How to determine the boot device?
Posted: Fri Dec 07, 2012 8:14 am
I have what is probably a basic question, but I can't seem to find the answer here.
The BIOS loads the first sector into 0x7C00, and executes it. But I can't do much with 512 bytes. So the first thing my boot code does is loads extended boot code from the next 15 sectors into memory, by calling the BIOS interrupt for disk access. The problem I am having is that the drive number is supposed to go into the DL register. This is fine if I know what I am booting from. For example, the code below works when I boot from the hard drive, but not from a floppy or CD. How do I determine which device was booted from, so I can load the extended boot code from the correct device? 512 bytes isn't enough to do hardware or file system probing, so I am hoping there is a BIOS call I can make to determine the boot device. Any help would be appreciated.
The BIOS loads the first sector into 0x7C00, and executes it. But I can't do much with 512 bytes. So the first thing my boot code does is loads extended boot code from the next 15 sectors into memory, by calling the BIOS interrupt for disk access. The problem I am having is that the drive number is supposed to go into the DL register. This is fine if I know what I am booting from. For example, the code below works when I boot from the hard drive, but not from a floppy or CD. How do I determine which device was booted from, so I can load the extended boot code from the correct device? 512 bytes isn't enough to do hardware or file system probing, so I am hoping there is a BIOS call I can make to determine the boot device. Any help would be appreciated.
Code: Select all
1: boot:
2: mov ah, 0x02 ; we are going to read
3: mov dl, 0x80 ; this is the problem
4: xor cx, cx ; we want cylinder 0
5: mov cl, 2 ; we want the second sector
6: mov dh, 0 ; we want head 0
7: mov al, 0x0F ; read 15 sectors
8: mov bx, ext_boot ; read them into our code
9: int 0x13
10: jmp ext_boot
11: times ((0x200 - 2) - ($ - $$)) db 0 ; pad with 0's
12: dw 0xAA55 ; boot identifier
13: ext_boot:
14: do_something