183 lines
5.7 KiB
Go
183 lines
5.7 KiB
Go
package main
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type state struct {
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registers registers
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memory memory
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}
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func step(s *state) {
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}
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const modeNone = -1
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const modeImmediate = 0
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const modeZeroPage = 1
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const modeZeroPageX = 3
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const modeZeroPageY = 6
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const modeAbsolute = 2
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const modeAbsoluteX = 4
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const modeAbsoluteY = 5
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const modeIndexedIndirectX = 7
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const modeIndirectIndexedY = 8
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const modeAccumulator = 9
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// https://www.masswerk.at/6502/6502_instruction_set.html
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// http://www.emulator101.com/reference/6502-reference.html
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// https://www.csh.rit.edu/~moffitt/docs/6502.html#FLAGS
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func getWordInLine(line []uint8) uint16 {
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return uint16(line[1]) + 0x100*uint16(line[2])
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}
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type opcode struct {
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name string
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bytes int
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cycles int
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action opFunc
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}
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type opFunc func(s *state, line []uint8, opcode opcode)
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func opNOP(s *state, line []uint8, opcode opcode) {}
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func buildOpTransfer(regSrc int, regDst int) opFunc {
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return func(s *state, line []uint8, opcode opcode) {
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value := s.registers.getRegister(regSrc)
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s.registers.setRegister(regDst, value)
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if regDst != regSP {
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s.registers.updateFlagZN(value)
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}
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}
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}
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func buildOpIncDecRegister(reg int, inc bool) opFunc {
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return func(s *state, line []uint8, opcode opcode) {
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value := s.registers.getRegister(reg)
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if inc {
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value++
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} else {
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value--
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}
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s.registers.setRegister(reg, value)
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s.registers.updateFlagZN(value)
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}
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}
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func resolveWithAddressMode(s *state, line []uint8, addressMode int) (value uint8, hasAddress bool, address uint16) {
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hasAddress = true
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switch addressMode {
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case modeAccumulator:
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value = s.registers.getA()
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hasAddress = false
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case modeImmediate:
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value = line[1]
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hasAddress = false
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case modeZeroPage:
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address = uint16(line[1])
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case modeZeroPageX:
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address = uint16(line[1] + s.registers.getX())
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case modeZeroPageY:
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address = uint16(line[1] + s.registers.getY())
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case modeAbsolute:
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address = getWordInLine(line)
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case modeAbsoluteX:
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address = getWordInLine(line) + uint16(s.registers.getX())
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case modeAbsoluteY:
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address = getWordInLine(line) + uint16(s.registers.getY())
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case modeIndexedIndirectX:
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addressAddress := uint8(line[1] + s.registers.getX())
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address = s.memory.getZeroPageWord(addressAddress)
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case modeIndirectIndexedY:
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address = s.memory.getZeroPageWord(line[1]) +
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uint16(s.registers.getY())
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}
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if hasAddress {
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value = s.memory[address]
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}
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return
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}
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func buildRotate(addressMode int, isLeft bool) opFunc {
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return func(s *state, line []uint8, opcode opcode) {
