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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)
}
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