From 06e0908642eb0e6c4d151c276f9976d7b1190862 Mon Sep 17 00:00:00 2001
From: Jeffrey Pfau <jeffrey@endrift.com>
Date: Wed, 20 Jan 2016 20:09:07 -0800
Subject: [PATCH] GB: Start on timers

---
 src/gb/gb.c    |  8 +++++
 src/gb/gb.h    |  2 ++
 src/gb/io.c    | 17 ++++++++++
 src/gb/timer.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/gb/timer.h | 38 +++++++++++++++++++++++
 5 files changed, 149 insertions(+)
 create mode 100644 src/gb/timer.c
 create mode 100644 src/gb/timer.h

diff --git a/src/gb/gb.c b/src/gb/gb.c
index 703e79003..c3de0599d 100644
--- a/src/gb/gb.c
+++ b/src/gb/gb.c
@@ -41,6 +41,8 @@ static void GBInit(struct LR35902Core* cpu, struct LR35902Component* component)
 	gb->video.p = gb;
 	GBVideoInit(&gb->video);
 
+	gb->timer.p = gb;
+
 	gb->romVf = 0;
 
 	gb->pristineRom = 0;
@@ -126,6 +128,7 @@ void GBReset(struct LR35902Core* cpu) {
 	}
 	GBMemoryReset(gb);
 	GBVideoReset(&gb->video);
+	GBTimerReset(&gb->timer);
 	GBIOReset(gb);
 }
 
@@ -176,6 +179,11 @@ void GBProcessEvents(struct LR35902Core* cpu) {
 		nextEvent = testEvent;
 	}
 
+	testEvent = GBTimerProcessEvents(&gb->timer, cycles);
+	if (testEvent < nextEvent) {
+		nextEvent = testEvent;
+	}
+
 	cpu->cycles -= cycles;
 	cpu->nextEvent = nextEvent;
 }
diff --git a/src/gb/gb.h b/src/gb/gb.h
index 680e807bd..9db0db0af 100644
--- a/src/gb/gb.h
+++ b/src/gb/gb.h
@@ -11,6 +11,7 @@
 #include "lr35902/lr35902.h"
 
 #include "gb/memory.h"
+#include "gb/timer.h"
 #include "gb/video.h"
 
 extern const uint32_t DMG_LR35902_FREQUENCY;
@@ -40,6 +41,7 @@ struct GB {
 	struct LR35902Core* cpu;
 	struct GBMemory memory;
 	struct GBVideo video;
+	struct GBTimer timer;
 
 	int* keySource;
 
diff --git a/src/gb/io.c b/src/gb/io.c
index 763c18e77..e7f669663 100644
--- a/src/gb/io.c
+++ b/src/gb/io.c
@@ -50,6 +50,18 @@ void GBIOReset(struct GB* gb) {
 
