Fully documented source code for Elite on the BBC Micro

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

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.

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 Micro 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
Browsing the source in an IDE
Building Elite from the source
Differences between the various source files
- ELITEC
- ELTB

Acknowledgements

The code on this site is identical to the version released on Ian Bell's personal website (it's just been reformatted to be more readable).

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 archives from Ian Bell's site form the basis for this project:

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 sources folder:
- The main game's source code is in the elite-source.asm file - 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 at the start of the elite-loader.asm file, 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 source code is peppered with a number of "deep dives", each of which goes into an aspect of the game in more detail. You can find deep dives in the source files by simply searching for Deep dive:
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 deep dive in the TT170 routine that has you covered
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.

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).

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 - An unencrypted version
encrypt - An encrypted version that exactly matches the released version of the game

The unencrypted version should be more useful for anyone who wants to make modifications to the game code. It includes a default commander with lots of cash and equipment, which makes it easier to test the game. As this target produces unencrypted files, the binaries produced will be quite different to the binaries on the original source disc, which are encrypted.

The encrypted version produces the released version of Elite, along with the standard default commander.

(Note that there is a third build target, extract, which is explained in the section below on differences between the various source files.)

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

Note that the build ends with a warning that there is no SAVE command in the source file. You can ignore this, as the source file contains a PUTFILE command instead, but BeebAsm still reports this as a warning.

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.exe).

All being well, doing one of the following:

make.bat build

make.bat encrypt

will produce a file called elite.ssd, 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, doing one of the following:

make build

make encrypt

will produce a file called elite.ssd, which you can then load into an emulator, or into a real BBC Micro using a device like a Gotek.

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 extracted 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 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 extracted folder were taken straight from the cassette sources disc image (though see the notes on ELTB below), while those in the 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:

[--extracted--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
a88ca82b   5426  a88ca82b   5426   Yes   ELITE.bin
0f1ad255   2228  0f1ad255   2228   Yes   ELTA.bin
e725760a   2600  e725760a   2600   Yes   ELTB.bin
97e338e8   2735  97e338e8   2735   Yes   ELTC.bin
322b174c   2882  322b174c   2882   Yes   ELTD.bin
29f7b8cb   2663  29f7b8cb   2663   Yes   ELTE.bin
8a4cecc2   2721  8a4cecc2   2721   Yes   ELTF.bin
7a6a5d1a   2340  7a6a5d1a   2340   Yes   ELTG.bin
01a00dce  20712  01a00dce  20712   Yes   ELTcode.bin
99529ca8    256  99529ca8    256   Yes   PYTHON.bin
49ee043c   2502  49ee043c   2502   Yes   SHIPS.bin
c4547e5e   1023  c4547e5e   1023   Yes   WORDS9.bin
*             *  f40816ec   5426    *    ELITE.unprot.bin
*             *  1e4466ec  20712    *    ELTcode.unprot.bin
*             *  00d5bb7a     40    *    ELThead.bin

All the compiled binaries match the extracts, so we know we are producing the same final game as the release version.

Log files

During compilation, details of every step are output in a file called compile.txt in the 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).

Differences between the various source files

ELITEC

It turns out that the cassette sources as text files do not contain identical code to the binaries in the cassette sources disc image. Specifically, there are some instructions in the ELTC binary that are different to the instructions in the ELITEC.TXT source file.

You can see these differences documented in the WARP routine in the elite-source.asm file. To find this, search the file for Name: WARP and follow the comments for mentions of ELITEC.TXT.

The instructions included in elite-source.asm are those that match the binary files rather than ELITEC.TXT, to ensure that the build process produces binaries that match the released version of the game.

ELTB

It also turns out there are two versions of the ELITEB BASIC source program on the cassette sources disc image, one called $.ELITEB and another called O.ELITEB. These two versions of ELITEB differ by just one byte in the default commander data. This byte controls whether or not the commander has a rear pulse laser. In O.ELITEB this byte is generated by:

EQUB (POW + 128) AND Q%

while in $.ELITEB, this byte is generated by:

EQUB POW

The BASIC variable Q% is a Boolean flag that, if TRUE, will create a default commander with lots of cash and equipment, which is useful for testing. You can see this in action if you build an unencrypted binary with make build, as the unencrypted build sets Q% to TRUE for this build target.

The BASIC variable POW has a value of 15, which is the power of a pulse laser. POW + 128, meanwhile, is the power of a beam laser.

Given the above, we can see that O.ELITEB correctly produces a default commander with no a rear laser if Q% is FALSE, but adds a rear beam laser if Q% is TRUE. This matches the released game, whose executable can be found as ELTcode on the same disc. The version of ELITEB in the cassette sources as text files matches this version, O.ELITEB.

In contrast, $.ELITEB will always produce a default commander with a rear pulse laser, irrespective of the setting of Q%, so it doesn't match the released version.

The ELTB binary file in the extracted folder of this repository is the release version, so we can easily tell whether any changes we've made to the code deviate from the release version. However, the ELTB binary file on the sources disc matches the version produced by $.ELITEB, rather than the released version produced by O.ELITEB - in other words, ELTB on the source disc is not the release version.

The implication is that the ELTB binary file on the cassette sources disc image was produced by $.ELITEB, while the ELTcode file (the released game) used O.ELITEB. Perhaps the released game was compiled, and then someone backed up the ELITEB source to O.ELITEB, edited the $.ELITEB to have a rear pulse laser, and then generated a new ELTB binary file. Who knows? Unfortunately, files on DFS discs don't have timestamps, so it's hard to tell.

To support this discrepancy, there is an extra build target for building the ELTB binary as found on the sources disc, and as produced by $.ELITEB. You can build this version, which has the rear pulse laser, with:

make extract

The ELTcode executable produced by this build target is different to the released version, because the default commander has the extra rear pulse laser. You can use the verify target to confirm this. Doing make encrypt verify shows that all the generated files match the extracted ones, while make extract verify shows that the all the generated files match the extracted ones except for ELTB and ELTcode.

Right on, Commanders!

Mark Moxon

jcnventura/elite-beebasm