Overview

This repository contains an annotated disassembly of the Contra (US) NES ROM and the build script(s) to reassemble the assembly into a byte-for-byte match of the game. This repo also contains supplemental documentation, diagrams, scripts, and tools to further understand the game.

A special thanks goes to Trax. This project would not have gotten started without the amazing initial disassembly by him for his Revenge of the Red Falcon project.

|-- docs - supplemental documentation
|   |-- attachments - files used in other documentation
|   |-- debugging - files used to help debugging
|   |-- diagrams - mermaid diagram files documenting program flow
|   |-- lua_scripts - lua scripts for mesen and fceux
|   |-- sprite_library - extracted sprites for ease of viewing
|-- src - the source code for the game
|   |-- assets - the compressed graphics data and encoded audio for the game
|-- assets.txt - a list of assets, their offset in baserom.nes and their length
|-- build.bat - build script for Windows cmd (no PowerShell)
|-- build.ps1 - recommended build script for Windows (PowerShell)
|-- build.sh - bash build script for Linux/mac
|-- contra.cfg - memory mapping of PRG banks used when building
|-- README.md
|-- set_bytes.vbs - script used by build.bat to extract data from baserom.nes

Building

In addition to Contra (US), this repo can also build the Probotector rom. To keep instructions simple, the section below only contains instructions for building Contra (US).

If you are interested in building a Probotector rom file, first read this section to understand what is needed, then follow the instructions in the section titled Probotector.

Prerequisites

• This repo does not include all assets (graphics data and audio data) necessary for assembling the ROM. An existing copy of the game is required. Place a copy of the US NES version of Contra with the name baserom.nes in the project root folder. The file will be used by the build script to extract the necessary assets. The MD5 hash of baserom.nes ROM should be 7BDAD8B4A7A56A634C9649D20BD3011B.
• Building requires the cc65 compiler suite to assemble and link the 6502 assembly files. Please install it and ensure the bin folder is added to your path.

Instructions

There are 3 build scripts in this repository. All of them do the same thing. To build the resulting .nes rom file, simply execute the appropriate build script based on your environment.

• build.ps1 - PowerShell script recommended for building on Windows machines. Some users have reported needing to run the script as admin to access the baserom.nes, although I haven't experienced this. Additionally, on older versions of PowerShell, you may need to adjust the execution policy with the command Set-ExecutionPolicy.
• build.bat - bat script that can be used on windows machines without PowerShell, but requires VBScript support.
• build.sh - bash script to be used in unix environments, or on Windows environments with bash support (Git bash, WSL, etc.)

Video Instructions

If you prefer video build instructions, @jdsilva has published a video on YouTube called Contra ROM Compiling Tutorial

Documentation

Supplemental materials have been added that help explain interesting features of the code. Below are some of the more important documents.

• docs/Aim Documentation.md - documentation on how enemy aiming works
• docs/Bugs.md - bugs identified while disassembling
• docs/Contra Control Flow.md - detailed look at the game routine and level routine flows.
• docs/Enemy Glossary.md - documentation on every enemy type in the game
• docs/Enemy Routines.md - documentation on level enemy configuration and random soldier generation.
• docs/Graphics Documentation.md - documentation on pattern tables, nametables, palettes, palette cycling, super-tiles, background collision, and compression.
• docs/Sound Documentation.md - documentation on the audio engine used as well as information on all sounds from the game.

All sprites were captured and labeled for easy reference in docs/sprite_library/README.md

Getting Started

At first, there is a lot to take in and it may be confusing where to start. The files in the repo under the src/ folder are broken up based on the memory banks in the game. The Contra cartridge is a UxROM cartridge. The game has 8, 16 KiB banks of memory for a total size of 128 KiB. Banks 0 through 6 are swapped in and out of memory at address $8000-$bfff and bank 7 is always in memory at $c000-$ffff. Knowing this helps understand the code since only 2 banks are available at any given time. The start of the game and most of the game engine logic exists in bank7.asm since that's in memory all of the time and where the 3 NES interrupts are.

