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[Ken Shirriff] does some of the most interesting teardowns. This time, he’s looking at a French-built minicomputer called the Mitra 125 MS from around 1980. In particular, it was the computer inside Spacelab, a European lab that could fit in the back of the Space Shuttle.
As you might expect, the computer doesn’t contain a microprocessor. Instead, it is a series of cards and, in this post, [Ken’s] looking at the ALU that allows the computer to perform math operations.
The Mitra was a descendant of a 1971 computer, and the “MS” indicated it was a military-grade variant of the computer. Spacelab had three of these. One operated the lab, another handled experiments, and the third was a backup.
At the heart of the board was the 74181 ALU. Well, actually, the 54S181, which was the military-grade high-speed part. Each chip handled four bits of addition, subtraction, or a few other logical operations. No multiply. No divide. Oddly, no right shift, either.
Although the computer is a 16-bit machine, it has a 32-bit ALU built from eight ‘181 chips, spread across several boards. Multiplexers allow the ALU to read different operands, and the result can go several different directions, also.
By 1991, these computers were obsolete. The IBM AP-101SL replaced the Mitra. It was basically the Shuttle’s new AP-101S computer with special microcode to pretend to be a Mitra 125.
We love [Ken’s] teardowns both at the macro and micro level.
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