Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!think.com!zaphod.mps.ohio-state.edu!rpi!uupsi!pbs.org!TALOS!jerry
From: jerry@TALOS.UUCP (Jerry Gitomer)
Newsgroups: comp.arch
Subject: Re: machines with some loadable microcode are easier to fix
Keywords: microcode hardware bugs
Message-ID: <777@TALOS.UUCP>
Date: 4 Jan 91 14:36:21 GMT
References: <71537@bu.edu.bu.edu> <1991Jan04.035359.12547@kithrup.COM>
Organization: NPRI, Alexandria VA
Lines: 34

In article <71537@bu.edu.bu.edu: gjc@buitc.bu.edu (George J. Carrette) writes:
:The subject line is a good summary, one reason that hardware designers
:and vendors prefered machines with loadable microstore (and by the same
:token machines that have microcode) was that it made it possible to fix
:some "hardware" bugs without replacing chips or boards.
:RISC implementations have no microcode, so fixing a hardware bug
:means replacing a chip.

Sorry guys, but the ability to fix bugs wasn't even a consideration in
deciding to use loadable microstore -- it was economics.  The computer
users demanded "families of computers", that is computer systems that
could run the same programs and varied only in price and performance. 
These demands were based on a desire to reduce the software costs
associated with upgrading to a faster machine.

Things were different when the first microcoded computers were built. 
Computer generations were five years apart.  By today's standards the
hardware was simplistic and slow.  Operating systems (and all the other
system software for that matter) were written in assembly language.  

Given these circumstances the solution was to use loadable microcode to
make a group of dissimilar computers look alike to the programmer.  The
best illustration was the IBM 360 family.  To the programmer each 360 was
a 32-bit word machine with 16 registers.  The 360/30 was an 8-bit machine,
the 360/40 was a 16-bit machine, the 360/50 was the only 32-bit machine in
the family, the 360/65 was a 64-bit(?) machine, and the 360/75 was even
bigger.  (Please don't flame me for ignoring the 360/20, 360/22, 360/25,
360/85, 360/90/91/95, 360/44, and the 360/67 since the additional detail
wouldn't add anything.)

-- 
Jerry Gitomer at National Political Resources Inc, Alexandria, VA USA
I am apolitical, have no resources, and speak only for myself.
Ma Bell (703)683-9090      (UUCP:  ...{uupsi,vrdxhq}!pbs!npri6!jerry