From a6bc2478526ef4768c6372d03b86d7296fe09d9b Mon Sep 17 00:00:00 2001
From: Stephen Anthony <sa666666@gmail.com>
Date: Mon, 19 Dec 2016 15:56:26 -0330
Subject: [PATCH] WSYNC should only work when the last access was a read.

---
 src/emucore/tia/TIA.cxx | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/emucore/tia/TIA.cxx b/src/emucore/tia/TIA.cxx
index 423fe3063..cf6a4f12b 100644
--- a/src/emucore/tia/TIA.cxx
+++ b/src/emucore/tia/TIA.cxx
@@ -347,9 +347,15 @@ bool TIA::poke(uInt16 address, uInt8 value)
   switch (address)
   {
     case WSYNC:
-      mySubClock += (228 - myHctr) % 228;
-      mySystem->incrementCycles(mySubClock / 3);
-      mySubClock %= 3;
+      // It appears that the 6507 only halts during a read cycle so
+      // we test here for follow-on writes which should be ignored as
+      // far as halting the processor is concerned.
+      if (mySystem->m6502().lastAccessWasRead())
+      {
+        mySubClock += (228 - myHctr) % 228;
+        mySystem->incrementCycles(mySubClock / 3);
+        mySubClock %= 3;
+      }
       break;
 
     case VSYNC: