initial commit

1 files changed, 138 insertions, 0 deletions

diff --git a/cpu/addressing.go b/cpu/addressing.go
new file mode 100644
index 0000000..f1b612a
--- /dev/null
+++ b/cpu/addressing.go
@@ -0,0 +1,138 @@
+package cpu
+
+type AddrMode int
+
+const (
+	ModeImplied AddrMode = iota
+	ModeAccumulator
+	ModeImmediate
+	ModeZeroPage
+	ModeZeroPageX
+	ModeZeroPageY
+	ModeAbsolute
+	ModeAbsoluteX
+	ModeAbsoluteY
+	ModeIndirect
+	ModeIndirectX
+	ModeIndirectY
+	ModeRelative
+)
+
+type operand struct {
+	addr        uint16
+	accumulator bool
+	pageCrossed bool
+}
+
+func samePage(a, b uint16) bool {
+	return a&0xFF00 == b&0xFF00
+}
+
+func (c *CPU) resolveOperand(mode AddrMode) operand {
+	switch mode {
+	case ModeImplied:
+		return operand{}
+
+	case ModeAccumulator:
+		return operand{accumulator: true}
+
+	case ModeImmediate:
+		addr := c.PC
+		c.PC++
+		return operand{addr: addr}
+
+	case ModeZeroPage:
+		addr := uint16(c.read(c.PC))
+		c.PC++
+		return operand{addr: addr}
+
+	case ModeZeroPageX:
+		addr := uint16(uint8(c.read(c.PC) + c.X))
+		c.PC++
+		return operand{addr: addr}
+
+	case ModeZeroPageY:
+		addr := uint16(uint8(c.read(c.PC) + c.Y))
+		c.PC++
+		return operand{addr: addr}
+
+	case ModeAbsolute:
+		addr := c.read16(c.PC)
+		c.PC += 2
+		return operand{addr: addr}
+
+	case ModeAbsoluteX:
+		base := c.read16(c.PC)
+		c.PC += 2
+		addr := base + uint16(c.X)
+		return operand{addr: addr, pageCrossed: !samePage(base, addr)}
+
+	case ModeAbsoluteY:
+		base := c.read16(c.PC)
+		c.PC += 2
+		addr := base + uint16(c.Y)
+		return operand{addr: addr, pageCrossed: !samePage(base, addr)}
+
+	case ModeIndirect:
+		ptr := c.read16(c.PC)
+		c.PC += 2
+		return operand{addr: c.readIndirectBug(ptr)}
+
+	case ModeIndirectX:
+		zp := uint8(c.read(c.PC) + c.X)
+		c.PC++
+		addr := c.read16zp(zp)
+		return operand{addr: addr}
+
+	case ModeIndirectY:
+		zp := c.read(c.PC)
+		c.PC++
+		base := c.read16zp(zp)
+		addr := base + uint16(c.Y)
+		return operand{addr: addr, pageCrossed: !samePage(base, addr)}
+
+	case ModeRelative:
+		offset := int8(c.read(c.PC))
+		c.PC++
+		addr := uint16(int32(c.PC) + int32(offset))
+		return operand{addr: addr}
+
+	default:
+		panic("cpu: unhandled addressing mode")
+	}
+}
+
+func (c *CPU) read16zp(addr uint8) uint16 {
+	lo := uint16(c.read(uint16(addr)))
+	hi := uint16(c.read(uint16(addr + 1)))
+	return hi<<8 | lo
+}
+
+// jmp (ind) hardware bug: if pointers low byte is 0xff, the high byte is
+// fetched from the start of the same page instead of the next page
+func (c *CPU) readIndirectBug(ptr uint16) uint16 {
+	lo := uint16(c.read(ptr))
+	var hiAddr uint16
+	if ptr&0x00FF == 0x00FF {
+		hiAddr = ptr & 0xFF00
+	} else {
+		hiAddr = ptr + 1
+	}
+	hi := uint16(c.read(hiAddr))
+	return hi<<8 | lo
+}
+
+func (c *CPU) operandValue(op operand) uint8 {
+	if op.accumulator {
+		return c.A
+	}
+	return c.read(op.addr)
+}
+
+func (c *CPU) writeOperand(op operand, v uint8) {
+	if op.accumulator {
+		c.A = v
+	} else {
+		c.write(op.addr, v)
+	}
+}