Fully documented source code for Elite on the NES

This repository contains source code for Elite on the Nintendo Entertainment System (NES), with every single line documented and (for the most part) explained. It has been reconstructed by hand from a disassembly of the original game binaries.

It is a companion to the bbcelite.com website.

See the introduction for more information, or jump straight into the documented source code.

Introduction
Acknowledgements
- A note on licences, copyright etc.
Browsing the source in an IDE
Folder structure
Building Elite from the source
Building different variants of NES Elite

Introduction

This repository contains source code for Elite on the NES, with every single line documented and (for the most part) explained.

You can build the fully functioning game from this source. Two variants are currently supported: the NTSC variant from Ian Bell's personal website, and the Imagineer PAL variant.

It is a companion to the bbcelite.com website, which contains all the code from this repository, but laid out in a much more human-friendly fashion. The links at the top of this page will take you to repositories for the other versions of Elite that are covered by this project.

If you want to browse the source and read about how Elite works under the hood, you will probably find the website is a better place to start than this repository.
If you would rather explore the source code in your favourite IDE, then the annotated source is what you're looking for. It contains the exact same content as the website, so you won't be missing out (the website is generated from the source files, so they are guaranteed to be identical). You might also like to read the section on Browsing the source in an IDE for some tips.
If you want to build Elite from the source on a modern computer, to produce a working ROM image that can be loaded into a real NES or an emulator, then you want the section on Building Elite from the source.

My hope is that this repository and the accompanying website will be useful for those who want to learn more about Elite and what makes it tick. It is provided on an educational and non-profit basis, with the aim of helping people appreciate one of the most iconic games of the 8-bit era.

Acknowledgements

The code on this site has been reconstructed from a disassembly of the version released on Ian Bell's personal website.

Huge thanks are due to the original authors for not only creating such an important piece of my childhood, but also for releasing the source code for us to play with; to Paul Brink for his annotated disassembly; and to Kieran Connell for his BeebAsm version, which I forked as the original basis for this project. You can find more information about this project in the accompanying website's project page.

The following archive from Ian Bell's personal website forms the basis for this project:

NES Elite, NTSC version

A note on licences, copyright etc.

This repository is not provided with a licence, and there is intentionally no LICENSE file provided.

According to GitHub's licensing documentation, this means that "the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work".

The reason for this is that my commentary is intertwined with the original Elite source code, and the original source code is copyright. The whole site is therefore covered by default copyright law, to ensure that this copyright is respected.

Under GitHub's rules, you have the right to read and fork this repository... but that's it. No other use is permitted, I'm afraid.

My hope is that the educational and non-profit intentions of this repository will enable it to stay hosted and available, but the original copyright holders do have the right to ask for it to be taken down, in which case I will comply without hesitation. I do hope, though, that along with the various other disassemblies and commentaries of this source, it will remain viable.

Browsing the source in an IDE

If you want to browse the source in an IDE, you might find the following useful.

The main game's source code is split across eight different ROM banks, which you can find in the main-sources folder. This is the motherlode and probably contains all the stuff you're interested in.
It's probably worth skimming through the notes on terminology and notations on the accompanying website, as this explains a number of terms used in the commentary, without which it might be a bit tricky to follow at times (in particular, you should understand the terminology I use for multi-byte numbers).
The accompanying website contains a number of "deep dive" articles, each of which goes into an aspect of the game in detail. Routines that are explained further in these articles are tagged with the label Deep dive: and the relevant article name.
There are loads of routines and variables in Elite - literally hundreds. You can find them in the source files by searching for the following: Type: Subroutine, Type: Variable, Type: Workspace and Type: Macro.
If you know the name of a routine, you can find it by searching for Name: <name>, as in Name: SCAN (for the 3D scanner routine) or Name: LL9 (for the ship-drawing routine).
The entry point for the main game code is the BEGIN routine in bank 7, which you can find by searching for Name: BEGIN. If you want to follow the program flow all the way from the title screen around the main game loop, then you can find a number of deep dives on program flow on the accompanying website.
The source code is designed to be read at an 80-column width and with a monospaced font, just like in the good old days.

I hope you enjoy exploring the inner workings of NES Elite as much as I have.

Folder structure

There are five main folders in this repository, which reflect the order of the build process.

1-source-files contains all the different source files, such as the main assembler source files, image binaries, fonts and so on.
2-build-files contains build-related scripts, such as the crc32 verification scripts.
3-assembled-output contains the output from the assembly process, when the source files are assembled and the results processed by the build files.
4-reference-binaries contains the correct binaries for each variant, so we can verify that our assembled output matches the reference.
5-compiled-game-discs contains the final output of the build process: an iNES ROM image that contains the compiled game and which can be run on real hardware or in an emulator.