I think a good way to start reading this code is to look into docs/Contra Control Flow.md. That file shows the responsibilities of the game engine loop routines and in what order those routines are called.

• bank0.asm - used exclusively for enemy routines. Enemy routines are the logic controlling enemy behaviors and settings: AI, movements, attack patterns, health, etc. Almost every enemy is coded in bank 0, but some enemy routines, usually those who appear in more than one level, are in bank7.asm.
• bank1.asm - responsible for audio and sprites. The audio code takes up about 3/4 of the bank. The remaining 1/4 of the bank is for sprite data and code to draw sprites.
• bank2.asm - starts with RLE-encoded level data (graphic super tiles for the level screens). It then contains compressed tile data and alternate tile data and occasional attribute table data. Then, bank 2 contains logic for setting the players' sprite based on player state. Next, bank 2 contains the level headers, which define specifics about each level. Bank 2 then has the data that specifies which enemies are on which screen and their attributes. Bank 2 also contains the soldier enemy generation code.
• bank3.asm - starts with the data that specifies which pattern table tiles comprises super-tiles along with the color palettes. This bank also has the routines to manage the end of levels.
• bank4.asm - mostly contains compressed graphic data. The rest of bank 4 is the code for the ending scene animation and the ending credits, including the ending credits text data.
• bank5.asm - mostly contains compressed graphic data. The rest of bank 5 is the code and lookup tables for automated input for the 3 demo (attract) levels.
• bank6.asm - contains compressed graphics data, data for short text sequences like level names and menu options. Bank 6 also contains the code for the players' weapons and bullets.
• bank7.asm - the core of the game's programming. Reset, NMI, and IRQ vectors are in this bank and is the entry point to the game. Bank 7 contains the code for drawing of nametables and sprites, bank switching, routines for the intro sequence, controller input, score calculation, graphics decompression routines, palette codes, collision detection, pointer table for enemy routines, shared enemy logic, score table, enemy attributes, and bullet angles and speeds, and the NES undocumented footer, among other things.

Annotations

While reviewing the assembly, I used a notation to categorize unexpected, or interesting observations. You can search the code for these annotations.

• !(BUG?) - a possible bug
• !(HUH) - unexpected code or logic used
• !(WHY?) - unsure of why the code is the way it is
• !(UNUSED) - unused code or data that is never executed nor read
• !(OBS) - observation or note

Debugging

One of the biggest advantages of building this rom is that the build scripts also build the debugging symbols. Debugging symbols can be read by emulators and be included in the source view while debugging the game. For example, Mesen will automatically load the debugging symbols as long as the contra.dbg file exists in the same folder as contra.nes. Below is a screenshot of what debugging looks like with symbols.

You can see that the debugging symbols file points to the source code to include comments and labels. If you move or delete the source files, you will lose all the debugging information.

FCEUX NameList

FCEUX doesn't support reading the debugger symbols, but you can load NameList (.nl) files. These files aren't as fully featured as the debugging symbols file, but does at least allow you to label jump/branch locations. It doesn't easily support comments on individual lines.

I've included the .nl files in docs/debugging/fceux_namelists. These files were generated from the -Ln contra.labels.txt command line parameter to ld65. Then the that file was converted to the .nl files with a custom script. I can't promise to keep these up to date as the source labels change, but I will try to.

Project History

I first became interested in this project after watching the Summoning Salt video The History of Contra World Records. In the video starting at 35m 19s, there was a section where a speedrunner named dk28 died in the middle of level 1, and was advanced to level 2 for their next life. The video explains that there still isn't a known explanation as to why this happened. This got me interested in the assembly and thus this project began. Spoilers, I have not found the cause of the level skip bug. It is still unknown whether the bug is from some bug in the code, or whether it was a hardware glitch.

The Contra (US) NES ROM was disassembled using a NES disassembly tool. This provided a starting point for code investigation. The disassembler incorrectly marked many lines of code as data and this had to be manually corrected. At this point, there was an .asm file for each ROM bank in Contra.

