Most simple instructions done.

2025-01-16 12:30:34 +00:00 · 2013-02-17 21:09:38 -08:00 · 2013-02-17 21:09:38 -08:00 · 7d26907926
commit 7d26907926
parent 90369c5a23
7 changed files with 6269 additions and 0 deletions
--- a/cpu/cpu.go
+++ b/cpu/cpu.go
@ -0,0 +1,145 @@
+/*
+  6502 implementation.
+
+  TODO(zellyn): Provide configurable options
+  TODO(zellyn): Implement IRQ, NMI
+*/
+
+package cpu
+
+import (
+	"fmt"
+)
+
+/* Bugs and Quirks.
+   See http://en.wikipedia.org/wiki/MOS_Technology_6502#Bugs_and_quirks
+*/
+const (
+	// TODO(zellyn) implement
+	// JMP xxFF reads xx00 instead of (xx+1)00
+	OPTION_BUG_JMP_FF                                = true
+	OPTION_BUG_INDEXED_ADDR_ACROSS_PAGE_INVALID_READ = true
+	OPTION_BUG_READ_MODIFY_WRITE_TWO_WRITES          = true
+	OPTION_BUG_NVZ_INVALID_IN_BCD                    = true
+	OPTION_BUG_NO_CLD_ON_INTERRUPT                   = true
+	OPTION_BUG_INTERRUPTS_CLOBBER_BRK                = true
+)
+
+type Cpu interface {
+	Reset()
+	Step() error
+	SetPC(uint16)
+	A() byte
+	X() byte
+	Y() byte
+	PC() uint16
+	P() byte // [NV-BDIZC]
+	SP() byte
+	// TODO(zellyn): Add signaling of interrupts
+}
+
+type Memory interface {
+	Read(uint16) byte
+	Write(uint16, byte)
+}
+
+type Ticker interface {
+	Tick()
+}
+
+const (
+	STACK_BASE   = 0x100
+	IRQ_VECTOR   = 0xFFFE
+	NMI_VECTOR   = 0xFFFA
+	RESET_VECTOR = 0xFFFC
+)
+
+const (
+	FLAG_C = 1 << iota
+	FLAG_Z
+	FLAG_I
+	FLAG_D
+	FLAG_B
+	FLAG_UNUSED
+	FLAG_V
+	FLAG_N
+)
+
+type registers struct {
+	A  byte
+	X  byte
+	Y  byte
+	PC uint16
+	P  byte // [NV-BDIZC]
+	SP byte
+}
+
+type cpu struct {
+	m Memory
+	t Ticker
+	r registers
+}
+
+func NewCPU(memory Memory, ticker Ticker) Cpu {
+	c := cpu{m: memory, t: ticker}
+	c.r.P |= FLAG_UNUSED // Set unused flag to 1
+	return &c
+}
+
+func (c *cpu) A() byte {
+	return c.r.A
+}
+func (c *cpu) X() byte {
+	return c.r.X
+}
+func (c *cpu) Y() byte {
+	return c.r.Y
+}
+func (c *cpu) PC() uint16 {
+	return c.r.PC
+}
+func (c *cpu) P() byte {
+	return c.r.P
+}
+func (c *cpu) SP() byte {
+	return c.r.SP
+}
+
+func (c *cpu) readWord(address uint16) uint16 {
+	return uint16(c.m.Read(address)) + (uint16(c.m.Read(address+1)) << 8)
+}
+
+func (c *cpu) Reset() {
+	c.r.SP = 0
+	c.r.PC = c.readWord(RESET_VECTOR)
+	c.r.P |= FLAG_I // Turn interrupts off
+	if !OPTION_BUG_NO_CLD_ON_INTERRUPT {
+		c.r.P &^= FLAG_D
+	}
+}
+
+func (c *cpu) Step() error {
+	i := c.m.Read(c.r.PC)
+	c.r.PC++
+	c.t.Tick()
+
+	if f, ok := opcodes[i]; ok {
+		f(c)
+		return nil
+	}
+
+	return fmt.Errorf("Unknown opcode at location $%04X: $%02X", c.r.PC, i)
+}
+
+func (c *cpu) SetPC(address uint16) {
+	c.r.PC = address
+}
+
+func (c *cpu) SetNZ(value byte) {
+	c.r.P = (c.r.P &^ FLAG_N) | (value & FLAG_N)
+	if value == 0 {
+		c.r.P |= FLAG_Z
+	} else {
+		c.r.P &^= FLAG_Z
+	}
+}
--- a/cpu/opcodes.go
+++ b/cpu/opcodes.go
@ -0,0 +1,366 @@
+package cpu
+
+import (
+	_ "fmt"
+)
+
+func samePage(a1 uint16, a2 uint16) bool {
+	return a1^a2&0xFF00 == 0
+}
+
+// Simple, one-off instructions ----------------------------------------------
+
+func clearFlag(flag byte) func(*cpu) {
+	return func(c *cpu) {
+		c.r.P &^= flag
+		c.t.Tick()
+	}
+}
+
+func setFlag(flag byte) func(*cpu) {
+	return func(c *cpu) {
+		c.r.P |= flag
+		c.t.Tick()
+	}
+}
+
+func dex(c *cpu) {
+	c.r.X--
+	c.SetNZ(c.r.X)
+	c.t.Tick()
+}
+
+func dey(c *cpu) {
+	c.r.Y--
+	c.SetNZ(c.r.Y)
+	c.t.Tick()
+}
+
+func inx(c *cpu) {
+	c.r.X++
+	c.SetNZ(c.r.X)
+	c.t.Tick()
+}
+
+func iny(c *cpu) {
+	c.r.Y++
+	c.SetNZ(c.r.Y)
+	c.t.Tick()
+}
+
+func pha(c *cpu) {
+	c.t.Tick()
+	c.m.Write(0x100+uint16(c.r.SP), c.r.A)
+	c.r.SP--
+	c.t.Tick()
+}
+
+func php(c *cpu) {
+	c.t.Tick()
+	c.m.Write(0x100+uint16(c.r.SP), c.r.P)
+	c.r.SP--
+	c.t.Tick()
+}
+func pla(c *cpu) {
+	c.t.Tick()
+	c.r.SP++
+	c.t.Tick()
+	c.r.A = c.m.Read(0x100 + uint16(c.r.SP))
+	c.t.Tick()
+}
+func plp(c *cpu) {
+	c.t.Tick()
+	c.r.SP++
+	c.t.Tick()
+	c.r.P = c.m.Read(0x100 + uint16(c.r.SP))
+	c.t.Tick()
+}
+
+func nop(c *cpu) {
+	c.t.Tick()
+}
+
+func tax(c *cpu) {
+	c.r.X = c.r.A
+	c.SetNZ(c.r.X)
+	c.t.Tick()
+}
+
+func tay(c *cpu) {
+	c.r.Y = c.r.A
+	c.SetNZ(c.r.Y)
+	c.t.Tick()
+}
+
+func tsx(c *cpu) {
+	c.r.X = c.r.SP
+	c.SetNZ(c.r.X)
+	c.t.Tick()
+}
+
+func txa(c *cpu) {
