Fully documented source code for Elite on the BBC Master

This repository contains source code for Elite on the BBC Master, 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
Flicker-free Elite
BBC Master Elite with music
Elite Compendium
Elite over Econet
Building Elite from the source
Building different variants of BBC Master Elite
Notes on the original source files
- Producing byte-accurate binaries

Introduction

This repository contains source code for Elite on the BBC Master, 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 Acornsoft SNG47 variant, and the Superior Software variant for the Master Compact.

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 game disc that can be loaded into a BBC Master 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:

BBC Elite, Master 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 most interesting files are in the main-sources folder:
- The main game's source code is in the elite-source.asm and elite-data.asm files (containing the game code and game data respectively) - this is the motherlode and probably contains all the stuff you're interested in.
- The game's loader is in the elite-loader.asm file - this is mainly concerned with setup and copy protection.
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 routine TT170, which you can find by searching for Name: TT170. 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 BBC 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, boot files and so on.
2-build-files contains build-related scripts, such as the checksum, encryption and 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 SSD disc image that contains the compiled game and which can be run on real hardware or in an emulator.

Flicker-free Elite

This repository also includes a flicker-free version, which incorporates a fix for planets so they no longer flicker. The flicker-free code is in a separate branch called flicker-free, and apart from the code differences for reducing flicker, this branch is identical to the main branch and the same build process applies.

The annotated source files in the flicker-free branch contain both the original Acornsoft code and all of the modifications for flicker-free Elite, so you can look through the source to see exactly what's changed. Any code that I've removed from the original version is commented out in the source files, so when they are assembled they produce the flicker-free binaries, while still containing details of all the modifications. You can find all the diffs by searching the sources for Mod:.

For more information on flicker-free Elite, see the hacks section of the accompanying website.

BBC Master Elite with music

This repository also includes a version of BBC Master Elite that includes the music from the Commodore 64 version. The music-specific code is in a separate branch called music, and apart from the code differences for adding the music, this branch is identical to the main branch and the same build process applies.

The annotated source files in the music branch contain both the original Acornsoft code and all of the modifications for the musical version of Elite, so you can look through the source to see exactly what's changed. Any code that I've removed from the original version is commented out in the source files, so when they are assembled they produce the music-enabled binaries, while still containing details of all the modifications. You can find all the diffs by searching the sources for Mod:.

The music itself is built as a sideways ROM using the code in the elite-music repository.

For more information on the music, see the hacks section of the accompanying website.

Elite Compendium

This repository also includes a version of BBC Master Elite for the Elite Compendium, which incorporates all the available hacks in one game. The Compendium version is in a separate branch called elite-compendium, which is included in the Elite Compendium repository as a submodule.

The annotated source files in the elite-compendium branch contain both the original Acornsoft code and all of the modifications for the Elite Compendium, so you can look through the source to see exactly what's changed. Any code that I've removed from the original version is commented out in the source files, so when they are assembled they produce the Compendium binaries, while still containing details of all the modifications. You can find all the diffs by searching the sources for Mod:.

For more information on the Elite Compendium, see the hacks section of the accompanying website.

Elite over Econet

This repository also includes a version of BBC Master Elite that loads over Econet and supports multiplayer scoreboards. The Elite over Econet version is in a separate branch called econet, which is included in the Elite over Econet repository as a submodule.

The annotated source files in the econet branch contain both the original Acornsoft code and all of the modifications for Elite over Econet, so you can look through the source to see exactly what's changed. Any code that I've removed from the original version is commented out in the source files, so when they are assembled they produce the Elite over Econet binaries, while still containing details of all the modifications. You can find all the diffs by searching the sources for Mod:.

For more information on Elite over Econet, see the hacks section of the accompanying website.

Building Elite from the source

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

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. The build process has only been tested on 3.x, but 2.7 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).

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.

Windows

For Windows users, there is a batch file called make.bat which you can use to build the game. 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, entering the following into a command window:

make.bat

will produce a file called elite-master-sng47.ssd in the 5-compiled-game-discs folder that contains the SNG47 variant, which you can then load into an emulator, or into a real BBC Micro using a device like a Gotek.

