14 Aug
2017
14 Aug
'17
7:16 p.m.
On Aug 14, 2017, at 9:09 AM, Paul Winalski
<paul.winalski(a)gmail.com> wrote:
And the 709 added CAS (compare accumulator with storage): it skips 0, 1, or 2 instructions
depending on whether the contents of the accumulator are algebraically greater, equal, or
less than the contents of the referenced memory location. (See page 49 of
http://www.bitsavers.org/pdf/ibm/709/709_RefMan_Fortran.pdf.)
> ...
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.