How was time represented in the five AP-101 general-purpose computers of the Space Shuttle?

  • How many bits?
  • Encoded as a plain binary count? Split into hours, minutes, seconds, subseconds? Binary-coded-decimal?
  • How were "T-minus" times internally represented?
  • How often did one "tick" occur?
  • Theoretically, how long until the representation would roll over?

Related: What operating system(s) were used in the space shuttle?

  • 2
    $\begingroup$ I would guess the internal time would be integer microseconds past a certain epoch, and reference times like MET (identical to the countdown) would be derived from that with accurate offsets. Parsing to a YMDHMS time would happen from that integer. $\endgroup$ Commented May 19, 2019 at 6:10

1 Answer 1


This answer is highly suspect and should be independently verified. Also, it only talks about the timers and the Master Timing Unit:

The computers themselves:

  1. The IBM System/4_PI architecture was used as the system architecture for the AP101 source
  2. The timer feature on this system was a full word (so, presumably 32 bits) Source, but not a primary source
  3. The timer counted down with the clock frequency, but unfortunately I don't know what that was

The Master Timing Unit

This was the thing that held all of the information about MET and so on. It also was used for synchronization of the 4 + 1 AP101 computers on board. It was an atomic clock and was read by each AP101 once per second to make sure that the AP101s' clocks stay in sync. source

It also held the MET stuff, which was stored in its accumulators (sorry, I don't know the number of bits). This timing information was stored in days, hours, minutes, seconds, and milliseconds, up to one year until it overflowed. I do not know if that means MET was stored as days as well as hours etc, or as days/hours/minutes, nor do I know whether there were multiple accumulators. source

I'm interested in knowing more about the answers, so I provide my answer in the hopes that someone else can fill in the details.

  • $\begingroup$ This may be a definition thing, but I'm pretty sure the MTU wasn't an "atomic clock". It had crystal oscillators inside of temperature-controlled "ovens". There is some information about it here archive.org/details/… Also the problem was not that the MTU (or GPC) overflowed at one year, it was rather that one of the devices rolled to Day 366 and the other rolled to Day 1, and then they detected a mismatch. Hence the name for the problem Year End RollOver YERO. The link has information about the accumulators. $\endgroup$ Commented Aug 1, 2019 at 22:02
  • $\begingroup$ There is also some good information in the flight rules about the MTU here: archive.org/details/flight_rules_generic/page/n1329 $\endgroup$ Commented Aug 1, 2019 at 22:15

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