by Berkeley Softworks, reverse engineered by Maciej 'YTM/Elysium' Witkowiak; Michael Steil
This is the reverse engineered source code of the KERNAL (plus disk and input drivers) of the English version of GEOS 2.0 for Commodore 64.
The source has been heavily reorganized and modularized, nevertheless, a standard compile will generate binaries that are identical with the GEOS 2.0 distrbution binaries.
- make, bash, dd
- cc65 for assembling and linking
- pucrunch for generating a compressed executable
- c1541 for generating the disk image
Without pucrunch/c1541, you can still build an uncompressed KERNAL binary image.
Run make to build GEOS. This will create the following files:
- raw KERNAL components:
kernal.bin,lokernal.bin,init.bin - disk drive drivers:
drv1541.bin,drv1571.bin,drv1581.bin - input drivers:
amigamse.bin,joydrv.bin,lightpen.bin,mse1531.bin,koalapad.bin,pcanalog.bin - combined KERNAL image (
SYS 49155):combined.prg - compressed KERNAL image (
RUN):compressed.prg - disk image:
geos.d64
If you have the cbmfiles.com GEOS64.D64 image in the current directory, the disk image will be based on that one, with the GEOS and GEOBOOT files deleted and the newly built kernel added. Otherwise, it will be a new disk image with the kernel, and, if you have a desktop.cvt file in the current directory, with DESK TOP added.
inc/equ.inc contains lots of compile time options. Most of them have not been tested recently.
./drv: disk drive driver source./inc: include files: macros and symbols./init: purgeable KERNAL init source./input: input driver source./kernal: kernal source./reference: original binaries from the cbmfiles.com version
The original GEOS was copy protected in three ways:
- The original loader decrypted the KERNAL at load time and refused to do so if the floppy disk was a copy. Like the cbmfiles.com version, this version doesn't use the original loader, and the kernel is available in plaintext.
- deskTop assigned a random serial number to the kernel on first boot and keyed all major applications to itself. This version comes with a serial number of 0x58B5 pre-filled, which matches the cbmfiles.com version.
- To counter tampering with the serial number logic, the KERNAL contained two traps that could sabotage the kernel. The code is included in this version, but can be removed by setting trap = 0.
- Farr, M.: The Official GEOS Programmer's Reference Guide (1987)
- Berkeley Softworks: The Hitchhiker's Guide to GEOS (1988)
- Boyce, A. D.; Zimmerman, B.: GEOS Programmer's Reference Guide (2000)
The original reverse-engineering was done by Maciej Witkowiak in 1999-2002, targeted the ACME assembler and was released as GEOS 2000, which included several code optimizations and code layout differences.
In 2015/2016, Michael Steil ported the sources to cc65, reconstructed the original code layout, did some more reverse-engineering and cleanups, and modularized the code aggressively.