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NROM template

This is a minimal working program for the Nintendo Entertainment System using the NROM-128 (nrom-template.nes) and NROM-256 (nrom-template256.nes) boards. NROM is the simplest circuit board used in NES Game Paks, with none of the mapper complexity that could confuse a beginner.

Concepts illustrated:

• system initialization code
• setting up a static background
• structure of a game loop
• controller reading, synchronized to OAM DMA to avoid a conflict with sample playback more reliably than most licensed games
• 8.8 fixed-point arithmetic
• acceleration-based character movement physics
• sprite drawing and animation, with horizontal flipping
• makefile-controlled conversion of sprite sheet images to NES format

Setting up the build environment

You'll need the following software installed to build this demo:

• ca65 and ld65, the assembly language tools that ship with the cc65 C compiler
• Python, a programming language interpreter
• Pillow (Python Imaging Library), a library to read and write bitmap images in Python programs
• GNU Make, a program to calculate which files need to be rebuilt when other files change
• GNU Coreutils, a set of simple command-line utilities for file management and text processing

It also requires general familiarity with the command prompt. Many tools used by programmers have a command-line interface (CLI). Though this may be unfamiliar to long-time users of graphical user interfaces (GUI), a CLI tool is easier to include in an automated process of building an NES game from source code.

If you've never programmed for a MOS 6502 processor, run through "Easy 6502" by skilldrick to familiarize yourself with its instructions. "Before the basics" on NESdev Wiki lists some informative articles about general computer science topics. You don't have to read them all, but the more you understand coming in, the easier time you'll have learning to program the NES.

On Linux

To install Make, Python 3, and Pillow under Debian or Ubuntu:

1. Open a terminal.

2. Type the following, followed by the Enter key:

    sudo apt install build-essential python3-pil
   

3. Type your password to authorize the installation.

To install Make, Python 3, and Pillow under Fedora: (instructions suggested by jroatch; not tested; maybe outdated)

1. Open a terminal and use su to become root.

2. Type the following, followed by the Enter key:

    yum install make automake gcc gcc-c++ python3 python3-pillow
   

Because cc65 is a fairly niche tool, and because part of the package once had non-free restrictions on distribution, your distribution's default repository might not provide cc65. Recent versions of Debian and Ubuntu provide it through sudo apt install cc65. If not, you can install it from source code.

1. Visit cc65 on GitHub.

2. Click Download ZIP

3. Unzip into a new folder.

4. In a terminal issue the following commands (suggested by jroatch):

    cd [path to where you unzipped cc65]
    make
    make install PREFIX="$HOME/.local"
   

   There's no ./configure step, and the PREFIX is case sensitive.

5. Insert the following in your .bash_profile or .bashrc file, to automatically add the local executables to your PATH the next time you log in.

    if [ -d "$HOME/.local/bin" ] ; then
        PATH="$HOME/.local/bin:$PATH"
    fi
   

Now that everything is set up, try to build the ROM. Close your session and log in again. Open a shell, use the cd command to navigate to the folder containing the file makefile if needed, and run make. You will probably have to edit makefile to specify an emulator to use in make run.

On Windows

PATH is an environment variable that tells the command prompt which folders to look in when you type a program name. The convention on UNIX is to install all programs to a handful of folders. On Windows, it's more common to install programs to separate folders, with one folder for each package, and then add each package's folder to Path for Windows to find it.

Because the steps for setting Path differ between Windows versions, you'll want to search the web for windows x.x path variable, replacing x.x with 7, 8.1, 10, etc. In particular, Windows 10 makes adding folders to Path more convenient than previous Windows versions. Architect Ryan's guide has screenshots of the following process:

1. Open the Start Menu.
2. Type environment.
3. Choose "Edit environment variables for your account".
4. Under "User variables", select Path and click "Edit..."
5. In "Edit Environment Variables", add %USERPROFILE%\bin.

User variables take effect after you restart the Command Prompt or Git Bash. System variables take effect after you restart Windows.

If your PC can run Windows Subsystem for Linux (WSL) 2, install WSL with Ubuntu and follow the Linux instructions above. Otherwise, you'll be installing MSYS2, a port of Make, Coreutils, Bash, and other key parts of the GNU operating environment to Windows. Git for Windows includes Bash and Coreutils. If you're installing Git for the first time, keep the default settings except for two.

• If you don't know what a :wq is, change the editor from Vim to something else.
• On "Configuring the terminal emulator to use with Git Bash", select "Use Windows' default console window" instead of MinTTY so that you can see the output of make without needing to prefix most commands with winpty.

To add Make, follow evanwill's instructions to download the latest Make without Guile from ezwinports and merge it into C:\Program Files\Git\mingw64. This method adds to Path a folder called bin directly inside your user profile folder, such as C:\Users\Pino\bin, so you can put things like ca65.exe there.

Another way to install MSYS is through the automated installer provided by devkitPro. This method may be more convenient if you also develop for Game Boy Advance or Nintendo DS.

1. Visit devkitPro Getting Started.
2. Follow the instructions there to download and run the devkitPro Automated Installer.
3. Uncheck devkitARM, devkitPPC, devkitPSP, and libraries for newer platforms (libnds, etc.) unless you plan to start developing for one of those soon. Leave Minimal System (MSYS) checked.