Build scripts were then created for both windows console (.bat), and powershell (.ps1). These scripts are equivalent and use ca65 to assemble the .asm files and cl65 to link the .o files into a single ROM file. Creating these build scripts required defining the memory addresses and bank layouts in the contra.cfg file.

At this point, I had a repository that could be built that matched the NES rom byte for byte. Then, I found and was able to incorporate the comments from Trax's IDA Pro disassembly. This was incredibly helpful, but more work had to be done as Trax's disassembly couldn't be used directly for reassembly. For instance, it had a label for every line and all jump and branch statements were hard-coded memory address offsets and not label references.

I then worked on updating all branch and jump offsets to point to label addresses. This helped ensure that the code was more readable. At this point, I also started updating data blocks that included memory offsets to use label offsets. The goal here was twofold:

• make the code more readable and similar to what the original developers would have written
• allow rom hacking without breaking the entire build due to breaking hard-coded memory offsets

When all of this was done, the project could assemble and be byte-for-byte exactly as the Contra (US) ROM. However, this was just the prerequisite to documenting the codebase. Every label had to be given an appropriate name, and each line of assembly had to be documented.

Probotector

This repo can also produce a Probotector rom file. Probotector is the PAL-specific variation of the Contra (US) game. It is mostly identical except different sprites. For a full list of differences, see the document docs/Probotector.md.

Building

First read the section above for building Contra (US) in general. The process is very similar. Place a Probotector rom in the root folder with the name baserom.nes. It should have a MD5 hash of 7127F616C13B58087481DD9E93AEB2FF.

Then, depending on your environment, run one of the following commands.

Notes of caution, especially for people building both Contra (US) and Probotector.

1. Be sure you have the correct baserom.nes for the game you are building. Otherwise, the generated rom will be incorrect.
2. If you have Contra (US) assets in the /src/assets/ folder and are building Probotector, or vice versa, the build scripts will delete the graphic asset data and re-extract it from the baserom.nes. This prevents accidentally using the wrong game's assets in the result rom.

Build Details

The build scripts accomplish the following tasks:

• extracts necessary data from baserom.nes into src/assets
• assemble each bank .asm file into a .o file
• assemble constants.asm and ines_header into .o files
• link all output .o files into a single .nes rom file

The build scripts all utilize cc65 compiler suite to assemble and link the 6502 assembly files. The asset data is pulled from baserom.nes as specified in assets.txt. Each line in assets.txt specifies the file name of the asset, its offset into baserom.nes, and its length.

1. Assembly

Each .asm file is assembled into a .o object file. This section outlines what is happening as part of that process.

Import Export Validation

During assembly, if any symbol is undefined and not explicitly defined in an .import directive, an error will be generated.

2. Linking

Linking is the concept of combining the various .o object files into a single .nes rom file. This is done by the cl65 linker. Its job is to replace labels with actual CPU memory addresses where the labels will exist when loaded. These are then stored in the resulting contra.nes rom file. This is aided by the linker configuration file contra.cfg.

This file specifies the layout of the .net rom file. Without this file, or with a misconfigured file, the linker would not generate an identical .nes file. There are two parts to the contra.cfg file: MEMORY layout and SEGMENTS layout.

Contra.cfg MEMORY Layout

This section tells the linker where in memory each bank will exist. The linker needs this to know to replace a label with the correct address. For example, the opcodes that get generated for jsr zero_out_nametables depend on where in memory the label zero_out_nametables will exist. This is what the configuration file memory section specifies.

Contra.cfg SEGMENTS Layout

This section specifies defines in which order the .o files should appear in the resulting .nes rom file.

References

• Trax's Disassembly of Contra (US)
• Trax's documentation on romhacking.net
• Tomorrow Corporation's Retro Game Internal Series on Contra
• Overview of NES Rendering by Austin Morlan
• Rom Detective article on Contra