+	c.r.A = c.r.X
+	c.SetNZ(c.r.A)
+	c.t.Tick()
+}
+
+func txs(c *cpu) {
+	c.r.SP = c.r.X
+	c.SetNZ(c.r.SP)
+	c.t.Tick()
+}
+
+func tya(c *cpu) {
+	c.r.A = c.r.Y
+	c.SetNZ(c.r.A)
+	c.t.Tick()
+}
+
+func jmpAbsolute(c *cpu) {
+	// T1
+	c.r.PC++
+	addr := uint16(c.m.Read(c.r.PC))
+	c.t.Tick()
+	// T2
+	c.r.PC++
+	addr |= (uint16(c.m.Read(c.r.PC)) << 8)
+	c.r.PC = addr
+	c.t.Tick()
+}
+
+func jmpIndirect(c *cpu) {
+	// T1
+	c.r.PC++
+	iAddr := uint16(c.m.Read(c.r.PC))
+	c.t.Tick()
+	// T2
+	c.r.PC++
+	iAddr |= (uint16(c.m.Read(c.r.PC)) << 8)
+	c.t.Tick()
+	// T3
+	addr := uint16(c.m.Read(iAddr))
+	c.t.Tick()
+	// T4
+	if (iAddr&0xff == 0xff) && OPTION_BUG_JMP_FF {
+		addr |= (uint16(c.m.Read(iAddr&0xff00)) << 8)
+	} else {
+		addr |= (uint16(c.m.Read(iAddr+1)) << 8)
+	}
+	c.r.PC = addr
+	c.t.Tick()
+}
+
+func jsr(c *cpu) {
+	// T1
+	c.r.PC++
+	addr := uint16(c.m.Read(c.r.PC)) // We actually push PC(next) - 1
+	c.t.Tick()
+	// T2
+	c.r.PC++
+	c.t.Tick()
+	// T3
+	c.m.Write(0x100+uint16(c.r.SP), byte(c.r.PC>>8))
+	c.r.SP--
+	c.t.Tick()
+	// T4
+	c.m.Write(0x100+uint16(c.r.SP), byte(c.r.PC&0xff))
+	c.r.SP--
+	c.t.Tick()
+	// T5
+	addr |= (uint16(c.m.Read(c.r.PC)) << 8)
+	c.r.PC = addr
+	c.t.Tick()
+}
+
+func rts(c *cpu) {
+	// T1
+	c.t.Tick()
+	// T2
+	c.r.SP++
+	c.t.Tick()
+	// T3
+	addr := uint16(c.m.Read(0x100 + uint16(c.r.SP)))
+	c.r.SP++
+	c.t.Tick()
+	// T4
+	addr |= (uint16(c.m.Read(0x100+uint16(c.r.SP))) << 8)
+	c.t.Tick()
+	// T5
+	c.r.PC = addr + 1 // Since we pushed PC(next) - 1
+	c.t.Tick()
+}
+
+func rti(c *cpu) {
+	// T1
+	c.t.Tick()
+	// T2
+	c.r.SP++
+	c.t.Tick()
+	// T3
+	c.r.P = c.m.Read(0x100 + uint16(c.r.SP))
+	c.r.SP++
+	// T4
+	addr := uint16(c.m.Read(0x100 + uint16(c.r.SP)))
+	c.r.SP++
+	c.t.Tick()
+	// T5
+	addr |= (uint16(c.m.Read(0x100+uint16(c.r.SP))) << 8)
+	c.r.PC = addr
+	c.t.Tick()
+}
+
+// Note that BRK skips the next instruction:
+// http://en.wikipedia.org/wiki/Interrupts_in_65xx_processors#Using_BRK_and_COP
+func brk(c *cpu) {
+	// T1
+	c.r.PC++
+	c.r.SP--
+	c.t.Tick()
+	// T2
+	c.m.Write(0x100+uint16(c.r.SP), byte(c.r.PC>>8))
+	c.r.SP--
+	c.t.Tick()
+	// T3
+	c.m.Write(0x100+uint16(c.r.SP), byte(c.r.PC&0xff))
+	c.r.SP--
+	c.t.Tick()
+	// T4
+	c.m.Write(0x100+uint16(c.r.SP), c.r.P|FLAG_B) // Set B flag
+	c.r.P |= FLAG_I                               // Disable interrupts
+	c.t.Tick()
+	// T5
+	addr := uint16(c.m.Read(IRQ_VECTOR))
+	c.t.Tick()
+	// T6
+	addr |= (uint16(c.m.Read(IRQ_VECTOR+1)) << 8)
+	c.r.PC = addr
+	c.t.Tick()
+}
+
+func branch(mask, value byte) func(*cpu) {
+	return func(c *cpu) {
+		offset := c.m.Read(c.r.PC)
+		c.r.PC++
+		c.t.Tick()
+		oldPC := c.r.PC
+		if c.r.P&mask == value {
+			c.t.Tick()
+			c.r.PC = c.r.PC + uint16(offset)
+			if offset >= 128 {
+				c.r.PC = c.r.PC - 256
+			}
+			if !samePage(c.r.PC, oldPC) {
+				c.t.Tick()
+			}
+		}
+	}
+}
+
+func immediate(f func(*cpu, byte)) func(*cpu) {
+	return func(c *cpu) {
+		// T1
+		value := c.m.Read(c.r.PC)
+		c.r.PC++
+		f(c, value)
+		c.t.Tick()
+	}
+}
+
+func lda(c *cpu, value byte) {
+	c.r.A = value
+	c.SetNZ(value)
+}
+
+func ldx(c *cpu, value byte) {
+	c.r.X = value
+	c.SetNZ(value)
+}
+
+func ldy(c *cpu, value byte) {
+	c.r.Y = value
+	c.SetNZ(value)
+}
+
+func ora(c *cpu, value byte) {
+	c.r.A |= value
+	c.SetNZ(c.r.A)
+}
+
+func and(c *cpu, value byte) {
+	c.r.A &= value
+	c.SetNZ(c.r.A)
+}
+
+func eor(c *cpu, value byte) {
+	c.r.A ^= value
+	c.SetNZ(c.r.A)
+}
+
+func cmp(c *cpu, value byte) {
+	v := c.r.A - value
+	c.SetNZ(v)
+}
+
+func cpx(c *cpu, value byte) {
+	v := c.r.X - value
+	c.SetNZ(v)
+}
+
+func cpy(c *cpu, value byte) {
+	v := c.r.Y - value
+	c.SetNZ(v)
+}
+
+var opcodes = map[byte]func(*cpu){
+	0x18: clearFlag(FLAG_C), // CLC
+	0xD8: clearFlag(FLAG_D), // CLD
+	0x58: clearFlag(FLAG_I), // CLI
+	0xB8: clearFlag(FLAG_V), // CLV
+	0x38: setFlag(FLAG_C),   // SEC
+	0xF8: setFlag(FLAG_D),   // SED
+	0x78: setFlag(FLAG_I),   // SEI
+	0xEA: nop,
+	0xAA: tax,
+	0xA8: tay,
+	0xBA: tsx,
+	0x8A: txa,
+	0x9A: txs,
+	0x98: tya,
+
+	0xCA: dex,
+	0x88: dey,
+	0xE8: inx,
+	0xC8: iny,
+	0x48: pha,
+	0x08: php,
+	0x68: pla,
+	0x28: plp,
+
+	0x4C: jmpAbsolute,
+	0x6C: jmpIndirect,
+	0x20: jsr,
+	0x60: rts,
+	0x40: rti,
+	0x00: brk,
+
+	0x90: branch(FLAG_C, 0),      // BCC
