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From: lindsay@gandalf.cs.cmu.edu (Donald Lindsay)
Newsgroups: comp.arch
Subject: Re: machines with some loadable microcode are easier to fix
Message-ID: <11521@pt.cs.cmu.edu>
Date: 5 Jan 91 23:33:17 GMT
References: <1991Jan04.205635.16420@iecc.cambridge.ma.us>
Organization: Carnegie Mellon Robotics
Lines: 21

In article <1991Jan04.205635.16420@iecc.cambridge.ma.us> 
	johnl@iecc.cambridge.ma.us (John R. Levine) writes:
>Indeed, the now ubiquitous diskette first appeared as the boot
>device for 370 microcode.

Strangely, it was actually the boot device for a 370 disk controller.
Which was, of course, microcoded. IBM's formatting standard came from
a subsequent short-lived data entry product; which explains why a 128
byte sector had to have 80 EBCDIC blanks, followed by 48 NULs.

Controllers, being less general-purpose than CPUs, are perhaps better
suited for LIW/horizontal-microcode implementations.  I believe that
IBM went with programmability because they wanted to play with
diagnostic features, and with seek optimization features.

Good microprocessors weren't available then, but now, the alternative
design is a micro or two + some sort of DMA/data-path hardware. Are
there any controller designers out there who'd like to compare and
contrast?
-- 
Don		D.C.Lindsay .. temporarily at Carnegie Mellon Robotics