[Simh] What is CDC OS?

Tue Feb 16 16:04:38 EST 2016

Will Senn <will.senn at gmail.com> asks on the list on Tue, 16 Feb 2016
12:47:12 -0600 about CDC operating systems.

I worked quite happily on a CDC 6400 from 1973 to 1977.  Initially, we
had the SCOPE operating system, and later, KRONOS.  The latter was,
from our viewpoint, a step back, because it had a flat file system
that in our case had to serve our entire campus.  As a result, we had
to adopt prefixes that encoded the department name and user name on
all filenames.

For its time, the CDC 6400 (and its bigger brothers, the 6600 and
7600) was quite fast.  It had interactive access, and a Unix-top-like
process display that you could run without logging in.

My previous machines were IBM and Amdahl mainframes that were only
accessible with batch jobs submitted on punched cards: for a long
time, users at that university were forbidden from storing files on
disks.  We solved that in our research group by renting our own IBM
2311 disk drive, and buying eventually a dozen or so removable disks.
According to Wikipedia,

	https://en.wikipedia.org/wiki/History_of_IBM_magnetic_disk_drives#IBM_2311

those disks with six LP-record-sized platters held about 7MB.  I did
hundreds of mounts of those disks myself.

The interactive features of the CDC 6400 had a huge beneficial impact
on my software productivity, compared to my previous batch
environment, and I regret that I have yet to find a CDC [67]x00
simulator with either SCOPE or Kronos.  I have one that boots up to
ROM level, but cannot get further with it.

The CDC machines had a small instruction set (64 different opcodes).
All integer arithmetic was done with floating-point instructions,
which offered both rounding and truncating modes.  The
integer-to-float-to-integer conversion meant that integers were
effectively limited to 48 bits of data stored in a 60-bit word.  There
were 18-bit An and Bn (n = 0..7) registers, and associated 60-bit Xn
registers.  A0 and X0 were scratch registers.  Loading an address into
A1 through A5 loaded a word from that memory word address into X1
through X5.  Putting an address in A6 or A7 stored the word in X6 or
X7.  All instruction mnemonics have hard-coded register names, so
there was no reasonable possibility of a macro language that could
usefully give symbolic names to registers.

The CDC floating-point arithmetic had both Indefinite and Infinity:
they were the inspiration for NaN and Infinity in IEEE 754 arithmetic
on almost all new post-1985 machine designs.

One nice feature of Indefinite was that with a job-control statement
immediately before your job, you could initialize all memory words to
Indefinite, with the word's address in the lower 18 bits.  Then, if
you accidentally used an uninitialized variable, you got a
floating-point exception, and the traceback showed where it came from,
and thus, which variable was unset.  We used that feature heavily, and
only a few modern systems provide something similar.

The address space was 2**18 = 262_144 words, and each process had a
contiguous block.  All physical addresses were computed dynamically
from the sum of a job base address and a relative-to-0 address, so the
O/S could slide entire jobs around in memory to achieve optimal
packing.  You could, and we did, adjust the process upper bound
dynamically during execution to free core memory for others.

The CDC [67]x00 family CPUs have no interrupts: instead, there are
several peripheral processors (PP's) that interface to external
devices such as disks and serial terminal lines.  The PP's are 12-bit
computers, each with 4096 words of memory. They communicate with the
main CPU by monitoring a couple of privileged memory locations that
hold data for a device operation.  At our site, the PPs were never
accessible to end users, so no one outside the computer center ever
knew their instruction set.

Niklaus Wirth and Urs Ammann have interesting, and sometimes negative,
comments about the CDC 6600 on which they developed the first Pascal
implementation.  Like Fortran and the IBM 709, and C and the DEC
PDP-11, the Pascal language also contains several influences of the
CDC 6600 architecture, although Wirth tried as much as possible to
hide the hardware from the programmer.

Here are some literature references about those machines, and early
Pascal:

papers:	James E. Thornton
	The CDC 6600 Project
	http://dx.doi.org/10.1109/MAHC.1980.10044

	Niklaus Wirth
	The Design of a PASCAL Compiler
	http://dx.doi.org/10.1002/spe.4380010403

	Urs Ammann
	On Code Generation in a PASCAL Compiler
	http://dx.doi.org/10.1002/spe.4380070311

book:	James E. Thornton
	Design of a Computer: the Control Data 6600
	Scott, Foresman (1970)
	LCCN TK7889.C6 T5 1970

Thornton was part of the 6600 architecture team.

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