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, entering the following into a terminal window:

make

will produce a file called elite-master-sng47.ssd in the 5-compiled-game-discs folder that contains the SNG47 variant, which you can then load into an emulator, or into a real BBC Micro using a device like a Gotek.

Build options

By default the build process will create a typical Elite game disc with a standard commander and verified binaries. There are various arguments you can pass to the build to change how it works. They are:

variant=<name> - Build the specified variant:
- variant=sng47 (default)
- variant=compact
commander=max - Start with a maxed-out commander (specifically, this is the test commander file from the original source, which is almost but not quite maxed-out)
encrypt=no - Disable encryption and checksum routines
match=no - Do not attempt to match the original game binaries (i.e. omit workspace noise)
verify=no - Disable crc32 verification of the game binaries

So, for example:

make variant=compact commander=max encrypt=no match=no verify=no

will build an unencrypted Master Compact variant with a maxed-out commander, no workspace noise and no crc32 verification.

The unencrypted version should be more useful for anyone who wants to make modifications to the game code. As this argument produces unencrypted files, the binaries produced will be quite different to the binaries on the original source disc, which are encrypted.

See below for more on the verification process.

Updating the checksum scripts if you change the code

If you change the source code in any way, you may break the game; if so, it will typically hang at the loading screen, though in some versions it may hang when launching from the space station.

To fix this, you may need to update some of the hard-coded addresses in the checksum script so that they match the new addresses in your changed version of the code. See the comments in the elite-checksum.py script for details.

Verifying the output

The default build process prints out checksums of all the generated files, along with the checksums of the files from the original sources. You can disable verification by passing verify=no to the build.

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, then this is the output of the verification process:

Results for variant: sng47
[--originals--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
d52370e7  27720  d52370e7  27720   Yes   BCODE.bin
86e9fa69  27720  86e9fa69  27720   Yes   BCODE.unprot.bin
bf10f02b  16896  bf10f02b  16896   Yes   BDATA.bin
f7a27087  16896  f7a27087  16896   Yes   BDATA.unprot.bin
6dce29cc    721  6dce29cc    721   Yes   M128Elt.bin

All the compiled binaries match the originals, so we know we are producing the same final game as the SNG47 variant.

Log files

During compilation, details of every step are output in a file called compile.txt in the 3-assembled-output folder. If you have problems, it might come in handy, and it's a great reference if you need to know the addresses of labels and variables for debugging (or just snooping around).

Auto-deploying to the b2 emulator

For users of the excellent b2 emulator, you can include the build parameter b2 to automatically load and boot the assembled disc image in b2. The b2 emulator must be running for this to work.

For example, to build, verify and load the game into b2, you can do this on Windows:

make.bat all b2

or this on Mac/Linux:

make all b2

If you omit the all target then b2 will start up with the results of the last successful build.

Note that you should manually choose the correct platform in b2 (I intentionally haven't automated this part to make it easier to test across multiple platforms).

Building different variants of BBC Master Elite

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

The Acornsoft SNG47 variant, which was the first appearance of BBC Master Elite, and the one included on all subsequent discs
The Superior Software variant for the Master Compact

By default the build process builds the SNG47 variant, but you can build a specified variant using the variant= build parameter.

Building the SNG47 variant

You can add variant=sng47 to produce the elite-master-sng47.ssd file that contains the SNG47 variant, though that's the default value so it isn't necessary. In other words, you can build it like this:

make.bat variant=sng47

or this on a Mac or Linux:

make variant=sng47

This will produce a file called elite-master-sng47.ssd in the 5-compiled-game-discs folder that contains the SNG47 variant.

The verification checksums for this version are as follows:

Results for variant: sng47
[--originals--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
d52370e7  27720  d52370e7  27720   Yes   BCODE.bin
86e9fa69  27720  86e9fa69  27720   Yes   BCODE.unprot.bin
bf10f02b  16896  bf10f02b  16896   Yes   BDATA.bin
f7a27087  16896  f7a27087  16896   Yes   BDATA.unprot.bin
6dce29cc    721  6dce29cc    721   Yes   M128Elt.bin

Building the Master Compact variant

You can build the Master Compact variant by appending variant=compact to the make command, like this on Windows:

make.bat variant=compact

or this on a Mac or Linux:

make variant=compact

This will produce a file called elite-master-compact.ssd in the 5-compiled-game-discs folder that contains the Master Compact variant.

