Path: utzoo!attcan!uunet!husc6!think!ames!hc!hi!erikj
From: erikj@hi.unm.edu (Erik Johannes)
Newsgroups: comp.sys.amiga
Subject: Re: Amiga UNIX
Message-ID: <23602@hi.unm.edu>
Date: 20 Jun 88 16:02:28 GMT
References: <211@laic.UUCP> <3663@cbmvax.UUCP> <1872@sugar.UUCP> <134@ssdis.UUCP> <2106@sugar.UUCP> <221@toylnd.UUCP>
Reply-To: erikj@hi.unm.edu (Erik Johannes)
Organization: U. of New Mexico, Albuquerque
Lines: 19

I believe one of the key issues to implementing UNIX or MINIX or any of the
other Unix clones is the fork command.  A fork command makes a copy of the
process.  This includes not only the code but also the data.  Copying the 
data is were a plain 68000 without an MMU runs into problem.  The 68000
doesn't necessarily know were all the data is.  It may know were the main
data segment is, but there can be pointers in there pointing else were.
In order for the fork command to function properly the 68000 would have to
chase down all the pointers and stuff, which would be extremely complex and
time consuming.  One of the principal foundations of UNIX is cheap processes,
and this is not possible with a plain 68000.  One of the work arounds for this
problem is the "forkexec" command.  I believe the Amiga has this.   But this
is not a true fork command and thus not compatable with UNIX.

	Intel got lucky with their convoluded segment registers in the 80x86
family.  Since there is a Data base register, they can make a copy of the
data segment and therefore have a true fork.  For a 68k system, one must
have an MMU or a 68020 (68030) which has a built in MMU.

			-Erik Johannes