You might think the DOS ROM of the Commodore 1541 disk drive has been analyzed to death. But here are two new resources:
- The original source of all versions of the 1540/1541/1541C/1541-II ROMs.
- A git repository with all versions in its history, each of them adapted to build with modern tools.
The Original Source
The Dennis Jarvis collection contains source that can build the 2031/1540 ROM. Using the 1571 source from the same collection and the ROM images, I reconstructed the source trees for all versions of the 1540/1541/1541C/1541-II ROMs:
| Directory | Part Numbers | Description |
|---|---|---|
| DOS_1540 | 325302-01, 325303-01 | 1540 |
| DOS_1541_01 | 325302-01, 901229-01 | 1541 (original) |
| DOS_1541_02 | 325302-01, 901229-02 | 1541 (update) |
| DOS_1541_03 | 325302-01, 901229-03 | 1541 (update) |
| DOS_1541_05 | 325302-01, 901229-05 | 1541 (short board) |
| DOS_1541_06 | 325302-01, 901229-06 | 1541 (short board, update) |
| DOS_1541C_01 | 251968-01 | 1541C (original) |
| DOS_1541C_02 | 251968-02 | 1541C (updated) |
| DOS_1541C_03 | 251968-03 | 1541-II |
All these directories can be found in the Commodore Source Code repository:
https://github.com/mist64/cbmsrc
Here is the output of “diff DOS_1540 DOS_1541”, for example:
The Buildable Source
This repository contains all versions of the 1540/1541/1541C/1541-II source, buildable with cc65:
https://github.com/mist64/dos1541
If you want to base your own development on this code:
- The head of the repository points to the 1541-II version, which contains all of Commodore’s bug fixes. It works on all hardware versions (1540/1541, 1541C/1541-II), but does not contain support for the 1541C track 0 sensor.
- 251968-02 is the last 1541C version and has track 0 sensor support, but one bug fix found in the 1541-II ROM is missing:
ptch15is supposed to jump tornget2, notrnget.