From 0e7d46c46f398c07adc5e30b7d1f8cbf11a313e5 Mon Sep 17 00:00:00 2001
From: Colin McMillen <c@16bit.cc>
Date: Sat, 30 May 2015 09:28:07 -0400
Subject: [PATCH] Don't read joysticks during auto-joypad read.

---
 pewpew.asm    | 10 +++++++---
 registers.asm | 14 ++++++++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/pewpew.asm b/pewpew.asm
index 5e30721..10f3f91 100644
--- a/pewpew.asm
+++ b/pewpew.asm
@@ -322,7 +322,7 @@ MainLoop:
     lda #%00000001  ; Disable NMI interrupt while processing.
     sta NMITIMEN
 
-    jsr JoypadDebug
+    jsr JoypadRead
     jsr JoypadHandler
     jsr UpdateWorld
     jsr FillSecondarySpriteTable
@@ -331,8 +331,12 @@ MainLoop:
 
 
 
-JoypadDebug:
-    ; Load joypad registers into RAM for easier inspection.
+JoypadRead:
+    ; Load joypad registers into RAM for easy inspection & manipulation.
+-
+    lda HVBJOY
+    bit #$01  ; If auto-joypad read is happening, loop.
+    bne -
     ldx JOY1L
     stx joy1
     ldx JOY2L
diff --git a/registers.asm b/registers.asm
index f350027..8477697 100644
--- a/registers.asm
+++ b/registers.asm
@@ -145,6 +145,20 @@
 ; Each bit that's set enables one channel: 76543210
 .define DMAENABLE $420B
 
+; $4212: VBlank/HBlank/Joypad status [HVBJOY]
+; vh-----a
+; v    = V-Blank Flag. If we're currently in V-Blank, this flag is set,
+;     otherwise it is clear. The setting seems to occur at H Counter
+;     about $16-$17 when V Counter is $E1, and the clearing at about $1E
+;     with V Counter 0.
+; h    = H-Blank Flag. If we're currently in H-Blank, this flag is set,
+;     otherwise it is clear. The setting seems to occur at H Counter
+;     about $121-$122, and the clearing at about $12-$18.
+; a    = Auto-Joypad Status. This is set while Auto-Joypad Read is in
+;     progress, and cleared when complete. It typically turns on at
+;     the start of V-Blank, and completes 3 scanlines later.
+.define HVBJOY $4212
+
 ; $4218: Joypad #1 status [JOY1L]
 ; Format: AXLR0000
 .define JOY1L $4218