CMOVNS - x86-64 instruction reference

cmovnsMove if not sign (SF == 0) 
Copies the source into the destination when the sign flag is clear (SF is 0).
Pentium Pro and later
Intel's reference: CMOVcc 
CFPFAFZFSFDFOF –––––––
Changes no flag
Reads SF
Forms  cmovns ,  163264 
Operands reg A general-purpose register matching the operation’s size, for example al, ax, eax or rax. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, which is required by spl, bpl, sil, dil and r8–r15. 
r/m A general-purpose register or memory location matching the operation’s size, for example rax or qword [rbp-8]. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, including when the memory address uses r8–r15. 
The condition reads the current flags, as the conditional jumps do; the condition-code table lists the tests and the other spellings of each.
The source is read whether or not the condition holds, so a memory source must be readable even when nothing moves. With 32-bit operands the destination is written either way: when the condition fails it keeps its low 32 bits, but the upper half of its 64-bit register is still cleared. The 16- and 64-bit forms leave a destination they do not move into untouched. There is no immediate source and no memory destination.
Every x86-64 processor has these instructions; some older 32-bit processors do not.
 global _start

section .text
_start:
        mov rax, 12
        mov rbx, 99
        cmp rax, 12
        cmovns rax, rbx      ; copies rbx into rax only when the condition holds

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