Fully documented source code for Elite-A on the BBC Micro

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

It is a companion to the bbcelite.com website.

See the introduction for more information.

Introduction
Acknowledgements
- A note on licences, copyright etc.
Browsing the source in an IDE
Folder structure
Flicker-free Elite
Building Elite-A from the source
Building different variants of Elite-A
Notes on the original source files
- Converting the original build process to BeebAsm

Introduction

This repository contains the original source code for Angus Duggan's Elite-A on the BBC Micro.

You can build the fully functioning game from this source. Three variants are currently supported: the officially released version from Angus's site, the variant produced by the original source discs (which was never released), and a variant that fixes various bugs.

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.

Elite-A is legendary amongst BBC Elite fans, and remains a deeply impressive project that has achieved almost mythical status in the Acorn retro scene (and deservedly so). Ian Bell, co-author of the original Elite, has this to say on his website:

Also available here is Angus Duggan's Elite-A, a comprehensive enhancement of BBC Elite. He created this by disassembling the object code and then reprogramming the resultant source. A significant achievement for which respect is due.

It's worth noting that Angus coded Elite-A back in the late 1980s, using a BBC Micro fitted with his own handwritten 6502 assembler ROM, which he used to disassemble the original, protected game binaries, and reassemble the enhanced version. He used Acornsoft's VIEW word processor as his IDE, and built the whole thing without the benefit of modern tooling. A significant achievement, indeed.

I am very grateful to Angus for giving me permission to analyse his work on Elite-A, and for providing me with the original source files.

See Angus's Elite-A site for more information on playing Elite-A.

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

Elite-A was written by Angus Duggan, and is an extended version of the BBC Micro disc version of Elite; the extra code is copyright Angus Duggan. The original Elite was written by Ian Bell and David Braben and is copyright © Acornsoft 1984.

The code on this site is identical to Angus Duggan's source discs (it's just been reformatted, and the label names have been changed to be consistent with the sources for the original BBC Micro disc version on which it is based).

Huge thanks are due to Angus Duggan for giving me permission to document his work in extending Elite; to the original authors of Elite 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.

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-flight.asm, elite-source-docked.asm and elite-source-encyclopedia.asm files (for when we're in-flight, docked or viewing the encyclopedia) - this is the motherlode and probably contains all the stuff you're interested in.
- The 6502 Second Processor version's source code is in the elite-6502sp-parasite.asm file (for the parasite, i.e. the Second Processor) and elite-6502sp-io-processor.asm (for the I/O processor, i.e. the BBC Micro).
- The game's loader is in the elite-loader.asm file - this is mainly concerned with setup and checking for Tube and BBC Master hardware.
The source files for Elite-A are unique amongst the annotated versions in this project, in that they contain inline diffs. Angus created Elite-A by taking the original disc version of Elite and modifying the code to include all his new features. The annotated source files in this repository contain both the original disc code and all of Angus's modifications, so you can look through the source to see exactly what Angus changed in order to create Elite-A. Any code that he removed from the disc version is commented out in the source files, so when they are assembled they produce the Elite-A binaries, while still containing details of Angus's modifications. You can find all the diffs by searching the sources for Mod:. (Note that this feature does not apply to the two 6502 Second Processor version source files, which just contain the Elite-A code.)
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 Elite-A 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 the backported flicker-free ship-drawing routines from the BBC Master. 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. Checksum values are different, but that's about it.

For more information on the flicker-free code, see the deep dives on flicker-free ship drawing and backporting the flicker-free algorithm.

Building Elite-A from the source

Requirements

You will need the following to build Elite-A 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).

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

Build targets

There are two main build targets available. They are:

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

Unlike the Acornsoft versions of Elite, Elite-A 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-a-released.ssd in the 5-compiled-game-discs folder that contains the released version of Elite-A, 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

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.