The verification checksums for this version are as follows:

Results for variant: compact
[--originals--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
d5cbbba9  27904  d5cbbba9  27904   Yes   BCODE.bin
bd689545  27904  bd689545  27904   Yes   BCODE.unprot.bin
8c9d6d1f  16896  8c9d6d1f  16896   Yes   BDATA.bin
5993627f  16896  5993627f  16896   Yes   BDATA.unprot.bin
107b98cc    740  107b98cc    740   Yes   M128Elt.bin

Differences between the variants

You can see the differences between the variants by searching the source code for _SNG47 (for features in the SNG47 variant) or _COMPACT (for features in the Master Compact variant). The main differences in the Master Compact variant compared to the SNG47 variant are:

Support for the Compact's digital joystick. The analogue stick is still supported, but if this variant is run on a Compact, then the digital stick is read instead.
Support for ADFS and the single disc drive on the Compact. This essentially replaces the "Which Drive?" prompt in the disc access menu with "Which Directory?", and changes the formatting of the disc catalogue to fit it on-screen. There is also additional code to claim and release the NMI workspace when disc access is required, as ADFS uses zero page differently to DFS.

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

Notes on the original source files

Producing byte-accurate binaries

Instead of initialising workspaces with null values like BeebAsm, the original BBC Micro source code creates its workspaces by simply incrementing the P% and O% program counters, which means that the workspaces end up containing whatever contents the allocated memory had at the time. As the source files are broken into multiple BBC BASIC programs that run each other sequentially, this means the workspaces in the source code tend to contain either fragments of these BBC BASIC source programs, or assembled code from an earlier stage. This doesn't make any difference to the game code, which either initialises the workspaces at runtime or just ignores their initial contents, but if we want to be able to produce byte-accurate binaries from the modern BeebAsm assembly process, we need to include this "workspace noise" when building the project. Workspace noise is only loaded by the encrypt target; for the build target, workspaces are initialised with zeroes.

You can disable the production of byte-accurate binaries by passing match=no to the build. This will omit most workspace noise, leaving workspaces initialised with zeroes instead.

Here's an example of how workspace noise is included, from the end of the main source in elite-source.asm:

IF _MATCH_ORIGINAL_BINARIES

 IF _SNG47

  EQUB &41, &23, &6D, &65, &6D, &3A, &53, &54  \ These bytes appear to be
  EQUB &41, &6C, &61, &74, &63, &68, &3A, &52  \ unused and just contain random
  EQUB &54, &53, &0D, &13, &74, &09, &5C, &2E  \ workspace noise left over from
  EQUB &2E, &2E, &2E, &0D, &18, &60, &05, &20  \ the BBC Micro assembly process
  EQUB &0D, &1A, &F4, &21, &5C, &2E, &2E, &2E
  EQUB &2E, &2E, &2E, &2E, &2E, &2E, &2E, &42
  EQUB &61, &79, &20, &56, &69, &65, &77, &2E
  EQUB &2E, &2E, &2E, &2E, &2E, &2E, &2E, &2E
  EQUB &2E, &0D, &1A, &FE, &05, &20, &0D, &1B
  EQUB &08, &11, &2E, &48, &41

 ELIF _COMPACT

  EQUB &2B, &26, &33    \ These bytes appear to be unused and just contain
                        \ random workspace noise left over from the BBC Micro
                        \ assembly process

 ENDIF

ELSE

 IF _SNG47

  SKIP 77               \ These bytes appear to be unused

 ELIF _COMPACT

  SKIP 3                \ These bytes appear to be unused

 ENDIF

ENDIF

Right on, Commanders!

Mark Moxon

Name		Name	Last commit message	Last commit date
Latest commit History 277 Commits
1-source-files		1-source-files
2-build-files		2-build-files
3-assembled-output		3-assembled-output
4-reference-binaries		4-reference-binaries
5-compiled-game-discs		5-compiled-game-discs
.gitattributes		.gitattributes
.gitignore		.gitignore
Makefile		Makefile
README.md		README.md
make.bat		make.bat

markmoxon/master-elite-beebasm

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