mirror of
https://github.com/zellyn/go6502.git
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76 lines
2.2 KiB
Go
76 lines
2.2 KiB
Go
/*
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Package asm provides routines for decomiling 6502 assembly language.
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*/
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package asm
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import (
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"fmt"
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"github.com/zellyn/go6502/cpu"
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)
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// bytesString takes three bytes and a length, returning the formatted
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// hex bytes for an instrction of the given length.
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func bytesString(byte0, byte1, byte2 byte, length int) string {
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switch length {
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case 1:
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return fmt.Sprintf("%02X ", byte0)
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case 2:
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return fmt.Sprintf("%02X %02X ", byte0, byte1)
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case 3:
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return fmt.Sprintf("%02X %02X %02X", byte0, byte1, byte2)
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}
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panic("Length must be 1, 2, or 3")
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}
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// addrString returns the address part of a 6502 assembly language
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// instruction.
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func addrString(pc uint16, byte1, byte2 byte, length int, mode int) string {
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addr16 := uint16(byte1) + uint16(byte2)<<8
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addrRel := uint16(int32(pc+2) + int32(int8(byte1)))
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switch mode {
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case cpu.MODE_IMPLIED:
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return " "
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case cpu.MODE_ABSOLUTE:
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return fmt.Sprintf("$%04X ", addr16)
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case cpu.MODE_INDIRECT:
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return fmt.Sprintf("($%04X)", addr16)
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case cpu.MODE_RELATIVE:
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return fmt.Sprintf("$%04X ", addrRel)
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case cpu.MODE_IMMEDIATE:
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return fmt.Sprintf("#$%02X ", byte1)
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case cpu.MODE_ABS_X:
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return fmt.Sprintf("$%04X,X", addr16)
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case cpu.MODE_ABS_Y:
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return fmt.Sprintf("$%04X,Y", addr16)
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case cpu.MODE_ZP:
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return fmt.Sprintf("$%02X ", byte1)
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case cpu.MODE_ZP_X:
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return fmt.Sprintf("$%02X,X ", byte1)
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case cpu.MODE_ZP_Y:
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return fmt.Sprintf("$%02X,Y ", byte1)
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case cpu.MODE_INDIRECT_Y:
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return fmt.Sprintf("($%02X,X) ", byte1)
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case cpu.MODE_INDIRECT_X:
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return fmt.Sprintf("($%02X),Y ", byte1)
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case cpu.MODE_A:
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return " "
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}
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panic(fmt.Sprintf("Unknown op mode: %d", mode))
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}
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// Disasm disassembles a single (up to three byte) 6502
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// instruction. It returns the formatted bytes, the formatted
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// instruction and address, and the length. If it cannot find the
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// instruction, it returns a 1-byte "???" instruction.
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func Disasm(pc uint16, byte0, byte1, byte2 byte) (string, string, int) {
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op, ok := cpu.Opcodes[byte0]
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if !ok {
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op = cpu.NoOp
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}
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length := cpu.ModeLengths[op.Mode]
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bytes := bytesString(byte0, byte1, byte2, length)
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addr := addrString(pc, byte1, byte2, length, op.Mode)
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return bytes, op.Name + " " + addr, length
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}
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