The Art of Assembly Language, 2nd Edition

10.8 Counting Bits

The last example in the previous section demonstrates a specific case of a very general problem: counting bits. Unfortunately, that example has a severe limitation: It only counts a single run of 1 bits appearing in the source operand. This section discusses a more general solution to this problem.

Hardly a week goes by that someone doesn't ask on one of the Internet newsgroups how to count the number of bits in a register operand. This is a common request, undoubtedly, because many assembly language course instructors assign this task as a project to their students as a way to teach them about the shift and rotate instructions. Undoubtedly, the solution these instructors expect is something like the following:

// BitCount1: // ...