Path: utzoo!utgpu!attcan!uunet!husc6!think!ames!elroy!gryphon!desint!geoff
From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: comp.sys.next
Subject: Re: portable a.out formats
Message-ID: <1805@desint.UUCP>
Date: 21 Oct 88 05:46:17 GMT
References: <909@goofy.megatest.UUCP> <23314@amdcad.AMD.COM>
Reply-To: geoff@desint.UUCP (Geoff Kuenning)
Organization: Interrupt Technology Corp., Manhattan Beach, CA
Lines: 28

In article <23314@amdcad.AMD.COM> tim@crackle.amd.com (Tim Olson) writes:

> It is very hard to create a truly portable object file format that can
> work on all machines.  The problem is the relocation info...
> ...
> Now we need a relocation type that can take a 32-bit absolute address
> and stuff the upper half into an instruction.
> 
> Because of this, most vendors extend a.out or COFF to fit their
> requirements.  A fully portable object file format would require a
> general-purpose linker "evaluation language" which allows not only +/-,
> but also <<, >>, &, |, etc.

And a lot of linkers have this.  Check out the DEC linkers, for
example, if I remember correctly.  It's not very hard to implement a
stack machine that can load from and store to arbitrary places in the
object file or the linked image.

The real advantage of such a linker is not portability, but the fact
that you are no longer restricted in the sort of link-time expressions you
can use in your code.  For example, you could write:

	int number_of_shorts = ((int) &a - (int) &b) / 2;

(Admittedly a contrived and stupid example, but when you *really* need
to do such a thing, it's a pain to have the linker prohibit it).
-- 
	Geoff Kuenning   geoff@ITcorp.com   uunet!desint!geoff