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From: rpw3@fortune.UUCP
Newsgroups: net.unix
Subject: Re: RE: tapes as mass storage (sob) - (nf)
Message-ID: <2464@fortune.UUCP>
Date: Tue, 7-Feb-84 11:05:36 EST
Article-I.D.: fortune.2464
Posted: Tue Feb  7 11:05:36 1984
Date-Received: Thu, 9-Feb-84 22:03:44 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
Lines: 44

#R:druky:-60800:fortune:26900023:000:1954
fortune!rpw3    Feb  7 04:45:00 1984

And I'll bet a nickel that the chairman of the session who
introduced Dave Custer to give that paper was John Alderman,
who never quit bragging about doing it too!

If Dave's scheme was the same one, it involved pre-formatting
the tape with short block headers alternated with longer data
blocks, with extended erase gaps after the data part:

<header1> <gap> <data1> <long gap> <header2> <gap> <data2> <long gap>...

You search for the header part (ignoring read errors but checking
block numbers of good headers in case you overshot), then immediately
turn on the write gate to write the data part. (The turnoff trash
ends up being short-gap written on top of long-gap. No problem.)

While working for John in 1971, we built a cassette tape controller
(Sykes tape) to run O/S-8 (for the PDP-8/e) on Phillips cassettes.
It ran, quite well in fact. About as fast as slow floppies.

It used similar formatting principles to the magtape scheme.
The controller was a micro-coded state machine using a hardwired
matrix of 7405 open-collector gates as a ROM! (Field blastable ROMs
didn't exist yet.) Was my first introduction to metastable logic
bugs. (Hours in front of a scope listening to the tape go "click,
hiss, whirr, click, hiss, click,...<silence>" hoping to see the
25ns glitch when the micro-PC on the controller got zapped causing
silent death.)

The trick to the controller was that the system device driver in O/S-8
has to fit in 92 (decimal) 12-bit words, thus the controller had to have
some smarts. (The boot code was 6 words, hand keyed!) One PDP-8 I/O
command ("SYBOOT") would halt the tape, wait until halted, rewind, wait
for clear leader, halt, wait 'til halted, go forward normal, wait until
non-leader, start data read. One instruction. (The other 5 were simpler.)

Rob Warnock

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