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It's GPC which is short for General Purpose Computer. THe orbiter have five identical GPCs, 4 running an redundant software called the Primary Avionics Software System(PASS) while the 5th is running a software called the Back-up Flight Software(BFS).This being the case, what functions are GPS-critical and which can be completed by the crew? I'm sure the lists are extensive, I'm just curious about the major items.
On a regular basis the GPCs vote among themselves and then run it by the BFS when the BFS GPC is running. If any GPC shows discrepancies during the voting, it is automatically declared faulty and a light on the GPC Matrix on panel O1 is illuminated to indicate which GPC has been declared faulty.
This gives MCC and the crew a chance to review the GPC problem and take the appropriate action such as shutting it down.
Edit: To add, the GPCs control everything on the orbiter. If you lose them all, including the final resort, the BFS, that's it, you're not coming home, especially if they die on-orbit as the only way to fire the OMS engines is by the PASS/BFS. There's no other way. Complete GPC-loss means loss of crew, especially during dynamic phases such as ascent and entry. Even bail-out is not an option as bail-out require the orbiter to be in controlled gliding flight which can only be done if the GPCs are working,
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