nesgodisasm

nesgodisasm - a tracing disassembler for NES ROMs

nesgodisasm allows you to disassemble programs for the Nintendo Entertainment System (NES).

Features

• Outputs asm6/ca65/nesasm compatible .asm files that can be used to reproduce the same original NES ROM
• Translates known RAM addresses to aliases
• Traces the program execution flow to differentiate between code and data
• Supports undocumented 6502 CPU opcodes
• Supports branching into opcode parts of an instruction
• Does not output trailing zero bytes of banks by default
• Batch processing mode to disassembling multiple ROMs at once
• Flexible architecture that allows it to create output modules for other assemblers

Support for mappers that use banking is currently experimental.

Installation

The tool uses a modern software stack that does not have any system dependencies beside requiring a somewhat modern operating system to run:

• Linux: 2.6.32+
• Windows: 10+
• macOS: 10.15 Catalina+

There are 2 options to install nesgodisasm:

1. Download and unpack a binary release from Releases

or

1. Compile the latest release from source:

go install github.com/retroenv/nesgodisasm@latest

Compiling the tool from source code needs to have a recent version of Golang installed.

To use the -verify option, the chosen assembler needs to be installed.

Usage

Disassemble a ROM:

nesgodisasm -a ca65 -o example.asm example.nes

The generated assembly file content will look like:

...
Reset:
  sei                            ; $8000 78
  cld                            ; $8001 D8
  lda #$10                       ; $8002 A9 10
  sta PPU_CTRL                   ; $8004 8D 00 20
  ldx #$FF                       ; $8007 A2 FF
  txs                            ; $8009 9A

_label_800a:
  lda PPU_STATUS                 ; $800A AD 02 20
  bpl _label_800a                ; $800D 10 FB

...
.segment "VECTORS"

.addr NMI, Reset, IRQ

Assemble an .asm file back to a ROM:

ca65 example.asm -o example.o
ld65 example.o -t nes -o example.nes 

Options

usage: nesgodisasm [options] <file to disassemble>

  -a string
    	Assembler compatibility of the generated .asm file (asm6/ca65/nesasm)
  -batch string
    	process a batch of given path and file mask and automatically .asm file naming, for example *.nes
  -cdl string
    	name of the .cdl Code/Data log file to load
  -debug
    	enable debugging options for extended logging
  -nohexcomments
    	do not output opcode bytes as hex values in comments
  -nooffsets
    	do not output offsets in comments
  -o string
    	name of the output .asm file, printed on console if no name given
  -q	perform operations quietly
  -verify
    	verify the generated output by assembling with ca65 and check if it matches the input
  -z	output the trailing zero bytes of banks