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Date: Sat, 13 Jul 1996 22:19:01 -0400 (EDT)
From: Henry Spencer <henry@zoo.toronto.edu>
Reply-To: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: cable vs. ISDN 
To: "Jacob M. Parnas" <jparnas@jparnas.cybercom.net>
cc: hardware@freebsd.org, bsdi-users@bsdi.com
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> ...Modems seem to have like a 2-4 Kbyte FIFO, even 
> inexpensive ones, made back in the mid 80's.  Why, except for poor design,
> weren't they at least an option on the computer side. (1 byte was normal, 16
> was "buffered")

The modems weren't single-chip devices.  They were a boardful of
electronics, typically including at least one microprocessor.  (At one
point, the modem I was using had considerably more computing power than
the computer it was connected to.)  In fact there was no actual FIFO on
them -- the microprocessor had a few KB of RAM, and used that for data
buffering among other things.

> I'm also confused as to why building FIFO's is difficult or complicated.
> I remember we built one as a project in my first hardware course back in 
> 1982-1983 and nobody found it hard (it was a beginning minor project nobody
> seemed to have problems with.

It's easy enough to do, but doing it on a chip does eat a fair bit of space.

> >Only when you start running a real operating system (or a kludged imitation
> >thereof :-)) do you start to care about buffering.
> 
> True, but even machines like Sun Sparc 2's or IBM RT's which only ran Unix
> had small FIFO's.  And they only ran Unix.

However, if you look inside them you will usually find that their serial-I/O
chips are off-the-shelf commercial designs built for other markets.

                                                           Henry Spencer
                                                       henry@zoo.toronto.edu