I am making a OS in 32-bit protected mode assembly and need to incrament (or decrament) a variable. My variable is defined like this:
Var: db 1
and I am trying to increment it like this:
mov ebx, [Var]
inc ebx
mov [Var], ebx
However, the variable is increased by a value WAY bigger then 1. Why is this happenng and how do I fix it. Best Regards, Markian.
>Solution :
Your problem is that you defined Var to be a byte-sized variable, but you operate on it as if it was a dword-sized variable. This causes you to read/write unrelated bytes around the variable, causing the strange numbers you observe.
To fix this, always operate on data with the correct data size. For example, do
movzx ebx, byte [Var]
inc ebx
mov [Var], bl
Note the asymmetry: we could have used mov bl, byte ptr [Var], but it’s slow to write to partial registers (i.e. bl being a part of ebx). The instruction movzx makes sure to write the full register while only fetching a single byte from memory.
Or even simpler
inc byte [Var]