Computer for the Moon

As the IL's Apollo project entered its initial design phase, the engineers began thinking about their computer. They started, logically enough, with the Mars probe computer and a good dose of Polaris. After a few prototypes, a computer design emerged that was designated ''Mod 3C.'' Typical computers of the time occupied buildings and burned enough electrical power to light a small town. These big machines, in universities, military installations, and corporations, tended to be ''numbers in, numbers out'' crunchers. But the Mod 3C, like MIT's Whirlwind, was a departure. In today's terms it resembled a ''microcontroller,'' the microprocessor embedded in everything from cell phones to automobiles, more than it did a scientific computer. It occupied a cubic foot rather than a tiny chip, but it shared with microcontrollers a modest calculation capacity melded to numerous, sophisticated channels for quickly getting data into and out of the machine. Like a microcontroller, the Apollo computer included an inter-ruptible processor so events could command the computer's attention in real-time. It had a ''night-watchman'' circuit to keep the computer from crashing and locking up (a feature still included in many embedded computers today as a ''watchdog''). In today's terms, the Apollo computer was ''embedded.''

The original Apollo machine used 16-bit words for its data and instructions, adequate for the local control functions, but the navigation calculations required ''double precision,'' more bits with more numerical precision, a feature implemented in software. The computer had two types of memory, ''erasable'' and ''fixed,'' what today are called RAM and ROM. Erasable memory was used as a kind of scratchpad for data and calculations while the programs were running (or for temporary programs), while the fixed memory contained the programs themselves. All was ''core'' memory;that is, bits were stored by magnetic fields on tiny cylindrical cores. The computer comprised 1,700 transistors, each in their own metal can, and nearly 20,000 metal and ferrite cores for memory.

Over the course of 1962, IL engineers refined and then actually built Mod 3C as a large prototype to enable further experiments. Eventually it had 12,288 words of permanent storage, 1,024 words of erasable memory, and a 16-bit architecture with 8 instructions. It included a variety of counters, pulse outputs, interrupts, and input-output registers for interfacing to the rest of the vehicle. For a control computer such as this, the interfaces were key—the machine would have to not only perform calculations, but also take data from the inertial platform, read the optics' angles, torque the gyros, fire the thrusters and the rocket engines, and, of course, interact with the astro-nauts.1 Often, these interfaces embodied the relationships with other organizations, companies that built sensors or engines. Most of the interfaces were analog, so the computer needed ways to accurately convert the discrete digital data to the continuous analog world and vice versa.

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