+	0xB0: branch(FLAG_C, FLAG_C), // BCS
+	0xF0: branch(FLAG_Z, FLAG_Z), // BEQ
+	0x30: branch(FLAG_N, FLAG_N), // BMI
+	0xD0: branch(FLAG_Z, 0),      // BNE
+	0x10: branch(FLAG_N, 0),      // BPL
+	0x50: branch(FLAG_V, 0),      // BVC
+	0x70: branch(FLAG_V, FLAG_V), // BVS
+
+	0x09: immediate(ora),
+	0x29: immediate(and),
+	0x49: immediate(eor),
+	// 0x69: immediate(adc),
+	0xC0: immediate(cpy),
+	0xC9: immediate(cmp),
+	0xA0: immediate(ldy),
+	0xA2: immediate(ldx),
+	0xA9: immediate(lda),
+	0xE0: immediate(cpx),
+	// 0xE9: immediate(sbc),
+}
--- a/docs/6502.org
+++ b/docs/6502.org
@ -0,0 +1,123 @@
+* References
+- Detailed per-instruction timing, etc: http://www.masswerk.at/6502/6502_instruction_set.html
+- aaabbbcc-type breakdown: http://www.llx.com/~nparker/a2/opcodes.html
+- 6502 Hardware manual (KIM-1): http://users.telenet.be/kim1-6502/6502/hwman.html#AA
+- Stack explanation (annoying writing): http://homepage.ntlworld.com/cyborgsystems/CS_Main/6502/6502.htm#STACK
+- For doublechecking cycle counts: http://www.obelisk.demon.co.uk/6502/reference.html
+
+* Reset sequences
+- http://www.pagetable.com/?p=410
+
+** Reset/interrupt vectors
+|---------+-------------|
+| Address | Description |
+|---------+-------------|
+| $FFFE/F | IRQ         |
+| $FFFA/B | NMI         |
+| $FFFC/D | Reset       |
+|---------+-------------|
+
+* Misc references
+- http://axis.llx.com/~nparker/a2/opcodes.html
+
+* Timing
+aaabbbcc
+
+** cc = 01
+
+| aaa | opcode | flags |
+|-----+--------+-------|
+| 000 | ORA    | NZ    |
+| 001 | AND    | NZ    |
+| 010 | EOR    | NZ    |
+| 011 | ADC    | NZCV  |
+| 100 | STA    |       |
+| 101 | LDA    | NZ    |
+| 110 | CMP    | NZC   |
+| 111 | SBC    | NZCV  |
+
+| bbb | addressing mode |
+|-----+-----------------|
+| 000 | (zero page,X)   |
+| 001 | zero page       |
+| 010 | #immediate      |
+| 011 | absolute        |
+| 100 | (zero page),Y   |
+| 101 | zero page,X     |
+| 110 | absolute,Y      |
+| 111 | absolute,X      |
+
+| Addressing Mode | ORA |      | AND |      | EOR |      | ADC |      | STA |     | LDA |      | CMP |      | SBC |      |
+|-----------------+-----+------+-----+------+-----+------+-----+------+-----+-----+-----+------+-----+------+-----+------|
+| (zp,X)          |  01 | 2/6  |  21 | 2/6  |  41 | 2/6  |  61 | 2/6  |  81 | 2/6 | A1  | 2/6  | C1  | 2/6  | E1  | 2/6  |
+| zp              |  05 | 2/3  |  25 | 2/3  |  45 | 2/3  |  65 | 2/3  |  85 | 2/3 | A5  | 2/3  | C5  | 2/3  | E5  | 2/3  |
+| #               |  09 | 2/2  |  29 | 2/2  |  49 | 2/2  |  69 | 2/2  |     |     | A9  | 2/2  | C9  | 2/2  | E9  | 2/2  |
+| abs             |  0D | 3/4  |  2D | 3/4  |  4D | 3/4  |  6D | 3/4  |  8D | 3/4 | AD  | 3/4  | CD  | 3/4  | ED  | 3/4  |
+| (zp),Y          |  11 | 2/5* |  31 | 2/5* |  51 | 2/5* |  71 | 2/5* |  91 | 2/6 | B1  | 2/5* | D1  | 2/5* | F1  | 2/5* |
+| zp,X            |  15 | 2/4  |  35 | 2/4  |  55 | 2/4  |  75 | 2/4  |  95 | 2/4 | B5  | 2/4  | D5  | 2/4  | F5  | 2/4  |
+| abs,Y           |  19 | 3/4* |  39 | 3/4* |  59 | 3/4* |  79 | 3/4* |  99 | 3/5 | B9  | 3/4* | D9  | 3/4* | F9  | 3/4* |
+| abs,X           |  1D | 3/4* |  3D | 3/4* |  5D | 3/4* |  7D | 3/4* |  9D | 3/5 | BD  | 3/4* | DD  | 3/4* | FD  | 3/4* |
+
+(*) - add 1 to cycles if page boundary is crossed
+
+** cc = 10
+
+| aaa | opcode | flags |
+|-----+--------+-------|
+| 000 | ASL    | NZC   |
+| 001 | ROL    | NZC   |
+| 010 | LSR    | NZC   |
+| 011 | ROR    | NZC   |
+| 100 | STX    |       |
+| 101 | LDX    | NZ    |
+| 110 | DEC    | NZ    |
+| 111 | INC    | NZ    |
+
+| bbb | addressing mode |                       |
+|-----+-----------------+-----------------------|
+| 000 | #immediate      |                       |
+| 001 | zero page       |                       |
+| 010 | accumulator     |                       |
+| 011 | absolute        |                       |
+| 101 | zero page,X     | STX,LDX: zero page, Y |
+| 111 | absolute,X      | LDX: absolute,Y       |
+
+|             | ASL |     | ROL |     | LSR |     | ROR |     | STX |     | LDX |      | DEC |     | INC |     |
+|-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------+-----+-----+-----+-----|
+| #           |     |     |     |     |     |     |     |     |     |     | A2  | 2/2  |     |     |     |     |
