Spacelab Mitra – A detailed teardown of the French-built Mitra 125 MS from around 1980 shows how Spacelab’s Space Shuttle-era minicomputer performed math—without a microprocessor—through a carefully built ALU made from military-grade “181” chips.
When Ken Shirriff cracked open a French-built minicomputer called the Mitra 125 MS from around 1980, the surprise wasn’t just what he found—it was what he didn’t. The machine didn’t rely on a microprocessor at all.
The Mitra 125 MS was the computer inside Spacelab, a European laboratory that could fit in the back of the Space Shuttle. Instead of a single chip doing the work, it used a series of cards—and Shirriff’s focus in this teardown was the ALU, the part that lets the computer perform math operations.
The Mitra itself was a descendant of a 1971 computer, and the “MS” designation pointed to a military-grade variant. Spacelab carried three of these computers: one to run the lab, one to handle experiments, and a third kept as a backup.
At the center of the ALU board was a 74181 ALU—though the military-grade high-speed version used here was the 54S181. Each chip handled four bits of operations, including addition, subtraction, and a few other logical functions. What the design didn’t do stands out just as much as what it did. There was no multiply. No divide. And oddly, no right shift.
Even with that limitation, the system could still operate as a 16-bit machine. It did it with a 32-bit ALU assembled from eight ‘181 chips, spread across several boards. Multiplexers let the ALU read different operands, and the result could be routed in several different directions.
Time eventually moved on. By 1991, these computers were obsolete. The IBM AP-101SL replaced the Mitra, effectively using a Shuttle-era AP-101S computer paired with special microcode designed to pretend to be a Mitra 125.
Shirriff’s teardown keeps swinging between the macro and the micro, but it lands on the same human point: even in space-grade hardware, the most important decisions can live in the quiet wiring of an ALU—and the trade-offs show up immediately when you look closely.
Spacelab Mitra 125 MS ALU 54S181 74181 Space Shuttle minicomputer IBM AP-101SL IBM AP-101S Shirriff teardown 16-bit machine