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NASA Switches Curiosity Rover To Backup Computer Following Glitch (extremetech.com)
NASA has switched its Curiosity rover over to its backup computer system after the main system started experiencing errors last month. "Many NASA spacecraft and surface missions have redundant systems built-in," reports ExtremeTech. "Once they've launched from Earth, there's no way to repair damage to critical systems, so it makes sense to double-up on the vital components. That includes Curiosity's computers, which were designed specifically for the harsh environment on Mars." From the report: The rover has a pair of identical brains running a 5-watt RAD750 CPU. This chip is part of the PowerPC 750 family, but it has been custom designed to survive high-radiation environments as you'd find on Mars or in deep space. These radiation-hardened CPUs cost $200,000 each, and NASA equipped the rover with two of them. Each computer also has 256 kB of EEPROM, 256 MB of DRAM, and 2 GB of flash memory. They run identical VxWorks real-time operating systems. When Curiosity landed on Mars in 2012, it used the "Side-A" computer. However, just a year later in 2013 (Sol 200), the computer failed due to corrupted memory. The rover got stuck in a bootloop, which prevented it from processing commands and drained the batteries. NASA executed a swap to Side-B so engineers could perform remote diagnostics on Side-A. In the following months, NASA confirmed that part of Side-A's memory was unusable and quarantined it. They kept Curiosity on Side-B, though. With Side-B experiencing problems preventing the rover from storing key science and engineering data, NASA switched Curiosity back to Side-A while it investigates the problem, which it can only do when the other computer is active. "NASA hasn't said how much of Side-A's RAM is bad, and it only had 256MB to start, but the team does intend to move Curiosity operations back to Side-B if possible," the report adds. "For now, the mission is functioning normally on Side-A."
Even if radiation on Mars surface is limited, the computers still have to survive the trip from Earth to Mars. The problem is that the cosmic radiation causes permanent damage to the crystalline structure of the semiconductors, building up over time until they stop working completely. COTS processors simply wouldn't survive, and redundancy doesn't help much if they are all getting damaged at the same rate.
Mythoughts as well.
I used to have a Dell CPx laptop with 256 MB in the early 2000's, and that worked quite well. Not to even mention Windows 3.1 on 4 MB.
Just out of cutiosity, I googled the RAM requirements for vxorks, and Version 6 from 2004, has the following requirements:
VxWorks CISC processors require 1 MB of RAM for a development system that includes the standard VxWorks features, such as the shell, network, file system, loader, and others.
RISC processors typically require more RAM space: 2 MB of RAM is the minimum; 4 MB is encouraged. For a scaled-down production system, the amount of RAM required depends on the application size and the options selected.
So 256 MB leaves most of the RAM available for the applications.