+| zp          |  06 | 2/5 |  26 | 2/5 |  46 | 2/5 |  66 | 2/5 |  86 | 2/3 | A6  | 2/3  | C6  | 2/5 | E6  | 2/5 |
+| A           |  0A | 1/2 |  2A | 1/2 |  4A | 1/2 |  6A | 1/2 |     |     |     |      |     |     |     |     |
+| abs         |  0E | 3/6 |  2E | 3/6 |  4E | 3/6 |  6E | 3/6 |  8E | 3/4 | AE  | 3/4  | CE  | 3/3 | EE  | 3/6 |
+| zp,X/zp,Y   |  16 | 2/6 |  36 | 2/6 |  56 | 2/6 |  76 | 2/6 |  96 | 2/4 | B6  | 2/4  | D6  | 2/6 | F6  | 2/6 |
+| abs,X/abs,Y |  1E | 3/7 |  3E | 3/7 |  5E | 3/7 |  7E | 3/7 |  ?? |     | BE  | 3/4* | DE  | 3/7 | FE  | 3/7 |
+
+(*) - add 1 to cycles if page boundary is crossed
+
+** cc = 00
+
+| aaa | opcode    | flags |
+|-----+-----------+-------|
+| 001 | BIT       | NZV   |
+| 010 | JMP       |       |
+| 011 | JMP (abs) |       |
+| 100 | STY       |       |
+| 101 | LDY       |       |
+| 110 | CPY       | NZC   |
+| 111 | CPX       | NZC   |
+
+
+| bbb | addressing mode |
+|-----+-----------------|
+| 000 | #immediate      |
+| 001 | zero page       |
+| 011 | absolute        |
+| 101 | zero page,X     |
+| 111 | absolute,X      |
+
+|       | BIT |     | JMP |     | JMP() |     | STY |     | LDY |      | CPY |     | CPX |     |
+|-------+-----+-----+-----+-----+-------+-----+-----+-----+-----+------+-----+-----+-----+-----|
+| #     |     |     |     |     |       |     |     |     | A0  | 2/2  | C0  | 2/2 | E0  | 2/2 |
+| zp    | 24  | 2/3 |     |     |       |     |  84 | 2/3 | A4  | 2/3  | C4  | 2/3 | E4  | 2/3 |
+| abs   | 2C  | 3/4 | 4C  | 3/3 | 6C    | 3/5 |  8C | 3/4 | AC  | 3/4  | CC  | 3/4 | EC  | 3/4 |
+| zp,X  |     |     |     |     |       |     |  94 | 2/4 | B4  | 2/4  |     |     |     |     |
+| abs,X |     |     |     |     |       |     |     |     | BC  | 3/4* |     |     |     |     |
--- a/docs/apple2.org
+++ b/docs/apple2.org
@ -0,0 +1,276 @@
+Apple II notes
+
+** Memory Map
+
+|---------+-------------------------------------------|
+| Page    | Use                                       |
+|---------+-------------------------------------------|
+| $00     | Page Zero                                 |
+| $01     | System stack                              |
+| $02     | Input buffer                              |
+| $03     | Monitor vector locations                  |
+| $04-$07 | Text/Lo-Res Primary                       |
+| $08-$0B | Text/Lo-Res Secondary                     |
+| $0C-$1F | Free RAM                                  |
+| $20-$3F | Hires Primary                             |
+| $40-$5F | Hires Secondary                           |
+| $60-$BF | Free RAM                                  |
+| $C0     | I/O and softswitches                      |
+| $C1-$C7 | ROM for seven peripheral cards            |
+| $C8-$CF | switchable ROM for peripheral cards       |
+| $D0-$D7 | empty ROM socket #1 / Programmer's Aid #1 |
+| $D8-$DF | empty ROM socket #2                       |
+| $E0-$F7 | Integer BASIC ROM                         |
+| $F8-$FF | Monitor ROM                               |
+|---------+-------------------------------------------|
+
+* Integer Basic Map
+
+|-------------+---------------------------------|
+| Address     | Use                             |
+|-------------+---------------------------------|
+| $E000-$F424 | BASIC                           |
+| $F425-$F4FB | Floating point math package 1/2 |
+| $F4FC-$F4FF | ??? (4 bytes)                   |
+| $F500-$F63C | Miniassembler                   |
+| $F63D-$F65D | Floating point math package 2/2 |
+| $F65E-$F665 | ??? (8 bytes)                   |
+| $F666-$F668 | Miniassembler entry point       |
+| $F669-$F688 | ??? (32 bytes)                  |
+| $F689-$F7FC | SWEET 16 interpreter            |
+| $F800-$FFFF | Monitor                         |
+|-------------+---------------------------------|
+
+* I/O and softswitches
+
+|---------+--------+------------------------------------------------------------------|
+| Address |    Dec | Use                                                              |
+|---------+--------+------------------------------------------------------------------|
+| $C00X   | -16384 | Read keyboard: High bit = key pressed                            |
