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From: bandy@amdcad.UUCP
Newsgroups: comp.sys.atari.st
Subject: Multitasking discussion
Message-ID: <14836@amdcad.UUCP>
Date: Wed, 18-Feb-87 18:15:17 EST
Article-I.D.: amdcad.14836
Posted: Wed Feb 18 18:15:17 1987
Date-Received: Fri, 20-Feb-87 01:05:20 EST
Followup-To: comp.sys.atari.st,comp.os.minix
Organization: AMDCAD, Sunnyvale, CA
Lines: 31
Keywords: minix, amiga's os


Well, as you may know, Andy Tanenbaum has mentioned a number of times
that a friend of his is working on a port of minix to the ST.  Well, I
was curious as to which method was being used (planned on being used? I
don't know how far along that port is), so I dropped a note off to him
and found out that when processes are loaded, they are relocated on the
fly -- apparently the same method that the Amiga folks use.  When a
process forks, the child is "swapped" to somewhere in memory (wherever
there is enough room).  When the scheduler starts the child, it swaps
out the parent (to somewhere else in memory), swaps the child to where
the parent is and starts running the child -- this way all the text and
data references are okay. 

In most cases, the child will exec so the new process being loaded in
can go anywhere and the parent process can go back to where it
belongs.  This all should happen within one scheduler tick, so you
won't have to do more than four copies of the parent's process image if
you're careful // however, if you're really careful, you could get away
with two copies (for the fork/exec case) -- copy the parent, run the
child (where the parent was), when the child execs, copy the parent
back to where it belongs You might be able to get away with not copying
the parent back *just shuffle the pointers), but with no read-only
pages (no mmu), you have no guarantees that the child hasn't scribbled
all over the data or text pages).

Yes, I know, this sounds gross, but it isn't too bad -- after all, if you do
want your machine to go faster, you can wait until one of the home-computer
manufacturers comes out with a cheap machine with an MMU and then run Minix
on it.
-- 
Andrew Scott Beals, {lll-crg,decwrl,allegra}!amdcad!bandy +1 408 749 3683