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value, hasAddress, address := resolveWithAddressMode(s, line, addressMode)
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oldCarry := s.registers.getFlagBit(flagC)
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var carry bool
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if isLeft {
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carry = (value & 0x80) != 0
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value <<= 1
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value += oldCarry
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} else {
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carry = (value & 0x01) != 0
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value >>= 1
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value += oldCarry << 7
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}
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s.registers.updateFlag(flagC, carry)
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s.registers.updateFlagZN(value)
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if hasAddress {
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s.memory[address] = value
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} else {
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s.registers.setA(value)
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}
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}
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}
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func buildOpLoad(addressMode int, regDst int) opFunc {
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return func(s *state, line []uint8, opcode opcode) {
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value, _, _ := resolveWithAddressMode(s, line, addressMode)
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s.registers.setRegister(regDst, value)
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s.registers.updateFlagZN(value)
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}
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}
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var opcodes = [256]opcode{
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0x26: opcode{"ROL", 2, 5, buildRotate(modeZeroPage, true)},
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0x2A: opcode{"ROL", 1, 2, buildRotate(modeAccumulator, true)},
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0x2E: opcode{"ROL", 3, 6, buildRotate(modeAbsolute, true)},
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0x36: opcode{"ROL", 2, 6, buildRotate(modeZeroPageX, true)},
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0x3E: opcode{"ROL", 3, 7, buildRotate(modeAbsoluteX, true)},
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0x66: opcode{"ROR", 2, 5, buildRotate(modeZeroPage, false)},
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0x6A: opcode{"ROR", 1, 2, buildRotate(modeAccumulator, false)},
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0x6E: opcode{"ROR", 3, 6, buildRotate(modeAbsolute, false)},
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0x76: opcode{"ROR", 2, 6, buildRotate(modeZeroPageX, false)},
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0x7E: opcode{"ROR", 3, 7, buildRotate(modeAbsoluteX, false)},
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0x88: opcode{"DEY", 1, 2, buildOpIncDecRegister(regY, false)},
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0x8A: opcode{"TXA", 1, 2, buildOpTransfer(regX, regA)},
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0x98: opcode{"TYA", 1, 2, buildOpTransfer(regY, regA)},
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0x9A: opcode{"TXS", 1, 2, buildOpTransfer(regX, regSP)},
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0xA0: opcode{"LDY", 2, 2, buildOpLoad(modeImmediate, regY)},
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0xA1: opcode{"LDX", 2, 6, buildOpLoad(modeIndexedIndirectX, regA)},
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0xA2: opcode{"LDX", 2, 2, buildOpLoad(modeImmediate, regX)},
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0xA4: opcode{"LDY", 2, 3, buildOpLoad(modeZeroPage, regY)},
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0xA5: opcode{"LDA", 2, 3, buildOpLoad(modeZeroPage, regA)},
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0xA6: opcode{"LDX", 2, 3, buildOpLoad(modeZeroPage, regX)},
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0xA8: opcode{"TAY", 1, 2, buildOpTransfer(regA, regY)},
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0xA9: opcode{"LDA", 2, 2, buildOpLoad(modeImmediate, regA)},
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0xAA: opcode{"TAX", 1, 2, buildOpTransfer(regA, regX)},
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0xAC: opcode{"LDY", 3, 4, buildOpLoad(modeAbsolute, regY)},
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0xAD: opcode{"LDA", 3, 4, buildOpLoad(modeAbsolute, regA)},
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0xAE: opcode{"LDX", 3, 4, buildOpLoad(modeAbsolute, regX)},
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0xB1: opcode{"LDX", 2, 5, buildOpLoad(modeIndirectIndexedY, regA)}, // Extra cycles
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0xB4: opcode{"LDY", 2, 4, buildOpLoad(modeZeroPageX, regY)},
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0xB5: opcode{"LDA", 2, 4, buildOpLoad(modeZeroPageX, regA)},
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0xB6: opcode{"LDX", 2, 4, buildOpLoad(modeZeroPageY, regX)},
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0xB9: opcode{"LDA", 3, 4, buildOpLoad(modeAbsoluteY, regA)}, // Extra cycles
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0xBA: opcode{"TSX", 1, 2, buildOpTransfer(regSP, regX)},
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0xBC: opcode{"LDY", 3, 4, buildOpLoad(modeAbsoluteX, regY)}, // Extra cycles
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0xBD: opcode{"LDA", 3, 4, buildOpLoad(modeAbsoluteX, regA)}, // Extra cycles
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0xBE: opcode{"LDX", 3, 4, buildOpLoad(modeAbsoluteY, regX)}, // Extra cycles
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0xC8: opcode{"INY", 1, 2, buildOpIncDecRegister(regY, true)},
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0xCA: opcode{"DEX", 1, 2, buildOpIncDecRegister(regX, false)},
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0xE8: opcode{"INX", 1, 2, buildOpIncDecRegister(regX, true)},
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0xEA: opcode{"NOP", 1, 2, opNOP},
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}
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func executeLine(s *state, line []uint8) {
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opcode := opcodes[line[0]]
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opcode.action(s, line, opcode)
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}
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