All CPUs have an ability to exchange values via the xchg instruction. This instruction is not allways the most optimal. It is the smallest but not the fastest - the instructions is not pairable and executes in 3 cycles. On the Pentium the following combination is faster:

        push    ax
        push    bx
        pop     ax
        pop     bx

Now AX,BX are swapped. This gem uses 2 cycles instead of 3 but are bigger. There is yet another way. It is equally fast on a 486 and may be faster on a Pentium if your code is interleaved properly. It is bigger than the above example:

        xor     ax,bx
	xor     bx,ax
        xor     ax,bx

Now AX,BX are once again swapped.

Gem writer: John Eckerdal
last updated: 1998-03-16