Building Elite from the source

Requirements

You will need the following to build Elite from the source:

BeebAsm, which can be downloaded from the BeebAsm repository. Mac and Linux users will have to build their own executable with make code, while Windows users can just download the beebasm.exe file.
Python. Both versions 2.7 and 3.x should work.
Mac and Linux users may need to install make if it isn't already present (for Windows users, make.exe is included in this repository).

You may be wondering why we're using BeebAsm - a BBC Micro assembler - to build the NES version of Elite. This is because NES Elite is a conversion of BBC Master Elite, which itself is a direct descendant of the original 1984 release for the BBC Micro and Acorn Electron (and the same is true of the Commodore 64 and Apple II versions of Elite - they are all cut from the same cloth). All of the older 6502 versions of Elite were built and assembled on a BBC Micro, including the Commodore and Apple versions, so BeebAsm is a good modern assembler to use for the NES version as well. The 1991 NES version was actually developed on the PC-based PDS development system, so the use of BeebAsm here isn't historically accurate, it's just a good fit for the source material.

For details of how the build process works, see the build documentation on bbcelite.com.

Let's look at how to build Elite from the source.

Build targets

There are two main build targets available. They are:

build - A version with a maxed-out commander
encrypt - A version that exactly matches the released version of the game

Unlike the Acornsoft versions of Elite on which it is based, the NES version is not encrypted, so there is no difference in encryption between the two targets. I have used the same target names for consistency, but the only difference is in the commander file.

Builds are supported for both Windows and Mac/Linux systems. In all cases the build process is defined in the Makefile provided.

Windows

For Windows users, there is a batch file called make.bat to which you can pass one of the build targets above. Before this will work, you should edit the batch file and change the values of the BEEBASM and PYTHON variables to point to the locations of your beebasm.exe and python.exe executables. You also need to change directory to the repository folder (i.e. the same folder as make.bat).

All being well, doing one of the following:

make.bat build

make.bat encrypt

will produce a file called elite-ntsc.NES in the 5-compiled-game-discs folder that contains the NTSC variant, which you can then load into an emulator, or into a real NES using a flash cart.

Mac and Linux

The build process uses a standard GNU Makefile, so you just need to install make if your system doesn't already have it. If BeebAsm or Python are not on your path, then you can either fix this, or you can edit the Makefile and change the BEEBASM and PYTHON variables in the first two lines to point to their locations. You also need to change directory to the repository folder (i.e. the same folder as Makefile).

All being well, doing one of the following:

make build

make encrypt

will produce a file called elite-ntsc.NES in the 5-compiled-game-discs folder that contains the NTSC variant, which you can then load into an emulator, or into a real NES using a flash cart.

Verifying the output

The build process also supports a verification target that prints out checksums of all the generated files, along with the checksums of the files from the original sources.

You can run this verification step on its own, or you can run it once a build has finished. To run it on its own, use the following command on Windows:

make.bat verify

or on Mac/Linux:

make verify

To run a build and then verify the results, you can add two targets, like this on Windows:

make.bat encrypt verify

or this on Mac/Linux:

make encrypt verify

The Python script crc32.py in the 2-build-files folder does the actual verification, and shows the checksums and file sizes of both sets of files, alongside each other, and with a Match column that flags any discrepancies. If you are building an unencrypted set of files then there will be lots of differences, while the encrypted files should mostly match (see the Differences section below for more on this).

The binaries in the 4-reference-binaries folder are those extracted from the released version of the game, while those in the 3-assembled-output folder are produced by the build process. For example, if you don't make any changes to the code and build the project with make encrypt verify, then this is the output of the verification process:

Results for variant: pal
[--originals--]  [---output----]
Checksum    Size  Checksum    Size  Match  Filename
-----------------------------------------------------
6a32bd20   16384  6a32bd20   16384   Yes   bank0.bin
1840f774   16384  1840f774   16384   Yes   bank1.bin
e08fa78a   16384  e08fa78a   16384   Yes   bank2.bin
e07c0f21   16384  e07c0f21   16384   Yes   bank3.bin
731cd900   16384  731cd900   16384   Yes   bank4.bin
fee7480c   16384  fee7480c   16384   Yes   bank5.bin
500f28cd   16384  500f28cd   16384   Yes   bank6.bin
8e1162f8   16384  8e1162f8   16384   Yes   bank7.bin
4cf12d39  131088  4cf12d39  131088   Yes   elite.bin
eb5e8763      16  eb5e8763      16   Yes   header.bin

All the compiled binaries match the originals, so we know we are producing the same final game as the PAL variant.

Log files

During compilation, details of every step are output in nine files called compile.txt (for the header) or compile0.txt through compile7.txt (for banks 0 to 7) in the 3-assembled-output folder. If you have problems, these might come in handy, and they're a great reference if you need to know the addresses of labels and variables for debugging (or just snooping around).

