From f6a55dc181cfde88b0d4505106d77e418139ec3f Mon Sep 17 00:00:00 2001
From: Peter Ferrie <peter.ferrie@gmail.com>
Date: Tue, 4 Oct 2022 13:37:33 -0700
Subject: [PATCH] work around IIe card VBL issue

---
 src/prelaunch/flapple.bird.a     | 17 +++++++++++++++--
 src/prelaunch/pipe.dream.a       |  8 +++++++-
 src/prelaunch/prince.of.persia.a | 23 +++++++++++++++++++----
 3 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/src/prelaunch/flapple.bird.a b/src/prelaunch/flapple.bird.a
index f356c7d6c..fcc8885ee 100644
--- a/src/prelaunch/flapple.bird.a
+++ b/src/prelaunch/flapple.bird.a
@@ -1,5 +1,5 @@
 ;license:MIT
-;(c) 2020 by Frank M.
+;(c) 2020, 2022 by Frank M., qkumba
 
 !cpu 6502
 !to "build/PRELAUNCH.INDEXED/FLAPPLE.BIRD",plain
@@ -7,7 +7,20 @@
 
     !source "src/prelaunch/common.a"
 
-         jmp   $800
+         +READ_RAM2_NO_WRITE
+         jsr   EnableAccelerator
+         lda   #$60
+         sta   $813
+         jsr   $800
+         +READ_RAM1_NO_WRITE
+         lda   WaitForVBL
+         eor   #$60
+         bne   @good_vbl  ; if VBL disabled in TR
+         sta   $2971      ; then disable VBL in game
+
+@good_vbl
+         +DISABLE_ACCEL
+         jmp   $2000
 
 !if * > $1C0 {
   !error "code is too large, ends at ", *
diff --git a/src/prelaunch/pipe.dream.a b/src/prelaunch/pipe.dream.a
index c86ca55bb..889e0b22d 100644
--- a/src/prelaunch/pipe.dream.a
+++ b/src/prelaunch/pipe.dream.a
@@ -10,17 +10,23 @@ my_vbl
     !source "src/prelaunch/common.a"
 
          +READ_RAM1_NO_WRITE
+         lda   WaitForVBL
+         cmp   #$60
+         beq   @skip_vbl
          lda   WaitForVBL+1
          sta   @copy_vbl+1
          lda   WaitForVBL+2
          sta   @copy_vbl+2
+
+@skip_vbl
          lda   MachineStatus
          and   #CHEATS_ENABLED
          php
          ldy   #0
 
 @copy_vbl
-         lda   $d1d1,y    ; SMC
+         lda   WaitForVBL,y
+                          ; SMC, default to no VBL
          sta   my_vbl,y
          iny
          cmp   #$60
diff --git a/src/prelaunch/prince.of.persia.a b/src/prelaunch/prince.of.persia.a
index f6296cece..844a72896 100644
--- a/src/prelaunch/prince.of.persia.a
+++ b/src/prelaunch/prince.of.persia.a
@@ -15,21 +15,31 @@
          and   #CHEATS_ENABLED
          sta   cheat_smc1+1
          sta   cheat_smc2+1
+         lda   WaitForVBL
+         cmp   #$60
+         php
          +READ_ROM_NO_WRITE
          lda   #$60
          sta   $13B8
          jsr   $1200      ; decompress
+
          lda   #<callback1
          sta   $FE06
+         lda   #<callback2
+         sta   $FD26
          lda   #>callback1
          sta   $FE07
+         sta   $FD27
+         plp
+         bne   @good_vbl
+         ldx   #<callback3
+         stx   $FD6A
+         sta   $FD6B
+
+@good_vbl
          jmp   $EE00
 
 callback1
-         lda   #<callback2
-         sta   $FD26
-         lda   #>callback2
-         sta   $FD27
 cheat_smc1
          lda   #0
          beq   +
@@ -50,6 +60,11 @@ cheat_smc2
 +
          rts
 
+callback3
+         lda   #$60       ; if VBL disabled in TR
+         sta   $4ED       ; then disable VBL in game
+         jmp   $F924
+
 !if * > $1C0 {
   !error "code is too large, ends at ", *
 }