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From: cruess@oakhill.UUCP (Michael Cruess)
Newsgroups: net.micro.68k
Subject: 24 vs. 32 Bits
Message-ID: <350@oakhill.UUCP>
Date: Mon, 4-Mar-85 11:21:55 EST
Article-I.D.: oakhill.350
Posted: Mon Mar  4 11:21:55 1985
Date-Received: Thu, 7-Mar-85 04:21:46 EST
Organization: Motorola Inc. Austin, Tx
Lines: 46

>From: david@daisy.UUCP (David Schachter)

>Chip designers have to make many trade-offs.  It appears that National chose
>speed-to-market and manufacturability..  Motorola chose prettiness.  Time will
>tell whose strategic planning department chose better, three or four years ago.

It may well be that National chose 24 bits because of  speed-to-market  and
manufacturability.  Those  were factors in the choice of 24 address pins on
the MC68000 several  years  ago.  For  the  MC68020,  they  were  not.  The
MC68020  already  had  32  bit ALU's, the die is not bonding pad limited in
size, and 8 fewer pins would not have put the part in a different  package.
To  restate  in  a  more  positive fashion: the MC68020 would not have been
smaller, sooner or cheaper if it only had 24  address  pins.  What  we  did
choose was not having to do it over again.

>Mr. Cruess (of Motorola) states that, in his experience, 24 bit addresses are
>a bit squeezed and that 28 bits is better.  Therefore, he concludes, the
>MC68020 is better than the NS32032.  In my experience, 24 bits is big enough
>to hold us workstation vendors until late 1987 or 1988.  Until then, given
>the real-world capabilities of microprocessors (as opposed to the claims of
>the marketing departments of the microprocessor vendors), anyone worrying
>about putting >16 MB on a microprocessor is wasting time.  You wouldn't
>put a 300 gallon gas tank in a Honda Civic, would you?  It would be overkill.

In other words, you will not be needing more than 24 bits of addressing for
3  years.  Considering  how much flak all microprocessor vendors take about
how long it takes to bring a product into full production,  you  should  be
glad  that we are starting now.  Also, re: ">16 MB":  I still maintain that
in  discussing  microprocessor  address  pins  we  are  discussing  logical
(virtual)  addresses  (as  opposed  to physical (real) addresses), and that
logical spaces of more that 2^24 bytes are required by our  customers.  The
amount  of  physical  memory is not relevant.  Please see other articles in
this group.

>By the time you need >16 MB in a workstation, National Semi will either have
>a part to handle your needs or be bankrupt.  Until then, the NS32032 gives you
>what you want (as does the Intel 80286) without paying for extra, unneeded
>address pins and without wasting expensive printed circuit board real estate,
>tranceivers, etc.

Bankrupt?  Do you know something  that  I  don't?  As  for  the  rest,  see
responses above.

Michael Cruess
Motorola Microprocessor Products Division
{ihnp4,seismo,ctvax,gatech}!ut-sally!oakhill!cruess