MOVZXB - x86-64 instruction reference

movzxbMove byte with zero extension 
Zero-extends the byte source to the width of the destination register, filling the high bits with zero.
386 and later
Intel's reference: MOVZX 
CFPFAFZFSFDFOF –––––––
Changes no flag
Forms  movzxb ,  163264 
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 unsigned byte and fills the added bits of a 16-, 32- or 64-bit destination with zeros. For example, movzxb rax, [rdi] reads one byte and produces a value from 0 to 255.
 global _start

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

        movzxb rax, bl

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