Art of Assembly Language, 1st Edition

11.10. 11.10 Reversing a Bit String

Another common programming project instructions 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 ...