The Art of Assembly Language, 2nd Edition

10.9 Reversing a Bit String

Another common programming project instructors assign, and a useful function in its own right, is a program that reverses the bits in an operand. That is, it swaps the L.O. bit with the H.O. bit, bit 1 with the next-to-H.O. bit, and so on. The typical solution an instructor probably expects for this assignment is the following:

// Reverse the 32-bits in eax, leaving the result in ebx:

               mov( 32, cl );
RvsLoop:       shr( 1, eax );  // Move current bit in eax to
                               // the carry flag.
               rcl( 1, ebx );  // Shift the bit back into
                               // ebx, backwards.
               dec( cl );
               jnz RvsLoop;

As with the previous examples, this code suffers from the fact that it repeats the loop 32 times, for a grand total of 129 instructions. By unrolling the loop you can ...