Path: utzoo!attcan!uunet!mitel!sce!cognos!geovision!gd
From: gd@geovision.uucp (Gord Deinstadt)
Newsgroups: comp.arch
Subject: Re: 68000 architecture
Message-ID: <660@geovision.UUCP>
Date: 7 Jun 89 02:14:08 GMT
References: <1989May30.171335.473@utzoo.uucp> <658@geovision.UUCP> <40938@bbn.COM>
Reply-To: gd@geovision.UUCP (Gord Deinstadt)
Organization: GeoVision Corp, Ottawa, Canada
Lines: 52

In article <40938@bbn.COM> slackey@BBN.COM (Stan Lackey) writes:
>In article <658@geovision.UUCP> gd@geovision.UUCP (Gord Deinstadt) writes:
>>In article <1989May30.171335.473@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>>>
>>>Just to be heretical, one can observe that the same thing is true of
>>>addresses.  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?
>
>I suspect that the reason 16-bit absolute addresses are signed is
>because all instruction stream data is sign extended, and the
>designers figured that 32Kbytes of scalar addressing was enough
>anyway, so why make an exception for addresses.

I suspect that's it.  From Henry Spencer's follow-up, it seems the
intent was that all your pointers would be 32-bit, and offsets
would likely only be used for accessing fields within a structure
and perhaps local variables off the frame pointer.

Someone also pointed out via e-mail that you can use the 16-bit
absolute mode for easy addressing into either very high or very low
memory.  This is handy because very low mem contains interrupt vectors
and the like, and i/o is usually mapped into very high mem.

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.  Especially
considering that this was in the days when people still wrote
a lot of assembler.  (Gosh, did people actually do that when
I was young? ;-P)

>The reason for the 16-bit absolute addressing mode was because in
>1977? when the 68000 was designed, 1) people cared about the amount of
>memory their application needed, and 2) lots and lots of applications
>used less than 32K bytes for scalar data.

And of course Intel was going around bragging about how compact 8086
code was.

>I feel that including 32-bit features at all showed great insight by
>its designers, and I also feel that the 68000 family was partially
>responsible for the emergence of the workstation as a market entity.

No argument about that from here.  I like the 68000, generally, and
often chafe at having to use 8086-family machines for the "wrong"
reasons.  My admiration for the 68000 only adds to my disappointment
at what I consider to be a flaw in the architecture.

Gord "an unsigned for the times" Deinstadt