Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: 68000 architecture
Message-ID: <1989Jun4.065416.14250@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1989May30.171335.473@utzoo.uucp> <658@geovision.UUCP>
Date: Sun, 4 Jun 89 06:54:16 GMT

In article <658@geovision.UUCP> gd@geovision.UUCP (Gord Deinstadt) writes:
>>... You can run the 68k family with 16-bit addresses (although
>>they have to be *signed* 16-bit addresses!).
>
>Can anyone tell my why Motorola did such a twisted, sick thing?

For much the same reason Intel gave us segments, but from the other side
of the fence:  it was a time of transition past the 16-bit boundary, with
nobody wanting to stay with a pure 16-bit solution but many customers
resisting going to a pure 32-bit solution.  Intel wanted bigger addresses
but didn't want to enlarge registers etc., so we got segments.  Motorola
originally started out with 16-bit data but larger addresses -- which is
why the A/D register split in the 68k -- but relatively late in design
they went 32 most places.  However, they wanted to preserve the ability
for people to work in a 16-bit world, with only two bytes needed to store
an address and only one fetch needed to get one across a 16-bit bus.  So
pretty well all the A-register manipulation instructions have variants
that only work with the bottom 16 bits.  They sign-extend, which is why
the addresses are signed.

We'll probably see the same approaches again when people start seriously
contemplating the 32-bit boundary.
-- 
You *can* understand sendmail, |     Henry Spencer at U of Toronto Zoology
but it's not worth it. -Collyer| uunet!attcan!utzoo!henry henry@zoo.toronto.edu