C64 PLA Replacement

The PLA chip (906114-01) used in the Commodore C64 is a generic 82S100 gate array with custom programming. Its logic functions were dumped and reverse engineered by the community and are available from a variety of sources [1], [2]. They're reused for this particular PLA replacement in the FPGA Arcade 28 pin DIL CPLD board.

Two approaches exist to build such a PLA replacement:

1. Implementation based on a truth table with 2¹⁶ entries, each entry consiting of 8 bits and programmed into an EPROM chip.

2. Reverse engineered logic equations programmed into a PLD.

Both approaches result in the same logic functionality when implemented in a CPLD. Since equations are more common for CPLDs, I chose this implementation style for the final design. However, there's a variant for the truth table available which has been verified in simulation but not in real C64 hardware. Following are descriptions for both of them.

We have run simulations proving that the equations perfectly match the truth table, so both compiled outputs are effectively identical in the C64 implementation discussed here.

Equation Based Desgin

The equation based design builds on the ABEL sources found in [3]. They were ported to VHDL and implement the core logic in file src/pla_c64-equ.vhd. It is instantiated in the top level hierarchy pla_c64_mj.vhd which contains the pin mapping and other glue logic that is required for the FPGA Arcade 28 pin DIL CPLD board.

A Quartus II project is available in syn/pla_c64_mj_equ. It's quite straigth forward and the resulting programming files are included in the archive. I've tested this design inside a real C64 and it worked for me. However, I cannot guarantee that it will match each and every fancy piece of software in the wild. Any feedback and suggestions are highly appreciated.

Truth Table Design

This one uses the same top level file as the equation based design. Instead of the logic equation,the ROM table wrapper hierarchy from src/pla_c64-rom.vhd is instantiated in this case. The truth table is supplied by a separate file in rom/rom_c64.vhd which basically contains a huge case statement for this purpose. The released file was generated from C64.BIN [2] with the command

hex2rom -b C64.BIN rom_c64 16b8a > rom_c64.vhd

A Quartus II project is available in syn/pla_c64_mj_equ. Truth tables for other commodore PLAs are available from [2], so it should be possible to replace these with the generic design as well.

Version 1.0 of the design and programming files can be downloaded here: pla_c64_28dil_1_0.zip
Parts of the designs are copyright 2009, Arnim Läuger.
Credits go to the creators of the equations according to [3].

Please note:
THE DESIGNS ARE PROVIDED AS THEY ARE. THERE IS NO WARRANTY HOWSOEVER THAT THEY WILL WORK WHEN PLUGGED INTO YOUR SPECIFIC PCB. YOU ARE SOLELY RESPONSIBLE FOR ANY HARM OR DAMAGE THEY INDUCE WHEN PUT INTO OPERATION.

References:
[1] http://www.zimmers.net/anonftp/pub/cbm/firmware/computers/c64/
[2] http://vic20.de/html/eprom_pla_8296_und_c64.html
[3] http://www.zimmers.net/cbmpics/cbm/c64/pla.txt