Teaching an Old PET New Tricks

Nearly half a century after it was first introduced, the Commodore PET isn’t going to impress anyone with its processing speed. Its 6502 processor stacked up well against the competition back in the day, but that day has long since passed. Nowadays these computers are used by enthusiasts who want to relive the experience of playing a PETSCII game or typing in a BASIC program from a magazine, but they have little practical value.

Even so, Ted Fried has a Commodore PET 4016 and wants to see just how far it can be pushed. This is a game without rules, so Fried had no qualms about swapping out the original processor for an MCL65+ 6502 CPU drop-in replacement board first. This board uses a Teensy 4.1 development board with an Arm Cortex-M7 processor and 1MB of memory, so it can do a lot more than just faithfully replicate the functionality of a real 6502.

However, simply ramping up the clock speed of the processor isn’t going to speed up a PET — at least not much. Other system components, especially RAM and ROM, have strict limits to how fast they can operate before timing constraints are violated and undefined behaviors start to break literally everything. The MCL65+ can get around at least some of this by also emulating system RAM and ROM with its onboard memory that is far faster.

This is accomplished via three acceleration modes: Mode-1, Mode-2, and Mode-3. Mode-1 perfectly replicates a stock 6502 with cycle-accurate reads and writes. Mode-2 accelerates reads, while Mode-3 accelerates both reads and writes by exclusively using the Teensy’s own memory, allowing it to run at up to 900MHz, which is 900 times faster than a normal PET.

Fried demonstrated all of these modes by running a variety of games and BASIC programs under each. Needless to say, the accelerated modes ran things far faster than an unmodified PET ever could. This just might be the world’s fastest Commodore PET ever to exist!