14 Aug
2017
14 Aug
'17
10:09 p.m.
On 8/13/17, Paul McJones <paul(a)mcjones.org> wrote:
On Sun, 13 Aug 2017, Dave Horsfall wrote:
> On Sat, 12 Aug 2017, Steve Johnson wrote:
>
>> A little Googling shows that the IF I mentioned was called the
>> "arithmetic IF".
>
> Ah yes. It was in FORTRAN II, as I recall.
The FORTRAN II arithmetic IF, with its three-way branch, was probably
introduced to facilitate use of the conditional branch instructions of
the IBM 704, which test the value of the accumulator and take a branch
accordingly:
TMI (transfer on minus)
TMP (transfer on plus)
TZE (transfer on zero)
TNZ (transfer on not zero)
It takes at most two of these instructions to implement the arithmetic IF.
-Paul W.
It turns out the original FORTRAN (manual published in October 1956; code
first shipped around April 1957) included the arithmetic IF as well as the
assigned and computed GOTO statements — see chapter 4 of this manual:
J.W. Backus, R.J. Beeber, S. Best, R. Goldberg, H.L. Herrick, R.A. Hughes,
L.B. Mitchell, R.A. Nelson, R. Nutt, D. Sayre, P.B. Sheridan, H. Stern, I.
Ziller. The FORTRAN Automatic Coding System for the IBM 704 EDPM :
Programmer's Reference Manual. Applied Science Division and Programming
Research Department, International Business Machines Corporation, October
15, 1956, 51 pages.
http://www.bitsavers.org/pdf/ibm/704/704_FortranProgRefMan_Oct56.pdf
(For more on the original FORTRAN compiler, see
http://www.softwarepreservation.org/projects/FORTRAN/.)