emulator step

chip8/chip8.go

@@ -1,5 +1,7 @@
 package chip8
 
+import "encoding/binary"
+
 const (
 	MemorySize      = 4096 // 4KB of memory
 	StackSize       = 16
@@ -41,3 +43,20 @@ func (emulator *Emulator) Reset() {
 	emulator.Timer.Sound = 0
 	copy(emulator.Memory[ProgramAddress:], emulator.ROM)
 }
+
+func (emulator *Emulator) Step() {
+	instruction := binary.BigEndian.Uint16(emulator.Memory[emulator.PC:])
+
+	switch instruction & 0xF000 {
+	case 0x1000: // 1nnn - JP addr
+		emulator.Jump(instruction & 0x0FFF)
+	case 0x2000: // 2nnn - CALL addr
+		emulator.Call(instruction & 0x0FFF)
+	case 0x3000: // 3xkk - SE Vx, byte
+		emulator.SkipEqual(uint8(instruction&0x0F00>>8), uint8(instruction&0x00FF))
+	case 0x4000: // 4xkk - SNE Vx, byte
+		emulator.SkipNotEqual(uint8(instruction&0x0F00>>8), uint8(instruction&0x00FF))
+	case 0x5000: // 5xy0 - SE Vx, Vy
+		emulator.SkipRegistersEqual(uint8(instruction&0x0F00>>8), uint8(instruction&0x00F0>>4))
+	}
+}

chip8/instructions.go

@@ -9,31 +9,50 @@ func (emulator *Emulator) Return() {
 	emulator.StackPop()
 }
 
-func (emulator *Emulator) Jump(addr uint16) {
+// Jump to location nnn.
-	emulator.PC = addr
+//
+// The interpreter sets the program counter to nnn.
+func (emulator *Emulator) Jump(nnn uint16) {
+	emulator.PC = nnn
 }
 
-func (emulator *Emulator) Call(addr uint16) {
+// Call subroutine at nnn.
+//
+// The interpreter increments the stack pointer, then puts the current PC
+// on the top of the stack. The PC is then set to nnn.
+func (emulator *Emulator) Call(nnn uint16) {
 	emulator.StackPush()
-	emulator.PC = addr
+	emulator.PC = nnn
 }
 
-func (emulator *Emulator) SkipEqual(x uint8, value uint8) {
+// Skip next instruction if Vx = kk.
-	if emulator.V[x] == value {
+//
+// The interpreter compares register Vx to kk, and if they are equal,
+// increments the program counter by 2.
+func (emulator *Emulator) SkipEqual(x uint8, kk uint8) {
+	if emulator.V[x] == kk {
 		emulator.PC += InstructionSize * 2
 	} else {
 		emulator.PC += InstructionSize
 	}
 }
 
-func (emulator *Emulator) SkipNotEqual(x uint8, value uint8) {
+// Skip next instruction if Vx != kk.
-	if emulator.V[x] != value {
+//
+// The interpreter compares register Vx to kk, and if they are not equal,
+// increments the program counter by 2.
+func (emulator *Emulator) SkipNotEqual(x uint8, kk uint8) {
+	if emulator.V[x] != kk {
 		emulator.PC += InstructionSize * 2
 	} else {
 		emulator.PC += InstructionSize
 	}
 }
 
+// Skip next instruction if Vx = Vy.
+//
+// The interpreter compares register Vx to register Vy, and if they are equal,
+// increments the program counter by 2.
 func (emulator *Emulator) SkipRegistersEqual(x uint8, y uint8) {
 	if emulator.V[x] == emulator.V[y] {
 		emulator.PC += InstructionSize * 2