x86 Assembly Add operands with different sizes

Question

I like to add a byte from a memory location to a 32bit register, is this possible in x86 assembly? add edx, byte [ebx] causes error: mismatch in operand sizes

Answer 1

You need to make sure that the operands are of the same size.

This involves a problem with the sign though. If you are working with signed integers you should use movsx, or use movzx if you are working with unsigned integers.

movsx/movzx eax, byte ptr [ebx]
add edx, eax