DEC - x86-64 instruction reference

decDecrement by 1 
Subtracts 1 from the operand and stores the result there, leaving the carry flag unchanged.
8086 and later·takes lock
Intel's reference: DEC 
Used in Loopsx86-64 assembly
Arithmetic, logic and bitsx86-64 assembly
Arrays, strings and the string instructionsx86-64 assembly
call, ret and the System V conventionx86-64 assembly
Length of a stringx86-64 assembly
Reverse a string in placex86-64 assembly
Even or odd, count the set bits, multiply by shiftingx86-64 assembly
Recursion: factorial and Fibonaccix86-64 assembly
Bubble sortx86-64 assembly
Print an unsigned number in bases 2 through 36x86-64 assembly
 
CFPFAFZFSFDFOF –●●●●–●
● set from the result
– not changed
Forms  dec  8163264 
Operands 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. 
It does not affect the
carry flag: to affect the carry flag, use SUB something,1. DEC affects all the other flags according to the result.
This instruction can be used with a LOCK prefix to allow atomic
execution.
See also INC.
 global _start

section .text
_start:
        mov rax, 12
        mov rbx, 5
        mov rcx, 3
        mov rsi, 9

        dec rax

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