+| $C01X   | -16368 | Set to 0 to clear keyboard strobe                                |
+| $C01A   | -16358 | Read: High bit 1 = TEXT switch "ON"                              |
+| $C01B   | -16357 | Read: High bit 1 = MIXED switch "ON"                             |
+| $C01C   | -16356 | Read: High bit 1 = PAGE2 switch "ON"                             |
+| $C01D   | -16355 | Read: High bit 1 = HIRES switch "ON"                             |
+| $C02X   |        | Casette output toggle                                            |
+| $C03X   | -16336 | Speaker toggle                                                   |
+| $C04X   |        | Output strobe to Game I/O connector                              |
+| $C050   | -16304 | Set graphics mode                                                |
+| $C051   | -16303 | Set text mode                                                    |
+| $C052   | -16302 | Set bottom 4 lines graphics                                      |
+| $C053   | -16301 | Set bottom 4 lines text                                          |
+| $C054   | -16300 | Display primary page                                             |
+| $C055   | -16299 | Display secondary page                                           |
+| $C056   | -16298 | Set HIRES graphics mode                                          |
+| $C057   | -16297 | Set color graphics mode                                          |
+| $CØ58   | -l6296 | Clear Game I/O ANØ output                                        |
+| $CØ59   | -l6295 | Set Game I/O ANØ output                                          |
+| $CØ5A   | -l6294 | Clear Game I/O ANl output                                        |
+| $CØ5B   | -l6293 | Set Game I/O ANl output                                          |
+| $CØ5C   | -l6292 | Clear Game I/O AN2 output                                        |
+| $CØ5D   | -l629l | Set Game I/O AN2 output                                          |
+| $CØ5E   | -l629Ø | Clear Game I/O AN3 output                                        |
+| $CØ5F   | -l6289 | Set Game I/O AN3 output                                          |
+| $C060/8 |        | Tape in (bit 7)                                                  |
+| $C061/9 | -16287 | "SW1": PDL(0) switch: High bit 1 = "on"                          |
+| $C062/A | -16286 | "SW2": PDL(1) switch: High bit 1 = "on"                          |
+| $C063/B | -16285 | "SW3": PDL(2) switch: High bit 1 = "on"                          |
+| $C064/C |        | Paddle 0 timer output: state of timer output in high bit         |
+| $C065/D |        | Paddle 1 timer output: state of timer output in high bit         |
+| $C066/E |        | Paddle 2 timer output: state of timer output in high bit         |
+| $C067/F |        | Paddle 3 timer output: state of timer output in high bit         |
+| $C064   |        | Paddle0                                                          |
+| $C07X   |        | Trigger paddle timers during \Zero_2                             |
+| $C08X   |        | Device select 0: Pin 41 on Peripheral Connector low during 0_2   |
+| $C09X   |        | Device select 1: Pin 41 on Peripheral Connector low during 0_2   |
+| $C0AX   |        | Device select 2: Pin 41 on Peripheral Connector low during 0_2   |
+| $C0BX   |        | Device select 3: Pin 41 on Peripheral Connector low during 0_2   |
+| $C0CX   |        | Device select 4: Pin 41 on Peripheral Connector low during 0_2   |
+| $C0DX   |        | Device select 5: Pin 41 on Peripheral Connector low during 0_2   |
+| $C0EX   |        | Device select 6: Pin 41 on Peripheral Connector low during 0_2   |
+| $C0FX   |        | Device select 7: Pin 41 on Peripheral Connector low during 0_2   |
+| $C10X   |        | Device select (8): Pin 41 on Peripheral Connector low during 0_2 |
+| $C11X   |        | Device select (9): Pin 41 on Peripheral Connector low during 0_2 |
+| $C12X   |        | Device select (A): Pin 41 on Peripheral Connector low during 0_2 |
+| $C13X   |        | Device select (B): Pin 41 on Peripheral Connector low during 0_2 |
+| $C14X   |        | Device select (C): Pin 41 on Peripheral Connector low during 0_2 |
+| $C15X   |        | Device select (D): Pin 41 on Peripheral Connector low during 0_2 |
