How to obtain the BIOS boot sequence?

Question

Is there any BIOS call that can be used in order to obtain the BIOS' boot sequence? The origin of this question was me trying to install Windows 7 on a very old Pentium III machine, where the installer kept insisting that "The computer's hardware may not support booting to this disk", in spite of the 1st HDD being set as the first boot device.

The only mention of programatically getting the boot sequence that I could find was in the DOS API reference, where int 21/AX=3305h in DOS 4.0+ is defined as "Get Boot Drive", with the dl register set on return to the drive the OS was booted from, 1 meaning A: and so on. However, it is obviously very doubtful that this call is used by the Windows 7 installer.

Answer 1

There is no (standard) BIOS call for obtaining the boot sequence, and as you say, DOS calls aren't relevant to Windows. However, it doesn't really matter in the case you're considering, because by convention only one hard drive (drive 0) is bootable.

Some BIOSes do allow you to select a particular hard drive as the boot drive. They do this by rearranging the drive order at boot time so that the selected drive is drive 0.

The warning message you describe was probably due to the size of the hard disk drive. Some older BIOSes can't boot to larger drives, and since the Windows installer had no way to tell whether your machine was one of the exceptions, it gave you the warning. (There are various other similar possibilities, such as the hard drive type.)

Assuming it did in fact boot, you don't really need to worry about it. :-)