I’ve been meaning to build myself a mouse adapter for the Commodore 64 for quite some time. The recent release of the unofficial port of “Eye of the Beholder” for the C64 was a welcome excuse to finally get to work on this project.
The Mouse 1351 made by Commodore can be plugged directly into the control port of the C64 or C128 and it has two different operating modes. In joystick mode, the input from the mouse is compatible with that of a regular, digital joystick. In proportional mode, which is the default, it behaves and feels like any mouse used on modern computers.
Commodore Mouse 1351. (Image source: Wikipedia, License: CC BY-SA 3.0)
Recently, I got hold of quite a few broken Commodore 64 motherboards. It has been a while since I last had the chance to repair one of those boards and it was fun! I dubbed this repair project the “Blank Screen Marathon” and documented the whole process in German on Forum64. I might still give a summary in English here at a later point. But today, I’d like to introduce a small side project that resulted from this repair marathon.
The 3 ROM chips on a C64 motherboard
Pagefox is a DTP (desktop publishing) software that comes as a cartridge for the expansion port of the Commodore 64. It was developed by Hans Haberl — who also created the graphics editor Hi-Eddi and the word processor Printfox — and it was published and sold by the small German company Scanntronik which is actually still in business today.
Pagefox cartridge pried open
The two 9-pin Control Ports located on the right-hand side of the C64 are used to connect joysticks and other input devices like paddles or even a mouse to the 8-bit computer.
The two Control Ports.
The digital input pins are directly connected to one of the two 6526 Complex Interface Adapters (CIA) inside the C64, without any protection on the motherboard.
The original external power supply for the Commodore 64 comes in different forms but they all have one thing in common: With increasing age they are prone to failure and when they finally break they are likely to destroy the precious computer they were powering.
An original PSU for the Commodore 64.
The power brick for the Commodore delivers both 9V AC and 5V DC, the latter being produced from 9V AC with a voltage rectifier and a linear voltage regulator. While this voltage regulator is generally very reliable, it has to operate at the upper limit of its specification which takes its toll after decades of use. When the regulator finally breaks, it tends to short internally, thus supplying the sensitive TTL ICs inside the C64 with up to 9V instead of 5V since there is no integrated over-voltage protection.
Cartridges for the C64 provide either 8K or 16K of ROM that will replace parts of the memory visible to the CPU when plugged into the expansion port. Either 8K or 16K, that is all the Commodore can see directly on a cartridge. Then there is an auto-start mechanism implemented in the Kernal that can be triggered to execute code contained in that ROM when the machine is powered on or reset. Jani features an excellent article about how to create such simple cartridges on his site which I have linked to before.
A simple ROM cartridge: the OpenC64Cart.
I’ve covered the PLA and the question how to replace a broken one before. First, when I repaired one of my C64 from back in the day, needed a quick replacement, and used an EEPROM on a simple adapter board. In a follow-up post I used a simple circuit suggested in a talk by Eslapion to test this PLA replacement for glitches and found plenty. Last but not least, I created a replica of the Super Zaxxon cartridge also to be used for testing the PLA. There is a lot more information online on this topic like the excellent Post on Breadbox64.
The broken PLA from a C64 mainboard.
A quick market survey on RAM expansions for the C64
The Commodore 64 features a whopping 64kB of RAM, hence its name. Most of this is actually usable at least when programming in assembler. While this used to be quite a lot when compared to competitors in the market, even back in the day there were different external RAM extensions. Some of the popular “freezer” cartridges like later versions of the Action Replay or the Super Snapshot feature some additional RAM of their own.
Commodore started selling their RAM Expansion Units (REU) in 1985 when the C128 came out. The REU was actually quite sophisticated, with its own controller — the MOS8726, REC — that provides fast memory-transfer by DMA.
The other day, I was researching the Jason-Ranheim Capture cartridges and browsing the DDI Projects on that topic. The capture cartridge is a freezer module for the C64 that allows the internal state of the machine to be saved or to be burned directly into an EPROM. These ROMs can then be installed in a cartridge that will run the frozen state. An interesting concept that I’ll probably explore closer at a later date.
In one of the archives I downloaded I came across a PDF document containing scans of a text, also by Jason-Ranheim Company, that caught my eye. The document is titled “Auto-Start BASIC Programs” and describes a simple process to convert a Commodore BASIC program into an auto-starting cartridge for the C64 or the VIC20. I got curious and decided to try this — but first I wanted to explore how it works in detail.
The Kernal is the low-level “operating system” of the Commodore 64. The functions it provides are the basis for higher level routines in Basic and most other software running on Commodore’s 8-bit machines. In case of the C64 it comes in the form of an 8k ROM chip or as part of a 16k ROM chip in later versions. So the good news is, the Kernal is immune to corruption though any kind of malware. The bad news is, there is no easy way of updating or replacing it although Commodore’s original Kernal leaves a lot of room for improvement.
Kernal ROM from a C64.