Building and Hacking on the C64 KERNAL & BASIC with Modern Tools
The original Commodore 64 KERNAL and BASIC source code has been available for a while. It used to be built using Commodore’s assembler of a PET.
A GEOS Speed Zone Bug? (Why Do C64 GEOS Boot Disks Break, Part 2)
I happened to come across 50 original German GEOS 2.0 disks that were broken and sent in for replacement. In the first part, I covered the disks that were broken probably due to user error. Now let’s look at the read errors on the remaining disks. As it turns out, there might be a bug in GEOS that caused the boot disks to break!
Why Do C64 GEOS Boot Disks Break? (Part 1)
The most fragile things in the universe (next to C64 PLA chips) are C64 GEOS boot disks. Because of its copy-protection, the “Graphic Environment Operating System” had to be booted from the original disk every time. The disk failed so often that the box came with a second “backup” boot disk.
Reading the Raw Bits of a C64/1541 Disk without a Parallel Cable
An unmodified Commodore 1541 disk drive cannot transfer the raw bits of a whole track to the computer it is attached to: The Commodore Serial Bus is too slow to transmit the data in real time as it arrives from the read head, and the drive only has 2 KB of RAM, which is not enough to buffer the 8 KB of a whole track.
Making of the Book "Anatomy of the 4040 Disk Drive"
To my surprise, Hilaire Gagne, the author of “Anatomy of the 4040 Disk Drive”, commented on my blog post about the reconstruction of his book.
Refilling a Commodore MPS 1550 C Ribbon Cartridge
An empty ribbon of the 9-pin printer Commodore MPS 1550 C (which is mechanically identical with the Olivetti DM 105) can easily be refilled using stamp pad ink. Here is how:
Fitting 44% More Data on a C64/1541 Floppy Disk
The physical data format on a Commodore 1541 5¼-inch floppy disk as used by the C64 is completely defined in software. The drive’s operating system fits 170 KB on a disk. This article explores different strategies, each with its pros and cons, to fit up to 246 KB.
The Ultimate Game Boy Talk [slides]
The Game Boy is turning 30. To celebrate this, I’m releasing the slides of my “Ultimate Game Boy Talk”, which you may use freely. (Please give credit & point me to derived content.)
Converting the "Competition Pro Extra USB" to C64/Amiga/Atari DB9
- Updated 2025-05-09: Added “SL-6602-TBE” pinout. Thanks a lot to Jörg Knitter!
- Updated 2022-03-09: Added “SL-6603-TWT” pinout. Thanks a lot to Risto K!
- Updated 2022-01-26: Added “V2” pinout.
- Updated 2022-01-22: Added “SL-6603-SPORTS-DE” pinout.
- Updated 2022-01-14: Added “SL-6603-SBK” and “KOKA” pinouts.
- Updated 2022-01-06: Added new “V3” and “V04T” pinouts and instructions on how to use a joystick extension cable.
Speedlink makes several versions of the Competition Pro joystick that have all the physical properties and the excellent switches of the original Competition Pro, but come with a USB interface:
Ultimate Commodore 64 BASIC & KERNAL ROM Disassembly
My side-by-side C64 ROM disassembly/commentary page has been completely redone!
Visualizing Commodore 1541 Disk Contents
G64 files are C64/1541 disk images that contain all bits as they are physically laid out on the 5¼-inch floppy disk. Let’s visualize them!
Building the Commodore TED Kernal with Modern Tools
The original Commodore TED (C16, C116, Plus/4) source code has recently appeared on zimmers.net. It is also available in my Commodore Source Code git repository.
Building the Original Commodore 1541 DOS Source
You might think the DOS ROM of the Commodore 1541 disk drive has been analyzed to death. But here are two new resources:
Book "Anatomy of the 4040 Disk Drive" by Hilaire Gagne
Hilaire Gagne: Anatomy of the 4040 Disk Drive
(HTML, 632K)
Final Cartridge III Monitor for the TED
In my quest to make the C16 more usable, i.e. more like the environment I’m used to, i.e. a C64 with a Final Cartridge III, I’ve ported the Final Cartridge III monitor to the TED series (C16, C116, Plus/4).
C64 Keyboard Layout for the C16
The Commodore 16 is an interesting machine, but the keyboard layout is infuriating. The cursor keys are at an awful place and… and… and… well, it just isn’t the C64 layout I’m used to!
Analyzing the ROMs of Third Party Disk Drives for the C64
Most Commodore 64 users had a 1541 disk drives, but there were always also third part options. Most of them claimed full 1541 compatibility, which sounds impossible without using the same ROM. Let’s analyze the ROMs of some third party drives!
Reconstructing the Unreleased C128 ROM Update
The C128 source dump over at zimmers.net that appeared recently contains source for a version 2 kernel, which was never released. The known versions are 0 and 1. Let’s see whether we can reconstruct the ROM image!
Original Commodore Source Code Collection
Over the years, the ROM source code of many Commodore computers and peripherals has appeared. I have been collecting them in a git repository here:
Commodore Peripheral Bus: Part 3: Commodore DOS
In the series about the variants of the Commodore Peripheral Bus family, this article covers the common layer 4: The “Commodore DOS” interface to disk drives.