SETNZ - x86-64 instruction reference

setnzSet byte if not zero (ZF == 0) 
Sets the destination byte to 1 when the zero flag is clear (ZF is 0), otherwise to 0.
386 and later
Intel's reference: SETcc 
CFPFAFZFSFDFOF –––––––
Changes no flag
Reads ZF
Forms  setnz   
Operands r/m8 An 8-bit general-purpose register or byte in memory, for example al or byte [rsi]. The high-byte registers ah, bh, ch and dh cannot be used with a REX prefix, including when the memory address uses an extended register such as r8. 
 global _start

section .text
_start:
        mov rax, 12
        cmp rax, 12
        setnz bl             ; bl becomes 1 when the condition holds, 0 when it does not

        mov rax, 60         ; syscall 60: exit
        xor rdi, rdi
        syscall