I don't have any real understanding of this, but this is interesting:"Every
serious hacker sooner or later needs the popcount instruction.
This "population count" instruction counts the set bits in a register, and is
so useful that the NSA demands that all computers they purchase implement it in
hardware."
Majek's technical blog: Magic popcount (popcnt) command
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| Majek's technical blog: Magic popcount (popcnt) commandFrom Frank de Groot
blog: Every serious hacker sooner or later needs the popcount instruction. |
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| View on blogger.popcnt.org | Preview by Yahoo |
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From: Robin Vowels <[email protected]>
To: [email protected]
Sent: Wednesday, March 11, 2015 3:25 PM
Subject: Re: LZRG??? Does this mean that 56-bit addressing is "a thing"?
From: "John McKown" <[email protected]>
Sent: Thursday, March 12, 2015 1:49 AM
> Given some of the new instructions, such as LGZR, I wish IBM would
> publish a manual with a title like: "What were the architects thinking
> of? Explanation of the reasons behind the instructions in the z
> architecture". Some are obvious, like L, ST, A. But why a single
> instruction to do this? Is it _that_ often used? And POPCNT is another
> one. Why do I need to know the number of 1 bits in each individual
> byte in a GPR?
Useful in some counting operations, where the bits represent a mask.
Though not as a hardware instruction, it was used by about 1961
on DEUCE using an automatic shift operation. It was 15 times
faster than the conventional way using a loop.
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