Building different variants of NES Elite

This repository contains the source code for two different variants of NES Elite:

The NTSC variant from Ian Bell's personal website
The Imagineer PAL variant, which is the only official release of NES Elite

By default the build process builds the NTSC variant, but you can build a specified variant using the variant= build parameter.

Building the NTSC variant

You can add variant=ntsc to produce the elite-ntsc.NES file that contains the NTSC variant, though that's the default value so it isn't necessary. In other words, you can build it like this:

make.bat encrypt verify variant=ntsc

or this on a Mac or Linux:

make encrypt verify variant=ntsc

This will produce a file called elite-ntsc.NES in the 5-compiled-game-discs folder that contains the NTSC variant.

The verification checksums for this version are as follows:

Results for variant: ntsc
[--originals--]  [---output----]
Checksum    Size  Checksum    Size  Match  Filename
-----------------------------------------------------
0560a52b   16384  0560a52b   16384   Yes   bank0.bin
c1239b33   16384  c1239b33   16384   Yes   bank1.bin
5e6c3bfb   16384  5e6c3bfb   16384   Yes   bank2.bin
54df916d   16384  54df916d   16384   Yes   bank3.bin
5953c5d4   16384  5953c5d4   16384   Yes   bank4.bin
0dd49e0c   16384  0dd49e0c   16384   Yes   bank5.bin
39255d4f   16384  39255d4f   16384   Yes   bank6.bin
26f0c7de   16384  26f0c7de   16384   Yes   bank7.bin
54386491  131088  54386491  131088   Yes   elite.bin
eb5e8763      16  eb5e8763      16   Yes   header.bin

Building the PAL variant

You can build the PAL variant by appending variant=ntsc to the make command, like this on Windows:

make.bat encrypt verify variant=pal

or this on a Mac or Linux:

make encrypt verify variant=pal

This will produce a file called elite-pal.NES in the 5-compiled-game-discs folder that contains the PAL variant.

The verification checksums for this version are as follows:

Results for variant: pal
[--originals--]  [---output----]
Checksum    Size  Checksum    Size  Match  Filename
-----------------------------------------------------
6a32bd20   16384  6a32bd20   16384   Yes   bank0.bin
1840f774   16384  1840f774   16384   Yes   bank1.bin
e08fa78a   16384  e08fa78a   16384   Yes   bank2.bin
e07c0f21   16384  e07c0f21   16384   Yes   bank3.bin
731cd900   16384  731cd900   16384   Yes   bank4.bin
fee7480c   16384  fee7480c   16384   Yes   bank5.bin
500f28cd   16384  500f28cd   16384   Yes   bank6.bin
8e1162f8   16384  8e1162f8   16384   Yes   bank7.bin
4cf12d39  131088  4cf12d39  131088   Yes   elite.bin
eb5e8763      16  eb5e8763      16   Yes   header.bin

Differences between the variants

You can see the differences between the variants by searching the source code for _PAL (for features in the PAL variant) or _NTSC (for features in the NTSC variant). The main differences in the NTSC variant compared to the PAL variant are:

The two versions count a different number of cycles in the NMI handler (7433 in the PAL version, 6797 in the NTSC version).
The NTSC version is missing the Imagineer and Nintendo headings from the Start screen.
The PAL version waits for longer before starting auto-play on the combat demo.
Each version has its own unique checksum algorithm for the save slots.
The internal version number is different (the PAL version is "<2.8>" while the NTSC version is "5.0")
The copyright message hidden in bank 3 is different (the PAL message is "NES ELITE IMAGE 2.8 - 04 MAR 1992" while the NTSC message is "NES ELITE IMAGE 5.2 - 24 APR 1992"
The first title in the combat demo scroll text is different (the PAL title is "IMAGINEER PRESENTS --- E L I T E --- (C)BRABEN & BELL 1991" while the NTSC title is "NTSC EMULATION --- E L I T E --- (C)BELL & BRABEN 1991")
A number of pixel y-coordinate constants in the PAL version are six pixels bigger than in the NTSC version, to cater for the taller screen height.
The interrupt vectors in banks 0 to 6 that are used during initialisation are subtly different.
The code for detecting double-taps of the B button when choosing buttons from the icon bar is a bit simpler in the NTSC version.

It's worth noting that the NTSC variant doesn't actually work on an NTSC machine. The NMI timings have been changed to work with some (but not all) emulators in NTSC mode, but it isn't a full NTSC conversion, it's an NTSC emulation (as per the scroll text).

See the accompanying website for a comprehensive list of differences between the variants.

Right on, Commanders!

Mark Moxon

markmoxon/nes-elite-beebasm

markmoxon

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