Re: C arithmetic, was Software proofs, was Are there different

(snip, I wrote)


> But also, the 704 Fortran, and I believe still the 7090,
> indexes arrays from the end of memory toward the beginning.


And or moderator wrote:
> [It did because for reasons I have never been able to figure out,
> the 70x series subtracted rather than added the contents of
> an index register to get the effective address. -John]


I suppose so, but it was also convenient. Compilers (or linkers)
generate code starting from the bottom of memory, and allocate
variables starting from the top. (Different for different sized
machines.) Since Fortran COMMON can be different length, or at least
blank COMMON in later versions, allocating it from the end of memory
works well.


You can extend common from one subroutine to the next, or with
chaining, to another whole program.


So, was Fortran designed around the hardware,
to allocate memory from the end?


Or maybe just lucky.
[Fortran was definitely designed around the 704. I have done a lot of
looking to try and find out where the subtracted index registers came
from, and while there are a lot of guesses, there is nothing written
down. The three index registers were 1,2, and 4, and if you specified
more than one, it OR'ed them together and subtracted the result, which
was really strange. There were a lot of other machines with index
registers but none I know of that subtracted or OR'ed. I have also
been unable to tell whether the OR'ing was deliberate or just a result
of minimizing the number of tubes that they then documented. It was
likely useless since the 7094 had 7 index registers and a flag in
case you wanted the old behavior. -John]

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