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.