Art of Assembly Language, 1st Edition

11.8. 11.8 Searching for a Bit

A very common bit operation is to locate the end of some run of bits. A very common special case of this operation is to locate the first (or last) set or clear bit in a 16- or 32-bit value. In this section we'll explore ways to accomplish this.

Before describing how to search for the first or last bit of a given value, perhaps it's wise to discuss exactly what the terms "first" and "last" mean in this context. The term "first set bit" means the first bit in a value, scanning from bit zero toward the high order bit, which contains a one. A similar definition exists for the "first clear bit." The "last set bit" is the first bit in a value, scanning from the high order bit toward bit zero, which contains a one. A similar ...