The Art of Assembly Language, 2nd Edition

10.11 Extracting Bit Strings

Of course, we can easily accomplish the converse of merging two bit streams; that is, we can extract and distribute bits in a bit string among multiple destinations. The following code takes the 32-bit value in EAX and distributes alternate bits among the BX and DX registers:

mov( 16, cl );   // Count the loop iterations.
ExtractLp: shr( 1, eax );   // Extract even bits to (e)bx.
           rcr( 1, ebx );
           shr( 1, eax );   // Extract odd bits to (e)dx.
           rcr( 1, edx );
           dec( cl );       // Repeat 16 times.
           jnz ExtractLp;
           shr( 16, ebx );  // Need to move the results from the H.O.
           shr( 16, edx );  // bytes of ebx/edx to the L.O. bytes.

This sequence executes 99 instructions. This isn't terrible, but we can probably do a little better by using an algorithm ...