MOVSXW - x86-64 instruction reference

movsxwMove word with sign extension 
Copies a word source into a register, sign-extending when the destination is wider.
386 and later
Intel's reference: MOVSX/MOVSXD 
CFPFAFZFSFDFOF –––––––
Changes no flag
Forms  movsxw ,  163264 
 movsxw eax, ax  
Operands reg The destination register. Its width is selected independently of the source width; widening uses a larger destination. 
r/m16 A 16-bit general-purpose register or word in memory, for example ax or word [rsi]. 
It reads 16 bits and sign-extends them into a 32- or 64-bit destination. For example, movsxw rax, [rdi] reads two bytes. NASM also accepts a 16-bit destination; that copies the word without widening it. A 32-bit destination clears the upper half of its 64-bit register.
 global _start

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

        movsxw rax, bx

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