[TUHS] Array index history

Greg 'groggy' Lehey grog at lemis.com
Fri Jun 9 10:03:08 AEST 2017

On Friday,  9 June 2017 at  1:04:39 +1000, Dave Horsfall wrote:
> And of course the afore-mentioned FORTRAN, which really reflects the
> underlying IBM 70x architecture (shudder).

I had promised myself that I wouldn't get involved in this thread, but
I had to step in here.

Yes, the IBM 704 had the CAS instruction (Compare Accumulator with
Storage), a three-way skip instruction that easily implemented the
FORTRAN Arithmetic IF.  But the "index" registers were correctly
called decrement registers: their content was subtracted from the base
address.  I assume that this was because the 704 was a ones-complement
machine whose basic arithmetic operation was subtraction, so this was
cheaper than adding the registers to the base address.

To implement indexing with the base 704 hardware, FORTRAN would have
had to have an upper bound of -0 (remember that number?).

Still, there were three decrement registers, and the original FORTRAN
was limited to three dimensions, evidently as a result.

> Speaking of Burroughs (which we weren't), their OS was MCP (Master
> Control Program) which we the great unwashed called Male Chauvinist
> Pig, no doubt to their distress :-)

This name (the original one) cropped up again in the 1982 film "Tron"
(http://www.imdb.com/title/tt0084827/).  It was singularly

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