Why can't I move directly a byte to a 64 bit register?

Question

Why can\'t I directly move a byte from memory to a 64-bit register in Intel x86-64 assembly?

For instance, this code:

extern printf

global main

seg         


        

Answer 1

Use move with zero or sign extension as appropriate.

For example: movzx eax, byte [rbp - 1] to zero-extend into RAX.

movsx rax, byte [rbp - 1] to sign-extend into RAX.