+| $C16X   |        | Device select (E): Pin 41 on Peripheral Connector low during 0_2 |
+| $C17X   |        | Device select (F): Pin 41 on Peripheral Connector low during 0_2 |
+| $C1XX   |        | I/O Select 1: Pin 1 on Peripheral Connector low during 0_2       |
+| $C2XX   |        | I/O Select 2: Pin 1 on Peripheral Connector low during 0_2       |
+| $C3XX   |        | I/O Select 3: Pin 1 on Peripheral Connector low during 0_2       |
+| $C4XX   |        | I/O Select 4: Pin 1 on Peripheral Connector low during 0_2       |
+| $C5XX   |        | I/O Select 5: Pin 1 on Peripheral Connector low during 0_2       |
+| $C6XX   |        | I/O Select 6: Pin 1 on Peripheral Connector low during 0_2       |
+| $C7XX   |        | I/O Select 7: Pin 1 on Peripheral Connector low during 0_2       |
+| $C8XX   |        | I/O Select (8): Pin 1 on Peripheral Connector low during 0_2     |
+| $C9XX   |        | I/O Select (9): Pin 1 on Peripheral Connector low during 0_2     |
+| $CAXX   |        | I/O Select (A): Pin 1 on Peripheral Connector low during 0_2     |
+| $CBXX   |        | I/O Select (B): Pin 1 on Peripheral Connector low during 0_2     |
+| $CCXX   |        | I/O Select (C): Pin 1 on Peripheral Connector low during 0_2     |
+| $CDXX   |        | I/O Select (D): Pin 1 on Peripheral Connector low during 0_2     |
+| $CEXX   |        | I/O Select (E): Pin 1 on Peripheral Connector low during 0_2     |
+| $CFXX   |        | I/O Select (F): Pin 1 on Peripheral Connector low during 0_2     |
+| $CFFF   |        | switchable peripheral ROM - find out more                        |
+|         |        |                                                                  |
+|         |        |                                                                  |
+
+* Page 00
+
+|---------+---------+----------------------------------------------------------------|
+| Address |     Dec | Description                                                    |
+|---------+---------+----------------------------------------------------------------|
+| $00-$1F |         | Register area for "sweet 16"                                   |
+| $18     |         | (DOS) first track of data                                      |
+| $19     |         | (DOS) first sector of data                                     |
+| $1A     |         | (DOS) number of sectors to load                                |
+| $1B     |         | (DOS) HIGH BYTE of buffer (LO is always 0)                     |
+| $1A/B   |         | Shape pointer used by DRAW and XDRAW                           |
+| $1C     |         | Last color used (HCOLOR converted to its color byte)           |
+| $20     |      32 | Left edge of window (0-39)                                     |
+| $21     |      33 | Width of the window (0 to 40 - Left)                           |
+| $22     |      34 | Top edge of window (0-23)                                      |
+| $23     |      35 | Bottom of window (0-23) Bottom > Top                           |
+| $24     |      36 | HTAB: 0-39                                                     |
+| $25     |      37 | VTAB: 0-23                                                     |
+| $2B     |      43 | Boot SLOT * 16                                                 |
+| $2C     |      44 | Lo-res line end-point                                          |
+| $30     |      48 | Lores color * 17                                               |
+| $32     |      50 | Inverse($3F), Flash($7F), Normal($FF), Invisible($80)          |
+| $33     |         | Prompt-char                                                    |
+| $4A/B   |   74-75 | LOMEM address (INT)                                            |
+| $4C/D   |   76-77 | HIMEM address (INT)                                            |
+| $4E-$4F |   78-79 | 16-bit number, randomized with each key entry                  |
+|         |     214 | Poke 255 to auto-run program                                   |
+|         |     212 | Error code flag in decimal                                     |
+|         |     216 | High bit set if error detected: zero to clear                  |
