A port of the original BBC Elite source code from BASIC assembler to BeebAsm assembler for modern PC development environment.
The original source files for BBC Elite can be found on Ian Bell's personal web.
The following archives are available:
- Cassette sources as DFS disk image
- Cassette sources as text files
- BBC 2nd processor sources as DFS disk image
- Original Elite ship sources as DFS disk image
- Elite 2 ship sources as DFS disk image
- Original BBC Disk version of Elite (game only)
- Master 128 and 2nd Processor versions of Elite (game only)
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.
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.
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:
ELITEAoutputtingELTAbinaryELITEBoutputtingELTBbinaryELITECoutputtingELTCbinaryELITEDoutputtingELTDbinaryELITEEoutputtingELTEbinaryELITEFoutputtingELTFbinaryELITEGoutputtingELTGbinary
It totals ~10,000 lines of 6502 assembler.
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.
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.
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
ELTcodebinary (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
ELITEbinary (protected)
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.
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
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
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.
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!