Commodore C64 PLA Die
An Outsanding Paper: "The C64 PLA Dissected"

An Excellent write-up on the Programmable Logic Array (PLA)  by Thomas 'skoe' Giesel.  This PLA is found in the Commodore C-64 of the early 1980's and  was the glue that held the video "VIC" chip, the microprocessor and the memory together to make a home computer. 

Those familiar with the PLA know that just as it gave, it also took away and at one time was the single highest failing component due to poor passivation - the final step in chip production meant to protect the die. In  section 4.3 Mr Giesel quotes Dam Morris that the failures were due to greater than 4% Boron in the Low Temp Oxide (probably activated further by the power density as these chips got a little toasty). He also goes to great length to discuss the geometric features of the chip and the differences in NMOS/HMOS processes.

Thomas did a great job in telling the story of a single chip, and I would say that almost every chip or piece of hardware or chuck of software has a story as well;  the total story of a single C64 could fill a book.  I am amazed at how the retro movements going on have revived some of the stories of the old days, sadly the players are taking their leave as time catches up.

The C64 PLA Dissected

Here the initials of one of the chip layout people: Joan Brenneke


Total PLA die:


Size differences between HMOSI and HMOSII processes

channel lengths.JPG

Block Diagram of the Input Cell


block diagram.JPG

input die.JPG


pla pinout.JPG

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bilherd's picture

Cluey fellow, Skoe..
At the time the C64 was new and at its zenith, the PLA was made out to be this mystical weird gizmo "where the magic happened". I guess it did, in a way - certainly it was the chip that made the C64 simple and cheap enough to be the success it was. Although my personal C64 was a 250407A board, the PLA was the original Signetics N82S100N chip, and is still working to this day (touchwood!)  I'm astounded that it was devised in 1975 - the bronze-age really of the microchip industry. 
Skoe goes further than just this authoratitive, brilliantly researched and deduced paper - HERE, he goes into how to manipulate the PLA and system buss signals into deducing kernal access without access to the HIRAM signal, and furthermore how to play switcheroo with ROM and address space, and provide a wire-free external kernal cartridge. This cartridge has found flesh in his brilliant easyflash3 cartridge. I have one - it's BRILLIANT!!!!!