Implement ADC, including Carry, Overflow, Decimal

addressing-modes.go

@@ -15,7 +15,7 @@ const (
 	relative
 	zeropage
 	zeropageX
-	zoerpageY
+	zeropageY
 )
 
 var addressingNames = [...]string{

cpu.go

@@ -7,6 +7,10 @@ type Cpu struct {
 
 	PC uint16 // 16-bit program counter
 	P  byte   // Status Register
+
+	A byte // Accumulator
+	X byte // X index register
+	Y byte // Y index register
 }
 
 const (
@@ -42,9 +46,49 @@ func (c *Cpu) LoadProgram(data []byte, location uint16) {
 func (c *Cpu) Step() {
 	instruction := c.readNextInstruction()
 	c.PC += uint16(instruction.Size)
-	fmt.Println(instruction)
+	// fmt.Println(instruction)
 	c.execute(instruction)
 }
 
 func (c *Cpu) execute(instruction Instruction) {
+	switch instruction.opcodeId {
+	case adc:
+		c.ADC(instruction)
+	}
+}
+
+func (c *Cpu) resolveOperand(in Instruction) uint8 {
+	switch in.addressingId {
+	case immediate:
+		return in.Op8
+	default:
+		return c.bus.Read(c.memoryAddress(in))
+	}
+}
+
+func (c *Cpu) memoryAddress(in Instruction) uint16 {
+	switch in.addressingId {
+	case absolute:
+		return in.Op16
+	case absoluteX:
+		return in.Op16 + uint16(c.X)
+	case absoluteY:
+		return in.Op16 + uint16(c.Y)
+	case indirect:
+		return c.bus.Read16(in.Op16)
+	case indirectX:
+		return c.bus.Read16(uint16(in.Op8 + c.X))
+	case indirectY:
+		return c.bus.Read16(uint16(in.Op8)) + uint16(c.Y)
+	case relative:
+		panic("Relative addressing not yet implemented.")
+	case zeropage:
+		return uint16(in.Op8)
+	case zeropageX:
+		return uint16(in.Op8 + c.X)
+	case zeropageY:
+		return uint16(in.Op8 + c.Y)
+	default:
+		panic(fmt.Errorf("Unhandled addressing mode. Are you sure you are running a 6502 ROM?"))
+	}
 }

cpu_instructions.go

@@ -0,0 +1,57 @@
+package i6502
+
+// Add Memory to Accumulator with Carry
+func (c *Cpu) ADC(in Instruction) {
+	operand := c.resolveOperand(in)
+	carryIn := c.getStatusInt(sCarry)
+
+	if c.getStatus(sDecimal) {
+		c.adcDecimal(c.A, operand, carryIn)
+	} else {
+		c.adcNormal(c.A, operand, carryIn)
+	}
+}
+
+// Performs regular, 8-bit addition
+func (c *Cpu) adcNormal(a uint8, b uint8, carryIn uint8) {
+	result16 := uint16(a) + uint16(b) + uint16(carryIn)
+	result := uint8(result16)
+	carryOut := (result16 & 0x100) != 0
+	overflow := (a^result)&(b^result)&0x80 != 0
+
+	// Set the carry flag if we exceed 8-bits
+	c.setCarry(carryOut)
+	// Set the overflow bit
+	c.setOverflow(overflow)
+	// Store the resulting value (8-bits)
+	c.A = result
+	// Update sZero and sNegative
+	c.setArithmeticFlags(c.A)
+}
+
+// Performs addition in decimal mode
+func (c *Cpu) adcDecimal(a uint8, b uint8, carryIn uint8) {
+	var carryB uint8 = 0
+
+	low := (a & 0x0F) + (b & 0x0F) + carryIn
+	if (low & 0xFF) > 9 {
+		low += 6
+	}
+	if low > 15 {
+		carryB = 1
+	}
+
+	high := (a >> 4) + (b >> 4) + carryB
+	if (high & 0xFF) > 9 {
+		high += 6
+	}
+
+	result := (low & 0x0F) | (high<<4)&0xF0
+
+	c.setCarry(high > 15)
+	c.setZero(result == 0)
+	c.setNegative(false) // BCD is never negative
+	c.setOverflow(false) // BCD never sets overflow
+
+	c.A = result
+}

