How to load a single byte from address in assembly

Question

How can I load a single byte from address? I thought it would be something like this:

mov      rax, byte[rdi]

Answer 1

mov al, [rdi]

Merge a byte into the low byte of RAX.

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Or better, avoid a false dependency on the old value of RAX by zero-extending into a 32-bit register (and thus implicitly to 64 bits) with MOVZX:

movzx  eax, byte [rdi]          ; most efficient way to load one byte on modern x86

Or if you want sign-extension into a wider register, use MOVSX. (On some CPUs this is just as efficient as MOVZX.)

movsx  eax, byte [rdi]    ; sign extend to 32-bit, zero-extend to 64
movsx  rax, byte [rdi]    ; sign extend to 64-bit

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The MASM equivalent replaces byte with byte ptr.

A mov load doesn't need a size specifier (al destination implies byte operand-size). movzx always does for a memory source because a 32-bit destination doesn't disambiguate between 8 vs. 16-bit memory sources.