The shuttle uses 4 separate 1.2MHz IBM AP-101 operating in lockstep / redundant systems, with a 5th running an independent system. I assume the fifth is setup in case of software failures.

http://en.wikipedia.org/wiki/IBM_AP-101

http://en.wikipedia.org/wiki/Space_Shuttle

PS: It's easy to forget just how old and hacked together the Space Shuttle is: Historically, the Shuttle was not launched if its flight would run from December to January (a year-end rollover or YERO). Its flight software, designed in the 1970s, was not designed for this, and would require the orbiter's computers be reset through a change of year, which could cause a glitch while in orbit. In 2007, NASA engineers devised a solution so Shuttle flights could cross the year-end boundary.[38]