+|         |     222 | Error code                                                     |
+|         | 224-226 | HIRES GR X&Y coordinates                                       |
+| $E4     |     228 | Color being used {0=0:42=1:85=2:127=3:128=4:170=5:213=6:255=7} |
+| $E6     |     230 | HIRES PLOTTING PAGE (32=1/64=2/96=3)                           |
+| $E7     |     231 | SCALE of shape                                                 |
+| $EA     |     234 | COLLISION COUNTER for shapes                                   |
+|         |     241 | 256-SPEED                                                      |
+| $F3     |     243 | FLASH MASK                                                     |
+| $F9     |     249 | ROT                                                            |
+
+* Page 03
+
+|-------------+--------------------------------------|
+| Address     | Description                          |
+|-------------+--------------------------------------|
+| $0320/1     | Low-endian HIRES X coordinate: 0-279 |
+| $0322       | HIRES Y coordinate: 0-159            |
+| $0324/5     | Start address of SHAPE TABLE         |
+| $032C       | COLOR for HIRES                      |
+| $03F8       | CTRL-Y in Monitor with JSR here      |
+| $03FB       | NMI                                  |
+| $03FE-$03FF | IRQ sent to this address             |
+|             |                                      |
+
+* Basic locations
+
+|---------+------+--------------------------------|
+| Address |  Dec | Description                    |
+|---------+------+--------------------------------|
+|         | 2049 | Set to 1 for "list protection" |
+
+* DOS routines
+|---------+-------+-------------|
+| Address |   Dec | Description |
+|---------+-------+-------------|
+|         | 42350 | Catalog     |
+
+* Monitor routines
+|---------+-------+-------+------------------------------------------------------------|
+| Address | Dec   |   Dec | Description                                                |
+|---------+-------+-------+------------------------------------------------------------|
+|         | $F3D4 | -3116 | HGR2                                                       |
+|         | $F3DE | -3106 | HGR                                                        |
+|         | $F3F2 | -3086 | CLEAR HIRES SCREEN                                         |
+|         | $F3F6 | -3082 | CLEAR HIRES SCREEN to HCOLOR                               |
+|         | $FB2F | -1233 | TEXT                                                       |
+|         | $FB40 | -1216 | GR                                                         |
+|         | $FB60 | -1184 | PRINT "APPLE ]["                                           |
+|         | $FBE4 | -1052 | RING BELL                                                  |
+|         | $FBF4 | -1036 | MOVE CURSOR RIGHT                                          |
+|         | $FC10 | -1008 | MOVE CURSOR LEFT                                           |
+|         | $FC1A |  -998 | MOVE CURSOR UP 1 LINE                                      |
+|         | $FC58 |  -936 | HOME                                                       |
+|         | $FD0C |  -756 | WAIT FOR KEYPRESS                                          |
+|         | $FD67 |  -665 | GET a LINE of input with PROMPT, LINE FEED, and wait.      |
+|         | $FD6A |  -662 | GET a LINE of input with PROMPT, NO LINE FEED, and wait.   |
+|         | $FD6F |  -657 | GET a LINE of input with NO PROMPT or LINE FEED, and wait. |
+|         | $FE80 |  -384 | INVERSE                                                    |
+|         | $FE84 |  -380 | NORMAL                                                     |
+|         | $FECA |  -310 | WRITE to tape                                              |
+|         | $FEFD |  -259 | READ from tape                                             |
+|         |       |       |                                                            |
+
+
+
+** References
+- http://apple2history.org/history/ah03/
+- http://www.textfiles.com/apple/peekpk.txt
+- http://www.textfiles.com/apple/peekpoke.app
+- http://www.textfiles.com/apple/peeks.pokes.2
+- http://www.applefritter.com/node/24236
+
+* Language card
+Install in slot 0.
