MOVSXB - x86-64 instruction reference

movsxbMove byte with sign extension 
Sign-extends the byte source to the width of the destination register and copies the result there.
386 and later
Intel's reference: MOVSX/MOVSXD 
CFPFAFZFSFDFOF –––––––
Changes no flag
Forms  movsxb ,  163264 
 movsxb ax, al   8086 and later 
Operands reg The destination register. Its width is selected independently of the source width; widening uses a larger destination. 
r/m8 An 8-bit general-purpose register or byte in memory, for example al or byte [rsi]. The high-byte registers ah, bh, ch and dh cannot be used with a REX prefix, including when the memory address uses an extended register such as r8. 
It reads an 8-bit register or memory value and sign-extends it into a 16-, 32- or 64-bit destination register. For example, movsxb rax, [rdi] reads one byte. A 32-bit destination clears the upper half of its 64-bit register after producing the 32-bit result.
 global _start

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

        movsxb rax, bl

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