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elite-beebasm

A port of the original BBC Elite source code from BASIC assembler to BeebAsm assembler for modern PC development environment.

Background

The original source files for BBC Elite can be found on Ian Bell's personal web.

The following archives are available:

As the game was written on 8-bit machines with very limited RAM and disk storage (the game started life on an Acorn Atom) the source code is incredibly terse, densely packed and effectively unreadable to anyone but the original authors (even then, I'd imagine both would struggle some 30+ years later..!)

This project aims to develop a readable, fast and reproducible build of Elite that can be used for learning and non-profit modification purposes.

Version

The BBC Cassette version of the game but built for disk was chosen as the initial starting point for simplicity. It generates just two binary executable files ELITE (the loader) and ELTcode (the game) and will run on a standard issue Model B with DFS, which is the most common configuration of any BBC system and easily emulated.

Future versions may include BBC Disk, Master and 2nd processor configurations.

Files

elite-source.asm

This is the main source for the Elite game. It is made up of 7 original BASIC source files concatenated and converted to BeebAsm assembler syntax:

  • ELITEA outputting ELTA binary
  • ELITEB outputting ELTB binary
  • ELITEC outputting ELTC binary
  • ELITED outputting ELTD binary
  • ELITEE outputting ELTE binary
  • ELITEF outputting ELTF binary
  • ELITEG outputting ELTG binary

It totals ~10,000 lines of 6502 assembler.

elite-bcfs.asm

The BASIC source file S.BCFS is responsible for creating the Big Code File, i.e. concatenating the ELTA..ELTG binaries plus the SHIPS data into a single executable for the Elite main game called ELTcode.

There is a simple checksum test added to the start of the code. The checksum function cannot be performed in the BeebAsm source so has been reproduced in the elite-checksum.py Python script described below.

elite-loader.asm

The BASIC source file ELITES creates the executable Elite loader ELITE. This is responsible for displaying the title screen and planet, drawing the initial (static) HUD, setting up interrupt routines (for the MODE 4/5 split in the HUD), relocating many routines to lower memory (below PAGE) and loading the main executable.

There are a number of checksum and protection routines that XOR the code and data with other parts of memory in an attempt to obfuscate and protect the game from tampering. This cannot be done in the BeebAsm source so has been reproduced in the elite-checksum.py Python script below.

elite-checksum.py

There are a number of checksum and simple XOR encryption routines that form part of the Elite build process. These are trivial to interleave with the assembly process in BBC BASIC but have had to be converted to a Python script to run as part of a modern development environment.

The script has two parts. Firstly performing some functions from the S.BCFS source to generate a protected version of the ELTcode binary:

  • Concatenate all Elite game binaries
  • Compute checksum for Commander data
  • Poke Commander checksum value into binary
  • Compute checksum for all game code except boot header
  • Poke checksum value into binary
  • Encrypt all game code except boot header with cycling XOR value (0-255)
  • Compute final checksum for game code
  • Output ELTcode binary (protected)

Secondly it performs the checksum and encryption functions from the ELITES loader source:

  • Reverse the bytes for a block of code that is placed on the stack
  • Compute checksum for MAINSUM ` Poke checksum value into binary
  • Comuter checksum for CHECKbyt
  • Poke checksum value into binary
  • Encrypt a block of code by XOR'ing with the code to be placed on the stack
  • Encrypt all code destined for lower RAM by XOR'ing with loader boot code
  • Encrypt binary data (HUD graphics etc.) by XOR'ing with loader boot code
  • Output ELITE binary (protected)

Build

For now a simple DOS Batch file make.bat will perform the following commands:

%BEEBASM% -i elite-source.asm -v
%BEEBASM% -i elite-bcfs.asm -v
%BEEBASM% -i elite-loader.asm -v
%PYTHON% elite-checksum.py
%BEEBASM% -i elite-disc.asm -do elite.ssd -boot ELITE

Simply define the location of your beebasm.exe and python.exe at the head of the batch file. All being well this will output elite.ssd which will boot the game.

Verify

The crc32dos.exe is included to verify that the output files are binary identical with the original sources.

The following built binaries were extracted from the Cassette sources disk image:

 CRC 32     File Size   Filename
-------------------------------------------------
a88ca82b         5426   \extracted\ELITE.bin
0f1ad255         2228   \extracted\ELTA.bin
0d6f4900         2600   \extracted\ELTB.bin
97e338e8         2735   \extracted\ELTC.bin
01a00dce        20712   \extracted\ELTcode.bin
322b174c         2882   \extracted\ELTD.bin
29f7b8cb         2663   \extracted\ELTE.bin
8a4cecc2         2721   \extracted\ELTF.bin
7a6a5d1a         2340   \extracted\ELTG.bin

The following binaries are output from the BeebAsm build process:

 CRC 32     File Size   Filename
-------------------------------------------------
a88ca82b         5426   \output\ELITE.bin
f40816ec         5426   \output\ELITE.unprot.bin
0f1ad255         2228   \output\ELTA.bin
3f107178         2600   \output\ELTB.bin
97e338e8         2735   \output\ELTC.bin
01a00dce        20712   \output\ELTcode.bin
8de09f8f        20712   \output\ELTcode.unprot.bin
322b174c         2882   \output\ELTD.bin
29f7b8cb         2663   \output\ELTE.bin
8a4cecc2         2721   \output\ELTF.bin
7a6a5d1a         2340   \output\ELTG.bin
00d5bb7a           40   \output\ELThead.bin

Differences

ELITEC

It was discovered that the Text Files archive does not contain the identical source to the Disk Image archive. A couple of small differences in code in the ELITEC source:

.WARP LDAMANY+AST
CLC
ADCMANY+ESC
CLC         ; NOT IN ELITEC.TXT but is in ELITE SOURCE IMAGE

ADCMANY+OIL
TAX
LDAFRIN+2,X
ORASSPR
ORAMJ
BNEWA1
LDYK%+8
BMIWA3
TAY
JSRMAS2
;LSRA
;BEQWA1
CMP #2
BCC WA1     ; NOT IN ELITEC.TXT but is in ELITE SOURCE IMAGE
.WA3 LDYK%+NI%+8
BMIWA2
LDY#NI%
JSRm
;LSRA
;BEQWA1
CMP #2
BCC WA1     ; NOT IN ELITEC.TXT but is in ELITE SOURCE IMAGE

ELTB

The ELTB binary output is not identical to the extracted file yet the final ELTcode binary is. What gives?

This comes down to a single byte in the default Commander data and is related to whether the BASIC variable Q% is TRUE or FALSE. This appears to be a cheat flag used during testing.

The implication is that the output binary files on the Cassette source disk were not produced at the same time but one was modified after the ELTcode file was created. (We can see that running the build process in an emulator results in a different output and checksum values.)

A decision was made to structure the elite-source.asm file so that the final ELTcode executable output is binary identical to the file extracted from the Cassette Source disk image.

Next Steps

Although the binary files output are identical, the build process is brittle meaning that the source cannot be altered. The main problem is that the encrytion process does not have knowledge of the symbols produced by the assembler, so these values have been hard coded for temporary convenience.

The next steps are:

  • Remove code requiring checksums and copy protection to allow source to be modified freely
  • Improve whitespacing for readability
  • Improve label names for readability
  • Commenting of critical functions
  • Add BBC Disk, Master and 2nd processor versions to build

I am fully open to PR's if anyone feels like contributing to this project!

Kieran Connell | July 2018