Question

I’m struggling with checking if Alt key is pressed in linux bootloader

int 0x16
mov ah, 0x0e
int 0x10

Above code prints ascii characters just fine, but how do I check for Alt key?

Answer 1

You test bit 3 of 0040h:0017h.

http://www.ousob.com/ng/bios/ng559a.php

Something like this (in AT/T syntax):

mov    $0x40,%ax
mov    %ax,%es
testb  $8,%es:0x17
jz     noAltKey
.... ;; Alt pressed
noAltKey:

According to objdump -d -M intel, that’s how it looks in Intel syntax:

0:   66 b8 40 00             mov    ax,0x40
4:   8e c0                   mov    es,eax
6:   26 f6 04 25 17 00 00    test   BYTE PTR es:0x17,0x8
d:   00 08 

UPD: The following should work for NASM:

mov    ax,0x40
mov    es,eax
test   [es:0x17],byte 0x8
jz     noAltKey
.... ;; Alt pressed
noAltKey: