Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!rice!titan!phil From: phil@titan.rice.edu (William LeFebvre) Newsgroups: sci.space.shuttle Subject: Re: Shuttle Computer Info? Message-ID: <3234@kalliope.rice.edu> Date: 8 May 89 21:52:28 GMT References: <24055@agate.BERKELEY.EDU> <3227@kalliope.rice.edu> <2645@ndsuvax.UUCP> Sender: usenet@rice.edu Reply-To: phil@Rice.edu (William LeFebvre) Organization: Rice University, Houston Lines: 49 In article <2645@ndsuvax.UUCP> ncoverby@ndsuvax.UUCP (Glen Overby) writes: >The flight software (both on the ground and on-board) gives me the >impression of being crufty. On the failed attempt last Friday, the >countdown was halted at 31 seconds; thats exactly when control of the launch >is turned over to the on-board computers. So why do they decide to hold the >launch at that point? Most of what causes the countdown to hold will have >existed for several minutes at least, so why don't the ground computers >decide to hold the launch then, or have the on-board computers say "I'm >gonna hold when I get control". I don't really understand how that qualifies the software as "crufty". It's just The Way It Is Done. If anything happens between T-5 minues and T-31 seconds that requires a hold, the hold won't happen until T-31 seconds. I remember for one of the recent flights (26, I believe), one of the launch controllers called that there would be a T-31s hold. This was around T-3 mins. But during the next minute, some people decided that the hold was not necessary. In other words, the situation that a computer had flagged as violating launch criteria was determined by the launch crew as OK, and the count was not held at T-31s. If the computer had held right away, it would have really disrupted the flow. [This following paragraph is my own ideas and theory:] Probably part of the reason they do things that way has to do with the wholw philosophy behind the countdown. Certain things are scheduled to take place at certain times in the count. If you can guaranteed that the could will proceed from T-5m to T-31s without interruption, then it seems to me that planning and executing procedures becomes much easier. I seriously doubt that they do it that way because of "crufty" software! >On another subject; I recall reading that the Shuttle's computers were so >stuffed that to add something, something else had to be removed. With a >16MB address space, it would seem that more memory could be added and this >could be avoided. I know this is true for ascent. The primary software (PASS) takes up all the available memory in the GPC (no virtual memory here). I virtually positive that that is NOT true for the software they run on orbit (it doesn't really have to do much anyway) and I believe that there is still a fair amount of room in the program they use for entry. They keep promising the great GPC upgrade: among other things it should include 256K words of battery backed static RAM. William LeFebvre Department of Computer Science Rice University "Flight, DPS.......GPC 4 failed to sync."