The Art of Assembly Language, 2nd Edition

10.7 Searching for a Bit

A very common bit operation is to locate the end of some run of bits. A 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 0 toward the high-order bit, which contains a 1. 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 0, which contains a 1. A similar definition exists for the last clear ...