tapes as mass storage (sob)

[Custer]

This is a true story, even though some folks might think
its in "The first guy hasn't got a chance!" cagegory.

In the late 60's, before the advent of OS8, DEC had
a DOS.  This dos used DF-32 disk drives.  32k of 12 bit words!

I was a grad student at University of Colorado, Boulder, at
the time & was doing some work at LASP (Laboratory for Atmospheric
and Space Physics, I think).  They had a an original (transistor, not
IC) PDP-8.  This PDP-8 had 4k memory, a Teletype ASR33 jimmied up
so the computer could control (turn on or off) the paper tape reader
(10 cps), a 12 bit AD converter, and
a 7 track mag tape (you know, the predecessor to a 9 track tape).
(Back then the characters were 6 bit, the tape wrote one char plus parity).
The machine was a $100,000 analog to digital converter.  The mag tapes
were hand carried to an IBM 4044 (later a CDC 6400) for analysis.

I got tired of assembly language programming on it (paper tape at 110 baud!).
Two pass assembler (read the tape twice) object on paper tape, core image
executable on paper tape.  YUK!

To make a looooong story short, I developed a scheme for random access
on the 7 track, including read after write (couldn't be done, **they** said).
Then I managed to get my hands on the source for DOS (including a funny
FORTRAN-II like compiler).  All it needed was a disk emulator for the 
mag tape!  In 1 plus page of 128 words!

I'll never forget the time it first ran.  The scientist in charge of the
machine (Jeff Pierce, Where Are You?) happened to be showing the lab to
a visitor.  The system booted, and started searching the tape for the
SAM block(s) so it could swap itself out.  It was really dramatic
(real sloooow too, as it did almost as many backspaces as it did forward
moves).  I explained to Jeff and his visitor that the OS was searching
the tape for a place to swap on, and to load the command line interpreter.
Jeff was awestruck.  He muttered something to the effect, "Can you imagine
a 12 bit computer doing that!?"

Even tho it was slow, it was **MUCH** better than paper tape on an
ASR33 TTY.

I left shortly thereafter, but before I did I noticed that the PDP-8 got
an increment of memory (for a total of 12K, I believe), a pen plotter,
a disk, and lots of other goodies.  

Turned it into a real computer, by golly.

Published the scheme in a DECUS proceeding.  The chairman introduced
my talk with the statement that it really worked:  he knew because he
didn't believe it until he tried it on his machine!  

It's interesting how people accept as gospel "conventional knowledge."
The conventional knowledge of the time was that you couldn't read after
write on an "industry standard" i.e., 7 track 1/2 inch, mag tape,
that you couldn't random access info on a mag tape,
that you couldn't write in the middle of a tape without destroying all
the information after the write, etc.

It was true too -- if you orgainized the tape in the conventional way.
Of course, the inventor of DecTape knew better.  All I did was adapt
the algorithms of DecTape to a 1/2" 7-track mag tape.  The only
trick was to get the tape driver to fit into < 128 machine instructions!

David Custer
AT&T ISL, Denver
druky!ddc
(303) 538-3517