1 files changed, 116 insertions, 0 deletions
diff --git a/cpu/cpu.go b/cpu/cpu.go
new file mode 100644
index 0000000..79199a2
--- /dev/null
+++ b/cpu/cpu.go
@@ -0,0 +1,116 @@
+package cpu
+
+const (
+	vectorNMI   uint16 = 0xFFFA
+	vectorReset uint16 = 0xFFFC
+	vectorIRQ   uint16 = 0xFFFE
+
+	stackBase uint16 = 0x0100
+)
+
+type CPU struct {
+	A, X, Y uint8
+	SP      uint8  // stack pointer
+	PC      uint16 // program counter
+	Status  uint8  // status flags
+
+	Bus    Bus
+	Cycles uint64
+
+	extraCycles int
+}
+
+func NewCPU(bus Bus) *CPU {
+	return &CPU{Bus: bus}
+}
+
+func (c *CPU) read(addr uint16) uint8 {
+	return c.Bus.Read(addr)
+}
+
+func (c *CPU) write(addr uint16, v uint8) {
+	c.Bus.Write(addr, v)
+}
+
+func (c *CPU) read16(addr uint16) uint16 {
+	lo := uint16(c.read(addr))
+	hi := uint16(c.read(addr + 1))
+	return hi<<8 | lo
+}
+
+func (c *CPU) push(v uint8) {
+	c.write(stackBase+uint16(c.SP), v)
+	c.SP--
+}
+
+func (c *CPU) pop() uint8 {
+	c.SP++
+	return c.read(stackBase + uint16(c.SP))
+}
+
+func (c *CPU) push16(v uint16) {
+	c.push(uint8(v >> 8))
+	c.push(uint8(v))
+}
+
+func (c *CPU) pop16() uint16 {
+	lo := uint16(c.pop())
+	hi := uint16(c.pop())
+	return hi<<8 | lo
+}
+
+func (c *CPU) Reset() {
+	c.A, c.X, c.Y = 0, 0, 0
+	c.SP = 0xFD
+	c.Status = FlagU | FlagI
+	c.PC = c.read16(vectorReset)
+	c.Cycles = 7
+}
+
+func (c *CPU) IRQ() {
+	if c.flag(FlagI) {
+		return
+	}
+	c.interrupt(vectorIRQ, false)
+	c.Cycles += 7
+}
+
+func (c *CPU) NMI() {
+	c.interrupt(vectorNMI, false)
+	c.Cycles += 7
+}
+
+func (c *CPU) interrupt(vector uint16, brk bool) {
+	c.push16(c.PC)
+	status := c.Status | FlagU
+	if brk {
+		status |= FlagB
+	} else {
+		status &^= FlagB
+	}
+	c.push(status)
+	c.setFlag(FlagI, true)
+	c.PC = c.read16(vector)
+}
+
+func (c *CPU) Step() int {
+	before := c.Cycles
+
+	opcode := c.read(c.PC)
+	c.PC++
+
+	entry := opcodeTable[opcode]
+	op := c.resolveOperand(entry.mode)
+
+	c.extraCycles = 0
+	entry.exec(c, op)
+
+	cycles := entry.cycles
+	if entry.pageCrossPenalty && op.pageCrossed {
+		cycles++
+	}
+	cycles += c.extraCycles
+	c.Cycles += uint64(cycles)
+
+	return int(c.Cycles - before)
+}