NASM 2.05 based x86 Instruction Reference[ch150]

A.5.129 JCXZ, JECXZ: Jump if CX/ECX Zero JCXZ imm ; a16 E3 rb [8086] JECXZ imm ; a32 E3 rb [386] JCXZ performs a short jump (with maximum range 128 bytes) if and only if the contents of the CX register is 0. JECXZ does the same thing, but with ECX.