The binaries in the 4-reference-binaries folder were taken straight from Angus Duggan's original source discs, 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 build verify, then this is the output of the verification process:

Results for variant: released
[--originals--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
26ae1732  19997  26ae1732  19997   Yes   1.D.bin
b1447e60  16778  b1447e60  16778   Yes   1.E.bin
14ee8b20  17432  14ee8b20  17432   Yes   1.F.bin
3d638042   1956  3d638042   1956   Yes   2.H.bin
1f1783e7  43141  1f1783e7  43141   Yes   2.T.bin
171ccea5   5363  171ccea5   5363   Yes   ELITE.bin
4f2febe4    256  4f2febe4    256   Yes   MISSILE.bin
678c1c7f   2560  678c1c7f   2560   Yes   S.A.bin
cae56eda   2560  cae56eda   2560   Yes   S.B.bin
7b56fbb5   2560  7b56fbb5   2560   Yes   S.C.bin
55e86dde   2560  55e86dde   2560   Yes   S.D.bin
be2665dd   2560  be2665dd   2560   Yes   S.E.bin
c0917c15   2560  c0917c15   2560   Yes   S.F.bin
80f4145e   2560  80f4145e   2560   Yes   S.G.bin
0d9fe4e8   2560  0d9fe4e8   2560   Yes   S.H.bin
31ea0782   2560  31ea0782   2560   Yes   S.I.bin
f444274e   2560  f444274e   2560   Yes   S.J.bin
b9672969   2560  b9672969   2560   Yes   S.K.bin
05f74f36   2560  05f74f36   2560   Yes   S.L.bin
39856010   2560  39856010   2560   Yes   S.M.bin
132980ad   2560  132980ad   2560   Yes   S.N.bin
26525e5c   2560  26525e5c   2560   Yes   S.O.bin
76097753   2560  76097753   2560   Yes   S.P.bin
6bd215b4   2560  6bd215b4   2560   Yes   S.Q.bin
bcd49589   2560  bcd49589   2560   Yes   S.R.bin
8b44b8b6   2560  8b44b8b6   2560   Yes   S.S.bin
155e6a6b   2560  155e6a6b   2560   Yes   S.T.bin
fab17499   2560  fab17499   2560   Yes   S.U.bin
8504604f   2560  8504604f   2560   Yes   S.V.bin
40f96e61   2560  40f96e61   2560   Yes   S.W.bin
b7b3c692   1024  b7b3c692   1024   Yes   WORDS.bin

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

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).

Building different variants of Elite-A

This repository contains the source code for three different variants of Elite-A:

The released version from Angus Duggan's site
The variant produced by the source disc (which was never released)
A variant that fixes two bugs in the original (splinters and Adder stats)

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

Building the released version

You can add variant=released to produce the elite-a-released.ssd file that contains the released version, though that's the default value so it isn't necessary.

The verification checksums for this version are shown above.

Building the source disc variant

You can build the source disc variant by appending variant=source-disc to the make command, like this on Windows:

make.bat build verify variant=source-disc

or this on a Mac or Linux:

make build verify variant=source-disc

This will produce a file called elite-a-from-source-disc.ssd in the 5-compiled-game-discs folder that contains the source disc variant.

The verification checksums for this version are as follows:

Results for variant: source-disc
[--originals--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
d1ca0224  19997  d1ca0224  19997   Yes   1.D.bin
b1447e60  16778  b1447e60  16778   Yes   1.E.bin
14ee8b20  17432  14ee8b20  17432   Yes   1.F.bin
3d638042   1956  3d638042   1956   Yes   2.H.bin
81d6d436  43141  81d6d436  43141   Yes   2.T.bin
171ccea5   5363  171ccea5   5363   Yes   ELITE.bin
4f2febe4    256  4f2febe4    256   Yes   MISSILE.bin
678c1c7f   2560  678c1c7f   2560   Yes   S.A.bin
cae56eda   2560  cae56eda   2560   Yes   S.B.bin
7b56fbb5   2560  7b56fbb5   2560   Yes   S.C.bin
55e86dde   2560  55e86dde   2560   Yes   S.D.bin
be2665dd   2560  be2665dd   2560   Yes   S.E.bin
c0917c15   2560  c0917c15   2560   Yes   S.F.bin
80f4145e   2560  80f4145e   2560   Yes   S.G.bin
0d9fe4e8   2560  0d9fe4e8   2560   Yes   S.H.bin
31ea0782   2560  31ea0782   2560   Yes   S.I.bin
f444274e   2560  f444274e   2560   Yes   S.J.bin
b9672969   2560  b9672969   2560   Yes   S.K.bin
05f74f36   2560  05f74f36   2560   Yes   S.L.bin
39856010   2560  39856010   2560   Yes   S.M.bin
132980ad   2560  132980ad   2560   Yes   S.N.bin
26525e5c   2560  26525e5c   2560   Yes   S.O.bin
76097753   2560  76097753   2560   Yes   S.P.bin
6bd215b4   2560  6bd215b4   2560   Yes   S.Q.bin
bcd49589   2560  bcd49589   2560   Yes   S.R.bin
8b44b8b6   2560  8b44b8b6   2560   Yes   S.S.bin
155e6a6b   2560  155e6a6b   2560   Yes   S.T.bin
fab17499   2560  fab17499   2560   Yes   S.U.bin
8504604f   2560  8504604f   2560   Yes   S.V.bin
40f96e61   2560  40f96e61   2560   Yes   S.W.bin
b7b3c692   1024  b7b3c692   1024   Yes   WORDS.bin