 void GBIOWrite(struct GB* gb, unsigned address, uint8_t value) {
 	switch (address) {
+	case REG_DIV:
+		GBTimerDivReset(&gb->timer);
+		return;
+	case REG_TIMA:
+		// ???
+		return;
+	case REG_TMA:
+		// Handled transparently by the registers
+		break;
+	case REG_TAC:
+		value = GBTimerUpdateTAC(&gb->timer, value);
+		break;
 	case REG_IF:
 		gb->memory.io[REG_IF] = value;
 		GBUpdateIRQs(gb);
@@ -108,6 +120,11 @@ uint8_t GBIORead(struct GB* gb, unsigned address) {
 		break;
 	case REG_IE:
 		return gb->memory.ie;
+	case REG_DIV:
+	case REG_TIMA:
+	case REG_TMA:
+	case REG_TAC:
+		break;
 	default:
 		// TODO: Log
 		if (address >= GB_SIZE_IO) {
diff --git a/src/gb/timer.c b/src/gb/timer.c
new file mode 100644
index 000000000..db6c149e4
--- /dev/null
+++ b/src/gb/timer.c
@@ -0,0 +1,84 @@
+/* Copyright (c) 2013-2016 Jeffrey Pfau
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "timer.h"
+
+#include "gb/gb.h"
+#include "gb/io.h"
+
+void GBTimerReset(struct GBTimer* timer) {
+	timer->nextDiv = GB_DMG_DIV_PERIOD; // TODO: GBC differences
+	timer->nextTima = INT_MAX;
+	timer->nextEvent = INT_MAX;
+}
+
+int32_t GBTimerProcessEvents(struct GBTimer* timer, int32_t cycles) {
+	if (timer->nextEvent == INT_MAX) {
+		return INT_MAX;
+	}
+	timer->eventDiff += cycles;
+	timer->nextEvent -= cycles;
+	if (timer->nextEvent < 0) {
+		timer->nextDiv -= timer->eventDiff;
+		if (timer->nextDiv <= 0) {
+			++timer->p->memory.io[REG_DIV];
+			timer->nextDiv = GB_DMG_DIV_PERIOD;
+		}
+		timer->nextEvent = timer->nextDiv;
+
+		if (timer->nextTima != INT_MAX) {
+			timer->nextTima -= timer->eventDiff;
+			if (timer->nextTima <= 0) {
+				++timer->p->memory.io[REG_TIMA];
+				if (!timer->p->memory.io[REG_TIMA]) {
+					timer->p->memory.io[REG_TIMA] = timer->p->memory.io[REG_TMA];
+					timer->p->memory.io[REG_IF] |= (1 << GB_IRQ_TIMER);
+					GBUpdateIRQs(timer->p);
+				}
+				timer->nextTima = timer->timaPeriod;
+			}
+			if (timer->nextTima < timer->nextEvent) {
+				timer->nextEvent = timer->nextTima;
+			}
+		}
+
+		timer->eventDiff = 0;
+	}
+	return timer->nextEvent;
+}
+
+void GBTimerDivReset(struct GBTimer* timer) {
+	timer->p->memory.io[REG_DIV] = 0;
+	// TODO: Do we need to reset the event?
+}
+
+uint8_t GBTimerUpdateTAC(struct GBTimer* timer, GBRegisterTAC tac) {
+	if (GBRegisterTACIsRun(tac)) {
+		switch (GBRegisterTACGetClock(tac)) {
+		case 0:
+			timer->timaPeriod = 1024;
+			break;
+		case 1:
+			timer->timaPeriod = 16;
+			break;
+		case 2:
+			timer->timaPeriod = 64;
+			break;
+		case 3:
+			timer->timaPeriod = 256;
+			break;
+		}
+		timer->nextTima = timer->eventDiff + timer->timaPeriod;
+		if (timer->nextTima < timer->nextEvent) {
+			timer->nextEvent = timer->nextTima;
+			if (timer->nextEvent < timer->p->cpu->nextEvent) {
+				timer->p->cpu->nextEvent = timer->nextEvent;
+			}
+		}
+	} else {
+		timer->nextTima = INT_MAX;
+	}
+	return tac;
+}
diff --git a/src/gb/timer.h b/src/gb/timer.h
new file mode 100644
index 000000000..19f4bcf80
--- /dev/null
+++ b/src/gb/timer.h
@@ -0,0 +1,38 @@
+/* Copyright (c) 2013-2016 Jeffrey Pfau
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef GB_TIMER_H
+#define GB_TIMER_H
+
+#include "util/common.h"
+
+DECL_BITFIELD(GBRegisterTAC, uint8_t);
+DECL_BITS(GBRegisterTAC, Clock, 0, 2);
+DECL_BIT(GBRegisterTAC, Run, 2);
+
+enum {
+	GB_DMG_DIV_PERIOD = 256
+};
+
+struct GB;
+struct GBTimer {
+	struct GB* p;
+
+	int mode;
+
+	int32_t nextEvent;
+	int32_t eventDiff;
+
+	int32_t nextDiv;
+	int32_t nextTima;
+	int32_t timaPeriod;
+};
+
+void GBTimerReset(struct GBTimer*);
+int32_t GBTimerProcessEvents(struct GBTimer*, int32_t cycles);
+void GBTimerDivReset(struct GBTimer*);
+uint8_t GBTimerUpdateTAC(struct GBTimer*, GBRegisterTAC tac);
+
+#endif