 CLOCKCTL	=	$C034	; bits 0-3 are IIgs border color
+RDVBLMSK	=	$C041	; read VBL interrupt (bit 7)
 SET_GR		=	$C050
 SET_TEXT	=	$C051
 FULLGR		=	$C052
@@ -21,17 +24,17 @@ PAGE0		=	$C054
 PAGE1		=	$C055
 LORES		=	$C056	; Enable LORES graphics
 HIRES		=	$C057	; Enable HIRES graphics
+CLRAN1		=	$C05A	; clear annunciator 1 (if IOUDIS off)
+DISVBL		=	$C05A	; disable VBLANK (iic) (if IOUDIS on)
+SETAN1		=	$C05B	; set annunciator 1   (if IOUDIS off)
+ENVBL		=	$C05B	; enable VBLANK (iic)  (if IOUDIS on)
 AN3		=	$C05E	; Annunciator 3
-
-
-
-
-
-
-
 PADDLE_BUTTON0	=	$C061
 PADDL0		=	$C064
-PTRIG		=	$C070
+PTRIG		=	$C070	; Analog input reset
+VBLINT		=	$C070	; Analog input reset + reset vblank (iic)
+IOUDISON	=	$C07E	; iic
+IOUDISOFF	=	$C07F	; iic
 
 ;; BASIC ROUTINES
 
diff --git a/games/lemm/intro_level.s b/games/lemm/intro_level.s
index d081b932..9dea1c2a 100644
--- a/games/lemm/intro_level.s
+++ b/games/lemm/intro_level.s
@@ -364,26 +364,48 @@ level_end_messages_h:
 
 .align $100
 
+.if 1
 	;==============
 	; this code from https://comp.sys.apple2.narkive.com/dHkvl39d/vblank-apple-iic
 	; as well as IIc technote #9
 split_screen_iic:
-	bit	$c041
+	bit	$c041		; RDVBLMSK Read VBL interrupt (bit 7)
 	bmi	rdvbl
-	sta	$c07f		; turn off IOUdis
+	sta	$c07f		; IOUDISOFF turn off IOUdis
 	sta	$c05b		; ENVBL enable vblank
-	sta	$c07e		; ??
+	sta	$c07e		; IOUDISON turn on IOUdis
 rdvbl:
-	bit	$c019
+	bit	$c019		; RSTVBL reset vblank (iic)
 	bmi	rdvbl
-	lda	$c070		; clear vblank
+	lda	$c070		; reset VBLINT vblank interrupt flag
 rdvbl2:
-	bit	$c019
+	bit	$c019		; RSTVBL reset vblank (iic)
 	bpl	rdvbl2
-	lda	$c070		; clears vblank
+	lda	$c070		; reset VBLINT vblank interrupt flag
 
 	jmp	start_vblank
 
+.else
+	; alternate implementation by Oliver Schmidt
+	; from https://github.com/cc65/cc65/blob/master/libsrc/apple2/waitvsync.s
+split_screen_iic:
+	sei			; disables interrupts
+	sta	IOUDISOFF
+	lda	RDVBLMSK
+	bit	ENVBL
+	bit	PTRIG		; Reset VBL interrupt flag
+:	bit	RDVBLBAR
+	bpl	:-
+	asl
+	bcs	:+		; VBL interrupts were already enabled
+	bit	DISVBL
+:	sta	IOUDISON	; IIc Tech Ref Man: The firmware normally leaves IOUDIS on.
+	cli			; re-enable interrupts
+
+	jmp	start_vblank
+.endif
+
+
 ; split screen?
 
 split_screen_iigs: