Read from keyboard with redirected stdin

[Daniela]

Hi!

I'm writing a program which acts as a filter, so stdin and stdout will be 
redirected most of the time. However, in one case I have to read some input 
from the keyboard. Well, I have stderr for displaying the prompt, but what 
file can I read from to get keyboard input in any case?
Please don't point me to libraries doing the job, as I'm doing it in ASM.
It's a quite simplistic program and I don't want it to grow unnecessarily.

Regards,
Daniela