cpu_status.go

@@ -0,0 +1,55 @@
+package i6502
+
+// Status Register bits
+const (
+	sCarry = iota
+	sZero
+	sInterrupt
+	sDecimal
+	sBreak
+	_
+	sOverflow
+	sNegative
+)
+
+func (c *Cpu) getStatusInt(bit uint8) uint8 {
+	return (c.P >> bit) & 1
+}
+
+func (c *Cpu) getStatus(bit uint8) bool {
+	return c.getStatusInt(bit) == 1
+}
+
+func (c *Cpu) setStatus(bit uint8, state bool) {
+	if state {
+		c.P |= 1 << bit
+	} else {
+		c.P &^= 1 << bit
+	}
+}
+
+func (c *Cpu) setCarry(state bool) {
+	c.setStatus(sCarry, state)
+}
+
+func (c *Cpu) setOverflow(state bool) {
+	c.setStatus(sOverflow, state)
+}
+
+func (c *Cpu) setZero(state bool) {
+	c.setStatus(sZero, state)
+}
+
+func (c *Cpu) setNegative(state bool) {
+	c.setStatus(sNegative, state)
+}
+
+func (c *Cpu) setArithmeticFlags(value uint8) {
+	// Set sZero if value is 0
+	c.setStatus(sZero, value == 0)
+
+	// Set sNegative if the 8th bit is 1, we're dealing with
+	// uint8's internally, and using two's complement to identify
+	// negatives
+	c.setStatus(sNegative, (value>>7) == 1)
+}

cpu_test.go

@@ -64,9 +64,197 @@ func TestProgramLoading(t *testing.T) {
 	assert.Equal(0x0300, cpu.PC)
 }
 
+//// NOP
+
 func TestNOP(t *testing.T) {
 	cpu, _, _ := NewRamMachine()
 	cpu.LoadProgram([]byte{0xEA}, 0x0300)
 	cpu.Step()
 	assert.Equal(t, 0x0301, cpu.PC)
 }
+
+//// ADC
+
+func TestADCImmediate(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0x53}, 0x0300)
+	cpu.A = 0x42
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x95, cpu.A)
+	assert.False(t, cpu.getStatus(sCarry))
+}
+
+func TestADCWithCarry(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0x53}, 0x0300)
+	cpu.A = 0xC0
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x13, cpu.A)
+	assert.True(t, cpu.getStatus(sCarry))
+}
+
+func TestADCWithCarryOver(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0x04}, 0x0300)
+
+	cpu.setStatus(sCarry, true)
+	cpu.A = 0x05
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x0A, cpu.A)
+	assert.False(t, cpu.getStatus(sCarry))
+}
+
+func TestADCWithOverflow(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0xD0}, 0x0300)
+
+	cpu.A = 0x90
+
+	// 0x90 + 0xD0 = 0x160
+	// 208 + 144 = 352 => unsigned carry
+	// -48 + -112 = 96 => signed overflow
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x60, cpu.A)
+	assert.True(t, cpu.getStatus(sCarry))
+	assert.True(t, cpu.getStatus(sOverflow))
+}
+
+func TestADCZero(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0x00}, 0x0300)
+	cpu.A = 0x00
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x00, cpu.A)
+	assert.True(t, cpu.getStatus(sZero))
+}
+
+func TestADCNegative(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0xF7}, 0x0300)
+	cpu.A = 0x00
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0xF7, cpu.A)
+	assert.True(t, cpu.getStatus(sNegative))
+}
+
+func TestADCDecimal(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x69, 0x28}, 0x0300)
+	cpu.setStatus(sDecimal, true)
+	cpu.A = 0x19
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x47, cpu.A)
+}
+
+func TestADCZeropage(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x65, 0x53}, 0x0300)
+	cpu.A = 0x42
+	cpu.bus.Write(0x53, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+func TestADCZeropageX(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x75, 0x53}, 0x0300)
+	cpu.A = 0x42
+	cpu.X = 0x01
+	cpu.bus.Write(0x54, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+func TestADCAbsolute(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x6D, 0x00, 0x80}, 0x0300)
+	cpu.A = 0x42
+	cpu.bus.Write(0x8000, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0303, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+func TestADCAbsoluteX(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x7D, 0x00, 0x80}, 0x0300)
+	cpu.A = 0x42
+	cpu.X = 0x02
+	cpu.bus.Write(0x8002, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0303, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+func TestADCAbsoluteY(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x79, 0x00, 0x80}, 0x0300)
+	cpu.A = 0x42
+	cpu.Y = 0x02
+	cpu.bus.Write(0x8002, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0303, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+func TestADCIndirectX(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x61, 0x80}, 0x0300)
+	cpu.A = 0x42
+	cpu.X = 0x02
+	cpu.bus.Write16(0x82, 0xC000)
+	cpu.bus.Write(0xC000, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+func TestADCIndirectY(t *testing.T) {
+	cpu, _, _ := NewRamMachine()
+	cpu.LoadProgram([]byte{0x71, 0x80}, 0x0300)
+	cpu.A = 0x42
+	cpu.Y = 0x02
+	cpu.bus.Write16(0x80, 0xC000)
+	cpu.bus.Write(0xC002, 0x12)
+
+	cpu.Step()
+
+	assert.Equal(t, 0x0302, cpu.PC)
+	assert.Equal(t, 0x54, cpu.A)
+}
+
+//

