[TUHS] RIP John Backus

[Paul McJones]

On 3/17/2018 12:22 PM, Arthur Krewat <krewat at kilonet.net> wrote:
> Leave it to IBM to do something backwards.
>
> Of course, that was in 1954, so I can't complain, it was 11 years before
> I was born. But that's ... odd.
>
> Was subtraction easier than addition with digital electronics back then?
> I would think that they were both the same level of effort (clock
> cycles) so why do something obviously backwards logically?

Subtraction was done by taking the two's complement and adding. I 
suspect the CPU architect (Gene Amdahl -- not exactly a dullard) 
intended programmers store array elements at increasing memory 
addresses, and reference an array element relative to the address of the 
last element plus one. This would allow a single index register (and 
there were only three) to be used as the index and the (decreasing) 
count. See the example on page 97 of:

James A. Saxon
Programming the IBM 7090: A Self-Instructional Programmed Manual
Prentice-Hall, 1963
http://www.bitsavers.org/pdf/ibm/7090/books/Saxon_Programming_the_IBM_7090_1963.pdf

The Fortran compiler writers decided to reverse the layout of array 
elements so a Fortran subscript could be used directly in an index register.
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