BSWAP - x86-64 instruction reference

bswapByte swap 
Reverses the byte order of a 32-bit or 64-bit register, so the lowest byte swaps places with the highest.
486 and later
Intel's reference: BSWAP 
CFPFAFZFSFDFOF –––––––
Changes no flag
Forms  bswap  3264 
 bswap ax   8086 and later 
 bswap cx   8086 and later 
 bswap dx   8086 and later 
 bswap bx   8086 and later 
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. 
For example, bswap rax exchanges byte 0 with byte 7, byte 1 with byte 6, and so on. A 32-bit result also clears the register's upper 32 bits. There is no defined 16-bit form; xchg al, ah swaps the bytes of ax.
 global _start

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

        bswap rax

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