The C64 PLA Dissected

The programmable logic array (PLA) in the Commodore 64 (C64) is used to create chip select signals from various other signals, e.g., from the current address. These signals control which chip is to be connected to the data bus. Therefore the PLA is responsible to implement the memory map of the C64.

If the PLA is broken, the CPU and the VIC-II direct memory access (DMA) can not access the right memory and I/O devices anymore. In this case some chips can not be selected or more than one chip is active at the same time. A total or partial malfunction of the computer is the result. If a PLA is replaced with a part which does not meet certain timing or electrical constraints of the chips connected to it, the computer becomes unstable, possibly depending on temperature or hardware extensions used or may refuse to work at all.

The logic implemented in the PLA defines the memory map. The logic equations are described in chapter 2, among a short overview of previous reverse engineering work related to it. Also the meaning of all signals connected to the PLA for the working of the C64 is explained there. Appendix A contains the actual memory maps in a readable form as a reference.

A list of PLAs found in C64s and measurements on some of them are shown in chapter 3. This chapter also describes which parameters are important when a PLA is replaced with a different part. Most C64s shipped with a PLA made by Commodore Semiconductor Group (CSG), better known under their original name MOS Technology (MOS). A PLA of this type was opened, photographed through a microscope and completely reverse engineered for this article, down to the transistor level. Chapter 4 contains information about the inner workings of the PLA, a redrawn chip layout and schematics. That chapter also contains some background information by former Commodore/MOS engineers.

An actual implementation of a PLA replacement called realPLA is introduced in chapter 5 which can be built with parts being in production at the time of writing. This implementation aims at highest compatibility possible while using low-cost parts only. The design files for the realPLA are available under a Creative Commons license. Finally, chapter 6 gives a conclusion about the findings in this document. It also evaluates the compatibility of different PLA replacements.