Wirt writes: > While no one could begin to count the number of computers that will be > used on tomorrow's flight, the number that were used on his first is well > known. The total count was zero. Of course how much guidance do you need on a cannonball :-) There was a quite interesting article many years ago in the Journal of the ACM that described the 5 main computers that controlled Space Shuttle flights at that time (and knowing NASA's conservatism, probably still do). Four redundant computers running the same software, plus a fifth backup running software with essentially the same specification but it was required to have been written by a different company. These systems were/are best described in units of KB and KHz rather than the MB/GB and 100s of MHz typical even of today's cheapest PCs. Different programs for different phases of flight are loaded from a tape drive under manual operator control. Each computer drives separate servos for the flight control surfaces, so that if one goes berserk and starts trying to push flight controls the wrong way, the other two systems will be able to overpower to actions of the malfunctioning one, which eliminates the need to combine the outputs of the multiple systems or even to accurately detect the malfunction of one computer. > TTL, the first truly successful small-scale integration series, didn't > appear until about 1964-1965, and these circuits only had a few dozen > components per chip, and yet TTL circuits prospered well into the 1980's > (and you can still buy them). In fact the first PA-RISC systems that shipped to customers (the HP3000 series 930 and HP9000 series 840) did not use microprocessors as their CPUs. These first PA-RISC processors were implemented as five boards full of discrete AS series TTL logic chips, still fundamentally the same as the ones Wirt describes (the 950 and later models did of course have microprocessor implementations). G.