79 lines
1.8 KiB
Go
79 lines
1.8 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 modeAbsolute = 2
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const modeAbsoluteX = 4
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const modeAbsoluteY = 5
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// https://www.masswerk.at/6502/6502_instruction_set.html
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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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mode int
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reg 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 opLDR(s *state, line []uint8, opcode opcode) {
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var value uint8
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switch opcode.mode {
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case modeImmediate:
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value = line[1]
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case modeZeroPage:
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address := line[1]
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value = s.memory[address]
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case modeZeroPageX:
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address := line[1] + s.registers.getX()
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value = s.memory[address]
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case modeAbsolute:
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address := getWordInLine(line)
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value = s.memory[address]
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case modeAbsoluteX:
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address := getWordInLine(line) + uint16(s.registers.getX())
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value = s.memory[address]
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case modeAbsoluteY:
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address := getWordInLine(line) + uint16(s.registers.getY())
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value = s.memory[address]
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}
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s.registers.setRegister(opcode.reg, value)
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}
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var opcodes = [256]opcode{
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0x00: opcode{"BRK", 1, 7, modeImmediate, regNone, opNOP},
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0xA0: opcode{"LDY", 2, 2, modeImmediate, regY, opLDR},
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0xA5: opcode{"LDA", 2, 3, modeZeroPage, regA, opLDR},
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0xB5: opcode{"LDA", 2, 4, modeZeroPageX, regA, opLDR},
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0xA9: opcode{"LDA", 2, 2, modeImmediate, regA, opLDR},
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0xAD: opcode{"LDA", 3, 4, modeAbsolute, regA, opLDR},
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0xB9: opcode{"LDA", 3, 4, modeAbsoluteY, regA, opLDR}, // Extra cycles
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0xBD: opcode{"LDA", 3, 4, modeAbsoluteX, regA, opLDR}, // Extra cycles
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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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