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I was reading about Cassini and learned that it used a CPU architecture I'd never heard of before, called MIL-STD-1750A. This got me thinking.

What are the most common (or otherwise notable) CPU architectures used in spacefaring flight systems? What about other parts of those computer systems (eg, core memory being used for its radiation resistance, even as recently as in the space shuttle)? Why were these systems chosen, and what advantages or disadvantages do they present over more conventional consumer systems?

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  • $\begingroup$ Core memory is out for use in space probes since decades. It is not possible to build a small lightweight core memory of at least one megabyte suitable for the use within a space probe. $\endgroup$ – Uwe Dec 20 '18 at 22:40
  • $\begingroup$ Core memory was used in the Space Shuttle about three decades ago, that is not recently. $\endgroup$ – Uwe Dec 21 '18 at 9:10
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    $\begingroup$ Nice question, I asked a more specific follow-up about GPUs in space. Thanks for the food-for-thought :). $\endgroup$ – Magic Octopus Urn Dec 21 '18 at 15:56
  • $\begingroup$ @Uwe I meant that the early Shuttle usage was more recent than I expected, since core memory in general fell out of favor much earlier, in the mid 70s $\endgroup$ – charliegreen Dec 22 '18 at 22:23
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http://www.cpushack.com/space-craft-cpu.html has a fairly comprehensive list of the CPUs used in various probes. Just listing the CPUs:

  • General Electric 18-bit TTL
  • Intel 8086
  • RCA 1802
  • Intel 80386
  • 2901 (bit-slice component)
  • Intel 80486
  • RAD 6000 (IBM RS/6000)
  • 1750A
  • RAD 750 (IBM PowerPC 750)
  • Mongoose V (Mips 3000)

This site doesn't talk about memory, but I guess that most of these use hardened versions of normal RAM chips.

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Why were these systems chosen, and what advantages or disadvantages do they present over more conventional consumer systems?

In general, spacecraft use radiation-hardened computers. CPUs are usually custom versions of architectures used on Earth (from the list in Eugene's answer, e.g. the RAD 6000 is a radiation-hardened version of the RS/6000 CPU, the RAD 750 is a radiation-hardened version of the PowerPC 750 CPU).

Creating a radiation-hardened CPU is a big investment for a very small market, so not every architecture gets a rad-hard variant. The chip has to be redesigned:

  • a different substrate is used, which means you can't use the standard production line
  • extra functions are added to increase radiation tolerance (e.g. memory scrubber circuits to check for bitflips in memory) so you can't reuse the design

The advantage is these systems are much more reliable when used in space than their consumer counterparts.

The disadvantages are cost (easily 1000x more expensive), and lower performance: rad-hard CPUs are several generations behind consumer CPUs. In part, this is because the smaller the transistors etc. are, the more difficult it is to make the chip reliable in a radiation environment.

The RAD 750 was introduced in 2001 and first flew in 2005. Unit cost: \$200,000. It's a derivative of the PowerPC 750 family, introduced in 1997 with a unit cost in the region of \$200.

It's common for bugs to be found in the first generation of a new CPU design. So for critical applications, it's smart to hold off on using a new design for a few years until the flaws have been found.

the high price is mainly due to radiation hardening revisions to the PowerPC 750 architecture and manufacturing, stringent quality control requirements, and extended testing of each processor chip manufactured.

The space industry was an early adopter of solid-state memory devices, to replace the tape recorders used in earlier spacecraft. They mostly skipped harddisks, which are sensitive mechanical instruments that are easily damaged by the vibrations of a launch. The first consumer SSD was introduced in 1991. SAMPEX was launched in 1995. Cassini, launched in 1997 was the first interplanetary mission to use an SSD.

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    $\begingroup$ This is the central issue. I am currently designing an optical system for a client to be sent to a solar system moon with about a 2 year journey. It's amazing how much the need for radiation hardening cuts down on the materials one can design with. That's just simple glass choices. Then there's the CCD camera: the only available ones are generations behind consumer goods. The abilities to withstand radiation the shaking throughout launch are the two biggest design hurdles. So yours is the best answer here: "what are some notable computer systems?" A: radiation hardened ones $\endgroup$ – WetSavannaAnimal Dec 29 '18 at 0:39
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«Аргон-11С» for Zond, L1, Soyuz spacecrafts https://habr.com/company/ua-hosting/blog/390035/ https://habr.com/company/ua-hosting/blog/276813/

The specifics of the special computers were as follows: the architecture of the machines being created corresponded to some specific functionality, the command system included special commands to perform a specific action

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