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Curiosity has two identical 'Rover Compute Element' (RCE) computers, based on BAE Systems-made RAD750 CPUs.
The RAD750 single-board system uses IBM's PowerPC architecture and is in some ways fairly similar to the chip that powered the original iMac back in 1998. It's clocked at 200MHz and uses 256MB of DRAM, along with just 2GB of flash memory.
As with much of the equipment onboard Curiosity, it seems very old fashioned now, but then again the rover had to be specified as long ago as 2004.
It's not quite an old iMac processor, though. The RAD750 is specifically designed to withstand the huge amounts of radiation it faces on the Martian surface, as well as temperatures ranging between -55°C and 70°C.
Image credit: BAE Systems via Presse Citron