> -----Original Message-----
> From: [EMAIL PROTECTED] On Behalf Of Rich Kulawiec
> Sent: Monday, November 02, 1998 6:42 PM
> In fact, that's one of the reasons that Unix has survived: because
> it separates kernel, shell, utilities, etc., in ways unlike some
> competing OS's, it *can* be changed and ugraded bit-by-bit. Imagine
> how different it would be if changing shells required modifying
> the scheduler, or using a different mailer required modifying the
> filesystem code, and so on. Heck, don't imagine it: go look at the
> architecture of some other OSs that have had a much shorter lifespan.
I have looked at most widely-used OSs; in fact, I've taught graduate
courses on OS internals since 1970. All of them that I know were
written the way you describe Unix. Modularity, low coupling,
high cohesion, black-box interfaces, data and algorithm hiding --
these have been known design principles since Andy van Dam taught me
programming in 1965. I was active in the HASP project, in which a
bunch of people at universities and companies all over the country
worked together to replace the Reader/Interpreter in OS/360,
essentially replacing the shell. A bunch of my students did a
term project that replaced the task/process scheduler and VM page
manager of CP-67, later MVS; I myself replaced its file system
with one based on memory-mapping. Of course, those systems weren't
up to the level of modularity of something like Choices or Chorus,
but I'd say they were about as modular as Unix.
There are systems that can have the kind of interdependencies
that you describe, sometimes called "crystal clock" architectures.
These divide the code into "major cycles" and "minor cycles"
based on some kind of mechanical timing like a rotation or
oscillation. I don't know of any operating systems built that
way, but some of the systems that are essentially include their
own OS, run on a bare machine.
So what are these "other OSs" that you're comparing to Unix?
Bob Munck
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