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Writing Code for Spacecraft
CowboyRobot writes "In an article subtitled, "And you think *your* operating system needs to be reliable."
Queue has an interview with the developer of the OS that runs on the Mars Rovers. Mike Deliman, chief engineer of operating systems at Wind River Systems, has quotes like, 'Writing the code for spacecraft is no harder than for any other realtime life- or mission-critical application. The thing that is hard is debugging a problem from another planet.' and, 'The operating system and kernel fit in less than 2 megabytes; the rest of the code, plus data space, eventually exceeded 30 megabytes.'"
> "The operating system and kernel fit in less than 2
> megabytes; the rest of the code, plus data space,
> eventually exceeded 30 megabytes." This should be used as
> the example for efficient coding
You've GOT to be kidding, right? 2 meg of OS code? That's ULTRABLOAT compared to most spacecraft. In fact, for the vast majority of the space age, that would have exceeded the resources of the computer by several orders of magnitude.
I've done this kind of programming for a living (for 10 years, moved up to controls design) but the last system I programmed for has 372k of memory, total. That includes data, code, OS, everything. Runs at 432 KIPS. And it performs what it probably one of the most complex in-flight autonomous control operations ever.
Most are even more restrictive. For example, 8K of PROM and 1k of volatile memory (and 28 WORDS) of non-volatile memory. This more than adequate for most applications, if you do it right.
Many spacecraft OS's are more akin to this:
hardware interrupt
external electronics power up processor.
external electronics set PC = 80hex
run
{execute all the code}
halt
power down
Once every 1/4 a second for 15 years.
The project I am currently working on uses VxWorks (and so we were quite interested in the Mars Rover problem) and it's so bloated with unnecessary features it's absurd. This is not a Windows box, it's a spacecraft processor.
I can't argue with the 30 meg of data space. Using the memory as a data recorder would be quite useful and a good picture takes a lot of space. But it's alarming to me that you could figure out how to waste maybe 4-5 meg on code. If you started with a bare home-brew OS, I would guess (and I get paid for this sort of guess) that you could do the entire flight code in 512K, with maybe 8k of data space, excluding the science data.
Only recently have space-qualified rad-hard processors with this kind of capability become available. Until then, if you said you needed 2 meg for the OS alone, you would have gotten fired on the sopt and referred to mental health professionals. The availability of these processors enabled people to use high-level languages with tremendous overhead (like C++) to be used. And this was only done for employee retention purposes during the bubble. For years it was done at the assembler or even machine level. It's still not at all uncommon to do, and we've done MANY flight code patches, with only a processor handbook, an engineering paper pad, and by setting individual bits one-by-one.
Brett