Implement timer, some IO operations and more CPU instructions

gb/timer.go

@@ -0,0 +1,125 @@
+package gb
+
+import (
+	"fmt"
+	"os"
+)
+
+var timer = Timer{DIV: 0xAC00}
+
+type Timer struct {
+	DIV  uint16
+	TIMA byte
+	TMA  byte
+	TAC  byte
+}
+
+func (timer *Timer) Tick() {
+	previousDIV := timer.DIV
+	timer.DIV++
+
+	timerNeedsUpdate := false
+
+	// Determine clock mode
+	// TAC (Timer control register)
+	// Bits 0-1 determines clock frequency
+	// 00: 4096 Hz
+	// 01: 262144 Hz
+	// 10: 65536 Hz
+	// 11: 16384 Hz
+	// Bit 2 determines whether timer is enabled or not
+
+	clockMode := timer.TAC & (0b11)
+	switch clockMode {
+	case 0b00:
+		// 4096 Hz
+		// Detect a falling edge by comparing bit 9 in the previous DIV value
+		// with bit 9 in the current DIV value
+		previouslyEnabled := previousDIV&(1<<9) != 0
+		currentlyDisabled := timer.DIV&(1<<9) == 0
+		timerNeedsUpdate = previouslyEnabled && currentlyDisabled
+
+	case 0b01:
+		// 262144 Hz
+		// Detect a falling edge by comparing bit 3 in the previous DIV value
+		// with bit 3 in the current DIV value
+		previouslyEnabled := previousDIV&(1<<3) != 0
+		currentlyDisabled := timer.DIV&(1<<3) == 0
+		timerNeedsUpdate = previouslyEnabled && currentlyDisabled
+
+	case 0b10:
+		// 65536 Hz
+		// Detect a falling edge by comparing bit 5 in the previous DIV value
+		// with bit 5 in the current DIV value
+		previouslyEnabled := previousDIV&(1<<5) != 0
+		currentlyDisabled := timer.DIV&(1<<5) == 0
+		timerNeedsUpdate = previouslyEnabled && currentlyDisabled
+
+	case 0b11:
+		// 16384 Hz
+		// Detect a falling edge by comparing bit 7 in the previous DIV value
+		// with bit 7 in the current DIV value
+		previouslyEnabled := previousDIV&(1<<7) != 0
+		currentlyDisabled := timer.DIV&(1<<7) == 0
+		timerNeedsUpdate = previouslyEnabled && currentlyDisabled
+	}
+
+	timerIsEnabled := timer.TAC&(1<<2) != 0
+	// If the timer needs to be updated based on the determined clock mode and the timer is enabled, increment the timer
+	if timerNeedsUpdate && timerIsEnabled {
+		timer.TIMA++
+
+		// Check if TIMA is going to wrap and trigger an interrupt if necessary
+		if timer.TIMA == 0xFF {
+			timer.TIMA = timer.TMA
+
+			fmt.Println("TODO: cpu_request_interrupt(IT_TIMER")
+		}
+	}
+
+}
+
+func (timer *Timer) Read(address uint16) byte {
+	switch address {
+	case 0xFF04:
+		return byte(timer.DIV >> 8)
+
+	case 0xFF05:
+		return timer.TIMA
+
+	case 0xFF06:
+		return timer.TMA
+
+	case 0xFF07:
+		return timer.TAC
+
+	default:
+		fmt.Printf("Reading from Timer: invalid address %X\n", address)
+		os.Exit(1)
+	}
+	return 0
+}
+
+func (timer *Timer) Write(address uint16, value byte) {
+	switch address {
+	case 0xFF04:
+		//DIV
+		timer.DIV = 0
+
+	case 0xFF05:
+		//TIMA
+		timer.TIMA = value
+
+	case 0xFF06:
+		//TMA
+		timer.TMA = value
+
+	case 0xFF07:
+		//TAC
+		timer.TAC = value
+
+	default:
+		fmt.Printf("Writing to Timer: invalid address %X\n", address)
+		os.Exit(1)
+	}
+}