Re: Why couldn't an operating system be written in ada

[Mark Eichin <eichin@cygnus.com>]

>   A neat idea whose time is long since gone.  The OS "wars" have been fought
>   and largely lost.... :-(

Well, the users lost anyway :-}  However, research certainly
continues -- and there need to be good arguments against doing OS
research using fragile, difficult to read languages like C...

MK, VSTA, and other operating systems exist to serve as research
platforms (and sometimes as operational platforms when "modern" 70's
and 80's systems just can't cut it.)  Surely there is justification
for using Ada in this context, at least?

I brought up this point with a well known linux kernel developer, and
between the two of us we came up with several things that I didn't
know Ada95 well enough to answer:
	1) Can you do efficient raw memory operations (ie. raw byte
arrays, explicit control of when an access can be eliminated, and no
overhead [or at least, not stored in-place, for an object like a page
or a video card?]
	2) Can you interface efficiently to machine code
(ie. equivalent of inline asm, where you *mostly* can code a high
level algorithm but the core needs to be a particular hardware level
instruction. A GNAT-specific answer is ok, in this case...)
	3) Can you work without tasking (since you're implementing the
scheduler!) [I'll take arguments that you should in fact use Ada
tasking within the kernel, if they're detailed...]
	4) Can you do complete memory management (without garbage
collection.) 

As you can see, I *don't* know much Ada, though I've gotten a start on
Barnes' excellent "Ada95" text.  My guesses are that (1) is possible,
but you must go out of your way to do it; (2) might be possible in
GNAT; (3) is probably obvious and (4) is true but the reasoning is
subtle.

I haven't thought of any other potential obstacles to OS work in Ada;
it would seem a good choice, based on the *intent* of the design, but
I can't yet judge the results.

					_Mark_ <eichin@cygnus.com>
					Cygnus Support, Eastern USA