x86 Assembly Language Reference Manual

Jump if ECX is Zero (jcxz)

jcxz    	disp8

jump to disp8 if (E)CX is 0


The jcxz instruction tests the contents of the CX or ECX register for 0. jcxz differs from other conditional jumps that it tests the flags, rather than (E)CX.

jcxz is useful at the beginning of a loop that terminates with a conditional loop instruction; such as:

loopne .-126

In this case, jcxz tests CX or ECX for 0 prior to entering the loop, thus executing 0 times:


jcxz .-126
loopne .-126