instruction.go

@@ -21,11 +21,11 @@ type Instruction struct {
 func (i Instruction) String() (output string) {
 	switch i.Size {
 	case 1:
-		output = fmt.Sprintf("~~ 0x%04X: 0x%02X - %s [%s] {%d}\n", i.Address, i.Opcode, instructionNames[i.opcodeId], addressingNames[i.addressingId], i.Cycles)
+		output = fmt.Sprintf("~~~ 0x%04X: 0x%02X - %s [%s] {%d}\n", i.Address, i.Opcode, instructionNames[i.opcodeId], addressingNames[i.addressingId], i.Cycles)
 	case 2:
-		output = fmt.Sprintf("~~ 0x%04X: 0x%02X - %s 0x%02X [%s] {%d}\n", i.Address, i.Opcode, instructionNames[i.opcodeId], i.Op8, addressingNames[i.addressingId], i.Cycles)
+		output = fmt.Sprintf("~~~ 0x%04X: 0x%02X - %s 0x%02X [%s] {%d}\n", i.Address, i.Opcode, instructionNames[i.opcodeId], i.Op8, addressingNames[i.addressingId], i.Cycles)
 	case 3:
-		output = fmt.Sprintf("~~ 0x%04X: 0x%02X - %s 0x%04X [%s] {%d}\n", i.Address, i.Opcode, instructionNames[i.opcodeId], i.Op16, addressingNames[i.addressingId], i.Cycles)
+		output = fmt.Sprintf("~~~ 0x%04X: 0x%02X - %s 0x%04X [%s] {%d}\n", i.Address, i.Opcode, instructionNames[i.opcodeId], i.Op16, addressingNames[i.addressingId], i.Cycles)
 	}
 
 	return
@@ -37,7 +37,7 @@ func (c *Cpu) readNextInstruction() Instruction {
 
 	optype, ok := opTypes[opcode]
 	if !ok {
-		panic(fmt.Sprintf("Unknown or unimplemented opcde 0x%02X", opcode))
+		panic(fmt.Sprintf("Unknown or unimplemented opcode 0x%02X", opcode))
 	}
 
 	instruction := Instruction{OpType: optype}

opcodes.go

@@ -143,4 +143,13 @@ type OpType struct {
 
 var opTypes = map[uint8]OpType{
 	0xEA: OpType{0xEA, nop, implied, 1, 2},
+
+	0x69: OpType{0x69, adc, immediate, 2, 2},
+	0x65: OpType{0x65, adc, zeropage, 2, 2},
+	0x75: OpType{0x75, adc, zeropageX, 2, 4},
+	0x6D: OpType{0x6D, adc, absolute, 3, 4},
+	0x7D: OpType{0x7D, adc, absoluteX, 3, 4},
+	0x79: OpType{0x79, adc, absoluteY, 3, 4},
+	0x61: OpType{0x61, adc, indirectX, 2, 6},
+	0x71: OpType{0x71, adc, indirectY, 2, 5},
 }