+Replaces RAM chip in E3 with a cable to the card.
+Contains "autostart ROM"
+Adds Esc-ijkm movement
+Ctrl-S stop/start program listings
+
+** Autostart ROM details
+$3F0/1 Break vector: $59,$FA
+$3F2/3 Reset vector: $03, $E0 (for non-disk systems after power-up reset)
+$3F4   Powered up mask: $45
+
+** Memory details
+|-------------+-------------------------|
+| Address     | Description             |
+|-------------+-------------------------|
+| $D000-$DFFF | 4K bank-switched memory |
+| $E000-$F7FF | RAM                     |
+| $F800-$FFFF | Language card ROM       |
+|-------------+-------------------------|
+
+* Control codes
+Bit 2 is ignored.
+Bit 3 chooses which RAM banks is switched in to $D000-$DFFF.
+When RAM is deselected (and write-enabled), it can be written but not read.
+When RAM is deselected, Language Card ROM is mapped to $F800-$FFFF.
+Power-on RESET initializes ROM to read mode, and RAM to write mode,
+and selects the second 4K bank for $D000-$DFFF.
+
+|----------------+---------------+---------------------------------------------------|
+| Second 4K Bank | First 4K Bank | Description                                       |
+|----------------+---------------+---------------------------------------------------|
+| $C080          | $C088         | Select RAM read. Write-protect RAM                |
+| $C081          | $C089         | Deselect RAM reaad (enable ROM)                   |
+|                |               | Two or more successive reads = write-enable RAM   |
+| $C082          | $C08A         | Deselect RAM read (enable ROM). Write-protect RAM |
+| $C083          | $C08B         | Select RAM read.                                  |
+|                |               | Two or more successive reads = write-enable RAM   |
+|----------------+---------------+---------------------------------------------------|
+
+* Revisions
+** II
+- Integer Basic
+- Revision 0
+ - Only 4 hires colors: black, white, violet, green
+- Revision 1
+ - 6 hires colors
+ - Color killer
+ - power-on-reset
+ - auto-start
+ - 24k memory map problem (?!)
+ - keyboard strobe flip flop connected to reset
+** II+
+- Applesoft
+- Usually with 16k "Language Card"
+** IIe
+- 80-column card
--- a/harness/6502_functional_test.a65
+++ b/harness/6502_functional_test.a65
--- a/harness/6502_functional_test.bin
+++ b/harness/6502_functional_test.bin
--- a/harness/harness.go
+++ b/harness/harness.go
@ -0,0 +1,52 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+
+	"github.com/zellyn/go6502/cpu"
+)
+
+type K64 [65536]byte
+
+func (m *K64) Read(address uint16) byte {
+	return m[address]
+}
+
+func (m *K64) Write(address uint16, value byte) {
+	m[address] = value
+}
+
+type CycleCount uint64
+
+func (c *CycleCount) Tick() {
+	*c += 1
+}
+
+func main() {
+	fmt.Println("Hello, world.")
+	bytes, err := ioutil.ReadFile("6502_functional_test.bin")
+	if err != nil {
+		panic("Cannot read file")
+	}
+	var m K64
+	var cc CycleCount
+	OFFSET := 0xa
+	copy(m[OFFSET:len(bytes)+OFFSET], bytes)
+	c := cpu.NewCPU(&m, &cc)
+	c.Reset()
+	c.SetPC(0x1000)
+	for {
+		oldPC := c.PC()
+		err := c.Step()
+		if err != nil {
+			fmt.Println(err)
+			break
+		}
+		if c.PC() == oldPC {
+			fmt.Printf("Stuck at 0x%X: 0x%X\n", oldPC, m[oldPC])
+			break
+		}
+	}
+	fmt.Println("Goodbye, world.")
+}