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The PLA20V8 is a aftermarket replacement for the C64's failure prone PLA custom chip. The PLA20V8 is designed to be an easy to install low-cost replacement for the original chip.
The PLA20V8 consists of two GAL20V8B chips from Lattice Semiconductor mounted on a small PCB. It's a programmable logic chip that, in certain modes, acts a lot like the C64's PLA. It is suspected that the GAL chip is the most PLA-like chip commonly available without resorting to FPGA. One limitation of the design is that a single GAL20V8B can't be programmed with all of the PLA chip's logic, so a pair of chips is required. The GAL20V8B is not still in production but is available on the secondary market in vast quantities.
The internal logic of the PLA20V89 is based upon the The C64 PLA Dissected document by Thomas Giesel. Alternate kernal implimentations such as JiffyDOS work just fine. Even the Epyx Fastload cartridge — which has a reputation for being a PLA chip torture test due to its design — works without issue with the PLA20V8. The only known compatibility issue is with the C64 Assy 250469 Rev. 4 board, which is expected, because the PLA was combined with several other chips into a single custom 64-pin ASIC.