Hi Jeremy, I vote for 2)
- use 0 or 80 only in the boot sector (or really any value the
user wants, but default to 0 or 80)
- the boot sector code has at a fixed location
useBIOSorNotFixedLocation:
mov [drive], dl
- SYS is then responsible for determining if BIOS provided value
is used or not by patching useBIOSorNotFixedLocation with NOPs
...
*but* with different SYS behaviour: Make "/B drivenumber" and
"/USEBPBDRIVE" work independent from each other. Sometimes you will
want "/B 81 /USEBPBDRIVE", sometimes "/USEBPBDRIVE" (with default
drive 0 or 80 that is), and in rare cases even "/B 81" (where the
81 is only read by disk tools but DOS uses the value passed by the
boot loader / MBR instead). And of course "" (use value passed by
MBR and write default 0 or 80 value into boot sector).
MS DOS 5 behaves as "/USEBPBDRIVE"
MS DOS 4 same
IBM DOS 3 probably the same
Win9x same but skips int 13.00 depending on drive number high bit?
WinNT same as DOS 5
... and so on.
According to:
http://home.att.net/~rayknights/pc_boot/pc_boot.htm
Win9x scans the MBR to find the primary partition which starts at
the same LBA offset as stored in the boot sector...
(if drive number is 0x80 or higher)
...
So all MS operating systems require that either the BIOS maps around
the drive numbers or that something stores 0x81 in the boot sector.
Otherwise they cannot boot from a secondary harddisk. They all ignore
the drive number passed from the BIOS / MBR / boot loader.
Or course this does not stop us from ACCEPTING the drive number passed
from the BIOS / MBR / boot loader :-). But it seems to be okay to make
/USEBPBDRIVE on by default. Then the user can use either the default
drive numbers (0 or 0x80) or select a different number (e.g. 0x81)
manually with the /B ... option. Some "/DONOTUSEBPBDRIVE" option would
only be needed for smart boot managers / BIOSes which can pass other
drive numbers than 0x80 to the boot sector. You would have to test
yourself if your BIOS is able to do that, and if not, how it implements
booting from secondary harddisks (e.g. by mapping drive numbers in a
different way or by patching around in boot sectors?).
Eric
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