Building the bug fix variant

You can build the source disc variant by appending variant=bug-fix to the make command, like this on Windows:

make.bat build verify variant=bug-fix

or this on a Mac or Linux:

make build verify variant=bug-fix

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

The verification checksums for this version are as follows:

Results for variant: bug-fix
[--originals--]  [---output----]
Checksum   Size  Checksum   Size  Match  Filename
-----------------------------------------------------------
0ce6de36  19945  0ce6de36  19945   Yes   1.D.bin
fdf8073b  16778  fdf8073b  16778   Yes   1.E.bin
14ee8b20  17432  14ee8b20  17432   Yes   1.F.bin
3d638042   1956  3d638042   1956   Yes   2.H.bin
1336cda8  43089  1336cda8  43089   Yes   2.T.bin
171ccea5   5363  171ccea5   5363   Yes   ELITE.bin
4f2febe4    256  4f2febe4    256   Yes   MISSILE.bin
678c1c7f   2560  678c1c7f   2560   Yes   S.A.bin
cae56eda   2560  cae56eda   2560   Yes   S.B.bin
7b56fbb5   2560  7b56fbb5   2560   Yes   S.C.bin
55e86dde   2560  55e86dde   2560   Yes   S.D.bin
a119f0d9   2560  a119f0d9   2560   Yes   S.E.bin
c0917c15   2560  c0917c15   2560   Yes   S.F.bin
d3224bc1   2560  d3224bc1   2560   Yes   S.G.bin
0d9fe4e8   2560  0d9fe4e8   2560   Yes   S.H.bin
31ea0782   2560  31ea0782   2560   Yes   S.I.bin
f444274e   2560  f444274e   2560   Yes   S.J.bin
ad98f535   2560  ad98f535   2560   Yes   S.K.bin
05f74f36   2560  05f74f36   2560   Yes   S.L.bin
39856010   2560  39856010   2560   Yes   S.M.bin
132980ad   2560  132980ad   2560   Yes   S.N.bin
a258d111   2560  a258d111   2560   Yes   S.O.bin
76097753   2560  76097753   2560   Yes   S.P.bin
6bd215b4   2560  6bd215b4   2560   Yes   S.Q.bin
2aefc58b   2560  2aefc58b   2560   Yes   S.R.bin
8b44b8b6   2560  8b44b8b6   2560   Yes   S.S.bin
155e6a6b   2560  155e6a6b   2560   Yes   S.T.bin
22336ce1   2560  22336ce1   2560   Yes   S.U.bin
8504604f   2560  8504604f   2560   Yes   S.V.bin
40f96e61   2560  40f96e61   2560   Yes   S.W.bin
b7b3c692   1024  b7b3c692   1024   Yes   WORDS.bin

Differences between the variants

You can see the differences between the variants by searching the source code for _RELEASED (for features in the released version), _BUG_FIX (for features in the buf fix version) or _SOURCE_DISC (for features in the source disc variant).

The only difference in the source disc variant is that the latter has considerably lower ship prices.

There is a bug in the original version of Elite-A that prevents splinters from displaying properly, which makes mining all but impossible. The bug fix variant fixes that bug, as well as an incorrect cargo capacity in the Adder ship card in the encyclopedia.

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

Notes on the original source files

Converting the original build process to BeebAsm

See the original source files for details about how Angus's original sources have been converted to assemble in BeebAsm, so Elite-A can be built on modern computers.

Right on, Commanders!

Mark Moxon

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markmoxon / elite-a-beebasm

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