To install Python under Windows:

1. Visit Python home page.
2. Under Downloads, click Windows.
3. Scroll down to Python 3.11.4 (or the latest 3.x release), then under that, click Windows installer (64-bit). (If you use 32-bit Windows, use the 32-bit installer instead. If you use Windows 7 or Windows 8.1, use Python 3.8.16.)
4. In your web browser's downloads folder, run the downloaded installer, whose name should resemble python-3.8.2-amd64.exe.
5. Follow the prompts through the installer wizard.

Python's installer also puts py.exe into Path for you.

To install Pillow under Windows, open a Command Prompt and enter the following command:

py -m pip install Pillow

To install cc65 under Windows:

1. Visit cc65 introduction.
2. Scroll to the bottom.
3. Click "Windows Snapshot" to download a zip file.
4. Open the zip file.
5. Inside the zip file, open the bin folder.
6. Drag ca65.exe, ld65.exe, and od65.exe into a new folder.

To make ca65 and ld65 available to Make, you'll need to add the folder containing ca65.exe and ld65.exe to Path or put them in a folder already on Path. Though od65.exe is not required to build nrom-template, some programs that use a mapper need od65 to determine which bank everything goes in.

Now that everything is set up, try to build the ROM. Open a command prompt, use the cd command to navigate to the folder containing the file makefile if needed, and run the command make.

• If make nrom-template.nes prints "up to date" and make run prints "command not found", you're almost there! The ROM was built successfully, and the makefile is trying to run the ROM an emulator. Open makefile in a text editor and change EMU to the path of your preferred emulator.
• If make prints nothing, not even "command not found" or "not an operable program" or "No targets specified and no makefile found", you may have accidentally downloaded and installed Make with Guile. Download the version without Guile and try again.

To get make dist to build a zipfile, you'll need to install the Zip and UnZip command-line tools published by Info-ZIP. Be careful, as unz600xn.exe is a self-extracting archive that extracts multiple files to the current directory, like a tarbomb, so run it in a new folder and then copy zip.exe and unzip.exe to a folder on Path.

Organization of the program

Include files

• nes.inc: Register definitions and useful macros
• global.inc: Global variable and function declarations

Source code files

Each source code file in the src folder is made up of subroutines that start with .proc and end with .endproc. See the ca65 Users Guide for what these mean.

• nrom.s: iNES header for NROM
• init.s: PPU and CPU I/O initialization code
• main.s: Main program
• bg.s: Background graphics setup
• player.s: Player sprite graphics setup and movement
• pads.s: Read the controllers in a DPCM-safe manner
• ppuclear.s: Useful subroutines for interacting with the NES PPU

Tile sheets

Pictures on the NES are made of tiles, each 8 by 8 pixels in size. To edit the tile sheet images in the tilesets folder, use a paint program that can save images using indexed color, such as LibreSprite or GIMP. Paint programs that save only in RGB format will not work.

• bggfx.png: Background tiles
• spritegfx.png: Moving object tiles

Linker configuration files

A linker configuration file (or "config") specifies to ld65 how a system's memory is organized. In the case of the NES, this includes RAM in the Control Deck, ROM in the Game Pak, and (occasionally) extra RAM in the Game Pak. Because each mapper organizes memory differently, each combination of mapper and memory size needs a specific config. The -C option to ld65 controls which config to use when producing a ROM. This template provides three:

• nrom128.cfg: NROM-128 board, 16 KiB PRG ROM, 8 KiB CHR ROM. A DMC segment is present for sampled sounds, such as the barks and quacks in Duck Hunt.
• nrom256-without-dmc.cfg: NROM-256 board, 32 KiB PRG ROM, 8 KiB CHR ROM. Most beginner projects can start with this config, which provides contiguous PRG ROM and no DMC segment.
• nrom256-with-dmc.cfg: NROM-256 board, 32 KiB PRG ROM, 8 KiB CHR ROM. This config treats PRG ROM as two 16 KiB areas, a lower half and an upper half. This helps ensure that the DMC segment for samples is in the upper half, as the NES sound hardware requires. Segments CODE and RODATA are placed in the upper half. If you see an error message overflows memory area 'ROMC0', you've run out of room in the upper half and can move things to segments CODE80 and RODATA80 in the lower half.

The tools

In my projects, the tools folder contains command-line programs written in Python to convert graphics into a form usable by the NES. The makefile contains instructions to run the converter program again whenever the original asset data changes.

• pilbmp2nes.py converts indexed bitmap images in PNG or BMP format into tile data usable by several classic video game consoles. It has several options to control the data format; use pilbmp2nes.py --help from the command prompt to see them all.

Greets

• NESdev Wiki and forum contributors
• FCEUX team
• Joe Parsell (Memblers) for getting me into NESdev in the first place
• Jeremy Chadwick (koitsu) for code organization tips
• Matt Hughson and Fiskbit for config feedback
• Greg Caldwell of Retrotainment Games for testing the Windows instructions
• velpachallenger for feedback on README.md

Legal

The demo is distributed under the following license, based on the GNU All-Permissive License:

Copyright 2011-2024 Damian Yerrick

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved in all source code copies. This file is offered as-is, without any warranty.