Mazen Gomaa's CMOS Take on Ben Eater's Eight-Bit CPU Is a Fully-Fledged Computing System
Based on Eater's famous SAP-1, this multi-board eight-bit marvel includes keypad inputs and display outputs.
Maker and vintage computing enthusiast Mazen Gomaa has become the latest to build a classic eight-bit computer based on Ben Eater's famous design — adding a few new features and tweaks along the way.
"A rebuild of Ben Eater's SAP-1 eight-bit processor architecture," Gomaa explains of his project, "but with mostly CMOS chips and some additional functionalities. The whole [purpose] is educational for those who really wish to understand how a processor works on a very basic and easy to understand and tinker-with scale. This thing can teach you all about logic, electronics, data, and even what 'compiling' really mean and machine language, since you can write programs for this computer in 0s and 1s."
The project uses an eight-bit discrete-logic eight-bit CPU design by Ben Eater which has served as a jumping-off point for a wide variety of retro-style computer projects. Where Eater's original used primarily 74-series TTL chips for its hardware, though, Gomaa's reinterpretation switches to CMOS parts instead. The breadboard is also gone, in favor of prototyping boards with hand-wired connections underneath.
There are a few improvements along the way, too. While Eater's design is more of a CPU than a fully-fledged computer, Gomaa's version adds more of the features you'd need to declare it a complete system — including keypads for entering instructions and switching memory locations, a dot-matrix display for reading memory contents, and a bootloader which populates the RAM with the contents of a ROM at boot time.
"This computer draws about 500mA at maximum current consumption and 310mA in idea with a clock speed that can go up to 100Hz," Gomaa writes. "Terrific, I know, right? With 128 bits (8×16) of RAM and a 64 bits (8×8)[of] ROM and [a] 10-instructions set. I'd like to add a bigger memory and more instructions in the future for more complex programs."
Gomaa has published images and a parts list for the build on the project's Hackaday.io page, but has not shared a wiring diagram. Eater's original design, meanwhile, can be found on his website.