The Art of Assembly Language, 2nd Edition

11.2 Performance of the 80x86 String Instructions

In the early 80x86 processors, the string instructions provided the most efficient way to manipulate strings and blocks of data. However, these instructions are not part of Intel's RISC Core instruction set, and as such, they can be slower than doing the same operations using discrete instructions. Intel has optimized the movs instruction on later processors so that it operates about as rapidly as possible, but the other string instructions can be fairly slow. As always, it's a good idea to implement performance-critical algorithms using different algorithms (with and without the string instructions) and compare their performance to determine which solution to use.

Keep in mind that the string instructions ...