SETNL - x86-64 instruction reference

setnlSet byte if not less (SF == OF) 
Sets the destination byte to 1 when the last comparison found not less (SF equals OF), otherwise to 0.
386 and later
Intel's reference: SETcc 
CFPFAFZFSFDFOF –––––––
Changes no flag
Reads SF OF
Forms  setnl   
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
        setnl bl             ; bl becomes 1 when the condition holds, 0 when it does not

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