Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcdc!hpislx!hplvli!boyne
From: boyne@hplvli.HP.COM (Art Boyne)
Newsgroups: comp.arch
Subject: Re: 68000 architecture
Message-ID: <310002@hplvli.HP.COM>
Date: 9 Jun 89 15:09:51 GMT
References: <658@geovision.UUCP>
Organization: Loveland Inst. Div
Lines: 26

gd@geovision.uucp (Gord Deinstadt) writes:

>Actually, I don't mind the absolute addresses being signed; it's a
>fairly minor issue.  It's the signed offsets in base+offset mode that
>burn me up.  You *can* work around it (ie. still get a 64k address
>range with 16-bit offsets) by fudging all your
>base addresses by 32768; but you shouldn't have to.

Signed offsets in address register direct with displacement mode are
useful when dealing with stack frames (LINK command).  Local variables
are located at negative offsets from the *stack frame* pointer, parameters
are at positive offsets.  True, one could argue that the local variables
are at positive offsets from the *stack* pointer, but it is a pain when
writing assembler to remember to kludge the offsets while pushing
parameters when calling another function (let's see... I've pushed
a word and three longs, so I need x+14(SP), I think).  It's a lot easier
to use the unchanging negative offsets from the *stack frame* pointer.

After writing >100K of 68000 assembler in the last 5 years, I have
good reason to want the signed offsets.  Yes, it means that you have
to kludge to get a 64k address range, but many smaller systems don't
have that much global/stack data, which is where most assembler is
written.  Bigger systems are written in HLL's and the compiler takes
care of this automatically. :-)

Art Boyne, boyne@hplvla.hp.com