RETNW - x86-64 instruction reference

retnwNear return, 16-bit 
Requests the 16-bit near-return encoding, with an optional unsigned cleanup byte count.
8086 and later
Intel's reference: RET 
CFPFAFZFSFDFOF –––––––
Changes no flag
Forms  retnw  
 retnw   
Operands imm16 An unsigned 16-bit byte count, 0 to 65535, added to the stack pointer after the return address is popped. 
NASM also accepts it in 64-bit source, with an operand-size prefix, but there it removes only 2 bytes from the stack and jumps to that value zero-extended, an address below 64 KiB: it cannot return to a 64-bit caller, and ordinary 64-bit functions return with ret. The optional sole immediate operand is an unsigned 16-bit additional cleanup count, not a signed stack adjustment.
 global _start

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

        retnw 5

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