[rescue] Finding antique machines
francini at mac.com
Fri Apr 20 06:25:04 CDT 2007
>Tue, 17 Apr 2007 @ 22:18 -0700, Sheldon T. Hall said:
>> And those things could handle a _lot_ of activity without breaking a sweat,
>> too. Remember when CompuServe was handling three million members with a
>> bunch of hopped-up TOPS-10 machines?
>I thought they ran the pre-cursor to Twenex (TOPS-20)?
>The story as I remember it is they bought PDP-10s, added paging and
>other functions that DEC refuse to add, and rewrote TOPS.
CompuServe's weird (to me) version of TOPS-10 branched off the 'real'
OS somewhere around the 5.07 or 6.01 release, mid-1970s.
>Time passes, and DEC basically starts from the timeshare system and
>creates the Decsystem-20 and TOPS-20.
Nope, nope, nope.
Here's the deal.
The TOPS-10 Monitor (the old term for what we call an "OS") dates
straight back to the original code written for the PDP-6 (DEC's first
36-bit product) in 1963. The first model of the PDP-10, the KA-10,
which debuted in 1968, didn't have virtual memory or paging. These
were new and (commercially) untested concepts, though they were
already operational on Multics at MIT.
Instead, the KA-10 had protection and relocation registers, which
allowed a user program to be split into a 'low segment' which the
program could write to, and a 'high segment', which was
write-protected. The high segment code could be shareable amongst
users. So, if your job (a process in modern parlance) were running,
say, a COBOL program, the COBOL run-time environment (libraries and
such) would live in the high segment, and your program and data would
live in the low segment. Segments were multiples of 1Kword (36-bit
words) in size, and were internally contiguous. The Monitor could
swap out an entire low segment to the swap area (a pre-allocated
contiguous spot on disk, or, earlier, drum storage). The high segment
stayed in core if there were other users who needed it; if not, the
high segment would get swapped out as well if necessary.
Computer scientists at Bolt, Berenek, and Newman (BBN), in Cambridge,
tried to get DEC to add the newfangled paging features to Monitor,
but to no avail. So they did the next best thing: they bought a
PDP-10 and then designed a box to add paging logic to the KA-10 CPU.
They then proceeded to write a new OS from the ground up for it. The
result was TENEX, or TEN EXecutive. This was the direct predecessor
to TOPS-20. Dan Murphy, one of the original programmers on the TENEX
project, was hired by DEC in 1972 to port TENEX to the
then-under-development KL-10 processor, which *would* support proper
paging via microcode. The eventual result of that port was TOPS-20.
The user community dubbed it "TWENEX", as an homage to its history.
Meanwhile, the original Monitor was dubbed "TOPS-10", and the system
as a whole was christened "DECsystem-10" when the second processor in
the family, the KI-10, came out in 1972. It was twice as fast as the
KA-10, used 512-word pages instead of the 1Kword-multiple memory
segments of the KA, would allow individual pages to be set
write-protected or write-enabled, and could support non-contiguous
job memory spaces. However, TOPS-10 still didn't support it.
The KL-10 Model A, which came out in 1974, was Digital's first
microcoded processor. It was twice as fast as the KI-10 it replaced.
It supported KI-10 style paging, which was just fine for TOPS-10. It
also supported external memory channels, which allowed it to be a
'drop-in' replacement for the KI-10.
The KL-10 Model B, which came out in 1976, was the processor on which
TOPS-20 debuted. The Model B supported internal MOS memory, and its
microcode also supported "real" (TENEX-style) paging, so it natural
that it was also the first computer that supported TOPS-20. The
system was dubbed the DECSYSTEM-20.
Later on, Digital came out with the DECsystem-1091, a re-packaging of
the KL-10 Model B in the same cabinetry as the DECSYSTEM-20 (and with
the same internal memory), but with blue top panels in place of the
orange used on the -20. After that point, the only difference
between a DECsystem-10 and a DECSYSTEM 20 were:
o The microcode loaded at boot time by the front-end PDP-11/40
o The OS loaded from disk
o The on-disk format of the filesystems
o A can of paint
Inside DEC, we had a collection of KL-10 computers available for
stand-alone developer use. These were usually orange-paneled. All
we needed to do to switch from TOPS-20 to TOPS-10 (or vice versa) was:
1. Shut down any OS running on the box.
2. Remove the previous user's system disk pack.
3. Install your own system disk pack.
4. Hit the "boot" button.
This would boot the PDP-11/40's console front-end software (a highly
modified version of RSX-11 called RSX-20F), which would then proceed
to load the CPU microcode and then boot the OS.
A single-box, cost-reduced (and 4x slower) model, the DECSYSTEM-2020
(also known as the KS-10), was released in 1978. It used a PDP-11
UNIBUS for its peripherals, and also supported both TOPS-10 and
TOPS-10 and TOPS-20 were very different beasts. TOPS-10 was always
the faster OS, while TOPS-20 was always the more feature-ful OS. Both
had their adherents, and both were in active development right to the
very end of the product's life in 1990.
There were plans for a 'unification' OS, dubbed TOPS-36, to take the
best parts of both Monitors and combine them into one system, but
that never really got beyond the investigational stage.
>Before the two systems could be merged into one product again, DEC
>dropped the whole 36-bit line in favor of VAX.
Partial credit here. (see above).
Indeed, the whole 36-bit line was indeed dropped because of the "one
architecture, one OS, one product" mantra. (We in TOPS dubbed that
"One chicken, egg, one basket".)
>For a lot of people, that was a black day.
Now THERE you're right on.
One longtime TOPS-20 user, Mark Crispin, to this day uses the
signature line "TOPS-20: a great improvement on its successors" in
his e-mails. And many of us still feel that way to this day.
John Francini, francini at mac.com
"The journey is more important than the destination -- that's part of life.
If you only live for getting to the end, you're almost always disappointed."
-- Donald Knuth
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