Method Step

nes/cpu.go:425–492

Step executes a single CPU instruction

()

// Step executes a single CPU instruction
func (cpu *CPU) Step() int {
	if cpu.stall > 0 {
		cpu.stall--
		return 1
	}

	cycles := cpu.Cycles

	switch cpu.interrupt {
	case interruptNMI:
		cpu.nmi()
	case interruptIRQ:
		cpu.irq()
	}
	cpu.interrupt = interruptNone

	opcode := cpu.Read(cpu.PC)
	mode := instructionModes[opcode]

	var address uint16
	var pageCrossed bool
	switch mode {
	case modeAbsolute:
		address = cpu.Read16(cpu.PC + 1)
	case modeAbsoluteX:
		address = cpu.Read16(cpu.PC+1) + uint16(cpu.X)
		pageCrossed = pagesDiffer(address-uint16(cpu.X), address)
	case modeAbsoluteY:
		address = cpu.Read16(cpu.PC+1) + uint16(cpu.Y)
		pageCrossed = pagesDiffer(address-uint16(cpu.Y), address)
	case modeAccumulator:
		address = 0
	case modeImmediate:
		address = cpu.PC + 1
	case modeImplied:
		address = 0
	case modeIndexedIndirect:
		address = cpu.read16bug(uint16(cpu.Read(cpu.PC+1) + cpu.X))
	case modeIndirect:
		address = cpu.read16bug(cpu.Read16(cpu.PC + 1))
	case modeIndirectIndexed:
		address = cpu.read16bug(uint16(cpu.Read(cpu.PC+1))) + uint16(cpu.Y)
		pageCrossed = pagesDiffer(address-uint16(cpu.Y), address)
	case modeRelative:
		offset := uint16(cpu.Read(cpu.PC + 1))
		if offset < 0x80 {
			address = cpu.PC + 2 + offset
		} else {
			address = cpu.PC + 2 + offset - 0x100
		}
	case modeZeroPage:
		address = uint16(cpu.Read(cpu.PC + 1))
	case modeZeroPageX:
		address = uint16(cpu.Read(cpu.PC+1)+cpu.X) & 0xff
	case modeZeroPageY:
		address = uint16(cpu.Read(cpu.PC+1)+cpu.Y) & 0xff
	}