Ted Fried's Teensy-Powered MCL65+ "Downgrades" an Apple II Into a Fully-Functional Apple I
If you hanker for a taste of Woz's eight-bit genius, an Apple II and Fried's Teensy 4.1-based "downgrade" board will get you there.
Embedded hardware engineer and vintage computing enthusiast Ted Fried is back with the aftermarket modifications for computers long since vanished from store shelves — but this time it's a downgrade rather than an upgrade, turning the relatively common Apple II into the functional equivalent of a rare Apple I.
Released in 1976, the Apple Computer — which would later be known as the Apple I, to differentiate it from its successor — would spend little over a year in production but had an oversized impact on the history of personal computing. Created by Steve Wozniak and marketed by Steve Jobs, the Apple I — an eight-bit single-board computer based on the MOS Technology 6502 processor — sold for $666.66 to an eager audience, but its limited production lifespan and a buyback program which saw boards destroyed in exchange for a discount on an Apple II mean working models are rare as hens' teeth today.
It's possible to emulate an Apple I, of course, but Fried decided to take a different approach to experimenting with the machine which launched the Apple empire: downgrading the considerably more common Apple II into its predecessor. "This project uses [my] MCL65+ to convert an Apple II into an Apple I," Fried explains. "The MCL65+ is a drop-in replacement the computer’s MOS 6502 which can emulate the microprocessor and much more."
In this case, the MCL65+ — powered by a Teensy 4.1 development board — provides both an emulated version of the MOS 6502 CPU which it replaces and the original Apple I programmable read-only memory (PROM) and RAM chips. "[These] contain the famous 256 byte Woz Monitor, 8kB of RAM, and a few conversion routines for differences in video display and keyboard handling. All keyboard and video I/O are also echoed to the Teensy’s UART so that code could be downloaded to the Apple I by simply cutting and pasting it into a terminal program."
This is far from Fried's first efforts at putting modern hardware into vintage systems. Late last month he unveiled the world's first Motorola 68000-powered IBM PC, again using a Teensy 4.1-powered emulated CPU replacement, and last year showcased the MCL64 Commodore 64 accelerator and MCL86jr FPGA-based IBM PCjr upgrade board.
More information on the Apple II to Apple I conversion is available on Fried's website, while the source code has been published to GitHub under an open-source license.