From 14adcd209642368e049def69bdae566262746400 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomasharte@clearchannel.com>
Date: Thu, 30 Jul 2015 16:09:32 -0400
Subject: [PATCH] Had a quick bash at timer overflow.

---
 Machines/Atari2600.cpp | 21 ++++++++++++++-------
 Machines/Atari2600.hpp |  1 +
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/Machines/Atari2600.cpp b/Machines/Atari2600.cpp
index 2c921d5f5..40cd0ae36 100644
--- a/Machines/Atari2600.cpp
+++ b/Machines/Atari2600.cpp
@@ -20,6 +20,7 @@ Machine::Machine()
 	_lastOutputStateDuration = 0;
 	_lastOutputState = OutputState::Sync;
 	_crt = new Outputs::CRT(228, 256, 1, 4);
+    _piaTimerStatus = 0xff;
 
 	reset();
 }
@@ -272,15 +273,15 @@ int Machine::perform_bus_operation(CPU6502::BusOperation operation, uint16_t add
 	if ((address&0x1280) == 0x280) {
 		if(isReadOperation(operation)) {
 			switch(address & 0xf) {
-//				case 0x00:
-				case 0x04: returnValue &= _piaTimerValue >> _piaTimerShift; break;
+				case 0x04: returnValue &= _piaTimerValue >> _piaTimerShift;             break;
+                case 0x05: returnValue &= _piaTimerStatus; _piaTimerStatus &= ~0x40;    break;
 			}
 		} else {
 			switch(address & 0x0f) {
-				case 0x04:	_piaTimerShift = 0;		_piaTimerValue = *value << 0; break;
-				case 0x05:	_piaTimerShift = 3;		_piaTimerValue = *value << 3; break;
-				case 0x06:	_piaTimerShift = 6;		_piaTimerValue = *value << 6; break;
-				case 0x07:	_piaTimerShift = 10;	_piaTimerValue = *value << 10; break;
+                case 0x04:	_piaTimerShift = 0;		_piaTimerValue = *value << 0;   _piaTimerStatus &= ~0xc0;   break;
+				case 0x05:	_piaTimerShift = 3;		_piaTimerValue = *value << 3;   _piaTimerStatus &= ~0xc0;   break;
+				case 0x06:	_piaTimerShift = 6;		_piaTimerValue = *value << 6;   _piaTimerStatus &= ~0xc0;   break;
+				case 0x07:	_piaTimerShift = 10;	_piaTimerValue = *value << 10;  _piaTimerStatus &= ~0xc0;   break;
 			}
 		}
 //		printf("Uncaught PIA %04x\n", address);
@@ -290,7 +291,13 @@ int Machine::perform_bus_operation(CPU6502::BusOperation operation, uint16_t add
 		*value = returnValue;
 	}
 
-	_piaTimerValue -= cycle_count;
+    if(_piaTimerValue < cycle_count) {
+        _piaTimerValue = 0x100 - cycle_count + _piaTimerValue;
+        _piaTimerShift = 0;
+        _piaTimerStatus |= 0xc0;
+    }
+    else
+        _piaTimerValue -= cycle_count;
 
 	return cycle_count;
 }
diff --git a/Machines/Atari2600.hpp b/Machines/Atari2600.hpp
index 375456304..5923c5f11 100644
--- a/Machines/Atari2600.hpp
+++ b/Machines/Atari2600.hpp
@@ -37,6 +37,7 @@ class Machine: public CPU6502::Processor<Machine> {
 		// the timer
 		unsigned int _piaTimerValue;
 		unsigned int _piaTimerShift;
+        uint8_t _piaTimerStatus;
 
 		// graphics registers
 		uint8_t _playfield[3], _playfieldControl;