mirror of
https://github.com/bradford-hamilton/apple-1.git
synced 2024-10-31 22:05:59 +00:00
199 lines
4.8 KiB
Go
199 lines
4.8 KiB
Go
package vm
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import (
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"errors"
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"fmt"
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)
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// Appleone represents the virtual Apple 1 computer
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type Appleone struct {
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cpu *Mos6502 // virtual mos6502 cpu
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mem block // available memory (64kiB)
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}
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// New returns a pointer to an initialized Appleone with a brand spankin new CPU
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func New() *Appleone {
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return &Appleone{
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cpu: newCPU(),
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mem: newBlock(),
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}
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}
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// load puts the provided data into the apple1's memory block starting at the provided address
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func (a *Appleone) load(addr uint16, data []byte) {
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a.mem.load(addr, data)
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a.cpu.pc = addr
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}
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func (a *Appleone) step() {
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operation, err := operationByCode(a.mem[a.cpu.pc])
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if err != nil {
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fmt.Println("TODO")
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}
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a.cpu.pc += uint16(operation.size)
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if err := operation.exec(a, operation); err != nil {
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fmt.Println("TODO")
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}
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}
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func (a *Appleone) getAddr(o operation) (uint16, error) {
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switch o.addrMode {
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// TODO: will these ever apply here?
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// case accumulator:
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//
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// case implied:
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//
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case absolute:
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return a.nextDWord(), nil
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case absoluteXIndexed:
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return a.nextDWord() + uint16(a.cpu.x), nil
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case absoluteYIndexed:
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return a.nextDWord() + uint16(a.cpu.y), nil
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case immediate:
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return a.cpu.pc - 1, nil
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case indirect:
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return uint16(a.nextWord()), nil
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case indirectXIndexed:
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addr := (uint16(a.nextWord()) + uint16(a.cpu.x)) & 0xFF
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return a.littleEndianToUint16(a.mem[addr+1], a.mem[addr]), nil
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case indirectYIndexed:
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addr := uint16(a.nextWord())
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val := a.littleEndianToUint16(a.mem[addr+1], a.mem[addr])
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return val + uint16(a.cpu.y), nil
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case relative:
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return a.cpu.pc - 1, nil
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case zeroPage:
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return uint16(a.nextWord()) & 0xFF, nil
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case zeroPageXIndexed:
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return (uint16(a.nextWord()) + uint16(a.cpu.x)) & 0xFF, nil
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case zeroPageYIndexed:
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return (uint16(a.nextWord()) + uint16(a.cpu.y)) & 0xFF, nil
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default:
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return 0, errors.New("unkown addressing mode")
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}
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}
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func (a *Appleone) getOperand(o operation) (byte, error) {
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if o.addrMode == accumulator {
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return a.cpu.a, nil
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}
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b, err := a.getAddr(o)
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if err != nil {
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return 0, err
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}
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return a.mem[b], nil
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}
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func (a *Appleone) littleEndianToUint16(big, little byte) uint16 {
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return uint16(a.mem[big])<<8 | uint16(a.mem[little])
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}
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// pushWordToStack pushes the given word (byte) into memory and sets the new stack pointer
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func (a *Appleone) pushWordToStack(b byte) {
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a.mem[StackBottom+uint16(a.cpu.sp)] = b
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a.cpu.sp = byte((uint16(a.cpu.sp) - 1) & 0xFF)
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}
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// pushWordToStack splits the high and low byte of the data passed in, and pushes them to the stack
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func (a *Appleone) pushDWordToStack(data uint16) {
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h := byte((data >> 8) & 0xFF)
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l := byte(data & 0xFF)
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a.pushWordToStack(h)
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a.pushWordToStack(l)
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}
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// popStackWord sets the new stack pointer and returns the appropriate byte in memory
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func (a *Appleone) popStackWord() byte {
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a.cpu.sp = byte((uint16(a.cpu.sp) + 1) & 0xFF)
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return a.mem[StackBottom+uint16(a.cpu.sp)]
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}
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// popStackDWord pops two stack words (a double word - uint16) off the stack
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func (a *Appleone) popStackDWord() uint16 {
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l := a.popStackWord()
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h := a.popStackWord()
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return (uint16(h) << 8) | uint16(l)
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}
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// nextWord returns the next byte in memory
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func (a *Appleone) nextWord() byte {
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return a.mem[a.cpu.pc-1]
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}
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// nextDWord returns the next two bytes (double word)
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func (a *Appleone) nextDWord() uint16 {
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return a.littleEndianToUint16(a.mem[a.cpu.pc-1], a.mem[a.cpu.pc-2])
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}
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// maybeSetFlagZero takes a single word (byte), clears flagZero, and sets flagZero if word is 0
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func (a *Appleone) maybeSetFlagZero(word byte) {
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a.clearFlag(flagZero)
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if word == 0 {
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a.setFlag(flagZero)
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}
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}
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func (a *Appleone) getFlag(flag byte) byte {
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return a.cpu.ps & flag
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}
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func (a *Appleone) setFlag(flag byte) {
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a.cpu.ps |= flag
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}
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func (a *Appleone) clearFlag(flag byte) {
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a.cpu.ps &^= flag
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}
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func (a *Appleone) maybeSetFlagOverflow(word byte) {
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a.clearFlag(flagNegative)
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if word > 127 {
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a.setFlag(flagNegative)
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}
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}
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// Branch offsets are signed 8-bit values, -128 ... +127, negative offsets in two's
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// complement. Page transitions may occur and add an extra cycle to the exucution
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func (a *Appleone) branch(o operation) error {
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offset, err := a.getOperand(o)
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if err != nil {
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return err
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}
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if offset > 127 {
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a.cpu.pc -= 256 - uint16(offset)
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} else {
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a.cpu.pc += uint16(offset)
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}
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return nil
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}
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// compare clears zero, carry, and negative flags, compares the two bytes, and sets the
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// appropriate flags based on the comparison between the bytes.
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func (a *Appleone) compare(b1, b2 byte) {
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a.clearFlag(flagZero)
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a.clearFlag(flagCarry)
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a.clearFlag(flagNegative)
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if b1 == b2 {
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a.setFlag(flagZero)
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a.setFlag(flagCarry)
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}
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if b1 > b2 {
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a.setFlag(flagCarry)
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}
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b := byte(uint16(b1) - uint16(b2))
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a.maybeSetFlagOverflow(b)
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}
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func (a *Appleone) setMem(o operation, operand byte) error {
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addr, err := a.getAddr(o)
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if err != nil {
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return err
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}
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a.mem[addr] = operand
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return nil
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}
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