Comments by "Mikko Rantalainen" (@MikkoRantalainen) on "MIT OpenCourseWare" channel.

  1. And the software is double expensive because for redundancy they have the software running on multiple computers. I couldn't find info about F-22 specifically (possibly classified) but according to some sources Airbus A320 run computers with i386 and m68k compatible hardware in parallel and both CPU architectures must compute the same results or computer is considered faulty. If I remember correctly, NASA had some programs that were designed and implemented by different teams to avoid logic errors – software designed and implemented by different teams of programmers were run in parallel computer system was considered faulty if the results of different programs were not identical. I wouldn't be surprised if these jets were running at least two variants of software in parallel on two different architectures in redundant mode just to avoid software bugs. That requires minimum of 4 computers executing the same program: for example, computer A executes x86 version of 1st team program, computer B executes x86 version of 2nd team program, computer C executes m68k version of 1st team program and computer D executes m68k version of 2nd team program. If one of the computers disagree with the rest, the majority result is considered correct. If two computers disagree the whole computer system is considered faulty (but you can still somewhat trust the result that at least 2 computers agree on).
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  2. It's great that they could tell about the plane even this much. Much of the stuff about F22 is classified.
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