Re: computer language used to program Mars Lander

[Paul Hsieh <websnarf@gmail.com>]

On Jul 14, 4:49 am, Nick Keighley <nick_keighley_nos...@hotmail.com>
wrote:
> On 14 Jul, 12:18, "jhc0...@gmail.com" <jhc0...@gmail.com> wrote:
>
> > Interesting article I came across on Slashdot:
>
> >http://developers.slashdot.org/developers/08/07/10/213211.shtml
>
> > They are using C at JPL to program Mars Lander and just about
> > everything now! Not Ada. Anyone got an explanation? I wonder also, do
> > they really mean C++ when they say C. In my experience, this is a
> > frequent, although disagreeable usage.
>
> contrary to popular expectations space probes are very
> conservative in their use of technology. Processors are old
> (and hence proven) memories are small (less vulnerable to radiation).

Indeed.  Ada is used by the military, because they have much more
sophisticated hardware in tanks and fighter jets.  I would expect that
perhaps NASA also uses it for the space shuttle.

> Remember if there's a bug they can't just upgrade the software.

They both can and *DO* do this.  They typically use VxWorks as the
main operating system, which comes with a fairly powerful command
console that makes it practical to actually debug, upload bug fixes
and reboot the systems all remotely.

> Well they can but it has to be mostly working for the download
> to work.

Its in the OS they use (and didn't make themselves.)

> These are essentially embedded systems and C is still popular
> in that world.

The small memory footprint and VxWorks environment probably makes C a
very attractive language for the NASA guys.  If they used Ada, they
would be slower, have much larger memory requirements and have a very
hard time debugging problems remotely.

> I suspect its a damn sight easier to prove C correct than C++.

Well by definition, since every C bug can be mapped to a C++ bug
(except for certain implicit type conversion failures; but that can be
dealt with by requiring that the C code also be compilable with a C++
compiler.)

--
Paul Hsieh
http://www.pobox.com/~qed/
http://bstring.sf.net/