23 Sep
2022
23 Sep
'22
11:21 a.m.
On Sep 23, 2022, at 2:14 PM, Randy Dawson via cctalk
<cctalk(a)classiccmp.org> wrote:
On the top secret number cruching....
The Cray had an instruction called 'population count'
asked for by the NSA.
The number of bits on in a word, not sure what this was used for.
Cray did? I didn't know that. It first appeared in the CDC 6600, and yes, according
to rumor at the request of NSA. I can imagine it being used for statistical analysis of
character patterns.
A non-classified example of its use is in the PLATO system for "fuzzy string
matching". PLATO needed to be able to recognize student answers that were correct
but misspelled; it would do that, roughly speaking, by taking the difference of the
expected string and the actual input and running population count on that. A difference
of less than n bits would be defined as a misspelled match.
paul