From: jgriffitts@spamcop.net   
      
   In article , Lex Spoon writes   
   >rk  writes:   
      
    . .   
      
   >> But on many spacecraft, the cost of an extra computer is "expensive"   
   >>and it's   
   >> worth some effort for the designers to spend some effort to eliminate   
   >> computers that aren't needed.   
   >   
   >I wasn't proposing to use more computers, but faster ones.   
   >   
   >This example was supposed to be illustrative.  A few decades ago,   
   >computers were expensive compared to programmers, and you tended to   
   >have lots of programmers hovering around each computer.  Try to   
   >picture that in a modern office -- it's hard, isn't it!  Nowadays it's   
   >the other way around, with computers all over the place.  Nowadays CPU   
   >time is cheap compared to programmer time, and so the appropriate   
   >design strategy is different: make things easy on the programmers, in   
   >order to conserve your most valuable resource.   
      
   That example works fine for office computers, but for "embedded"   
   applications there are usually some extra constraints.   
      
   A faster CPU or larger memory probably uses extra power and generates   
   more heat.  If you are really going upscale it may cost board space and   
   mass.  It possibly generates more electrical noise.  If IC geometry is   
   smaller it may translate to more radiation sensitivity.  If you're   
   getting close to current state of the art, the fast processor may not   
   yet be qualified for high-reliability applications, or it may even be   
   buggy.  Any of these factors could be critically important.   
      
   A faster CPU will usually also cost extra money.  Parts cost may be not   
   so important in most spacecraft but it can sure be a showstopper in some   
   applications.   
      
      
      
   You're thinking in terms of office computers, but we're discussing   
   deep-space probes.  I find it particularly easy to picture a spacecraft   
   computer under development with "lots of programmers hovering around"   
   it.   
      
   The programming of embedded processors requires a different mindset, and   
   often needs "obsolete" skills like optimizing for speed and memory use.   
      
   --   
                           Jonathan Griffitts   
   AnyWare Engineering                             Boulder, CO, USA   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)