What do you mean "write the key"? Post_out is used entirely for debugging purposes, it's to know where/when the startup failed and what was the cause of it... nothing else... well, for microsoft anyways... for us, it's to know where we are in the boot process, making it possible for us to determine when to send our reset pulse
If you sit back and read the name one more time you'll also understand that it's output, not input... output means that whatever is going on is sending something here, not accepting information to use in some way...
Segnalibri