This way of operation is more robust than the "AI" used
before.
Add flags to mbr accessible from make.conf as BOOT_MBR_FLAGS.
Only one flag is defined now, "allow using packet mode", which
is 0x80 in accord with the rest of i386 boot code. The "packet"
flag is on by default.
PR: i386/70241
Submitted by: Valentin Nechayev <netch <@> netch.kiev.ua> (inital version)
Discussed with: jhb (by Valentin Nechayev)
Tested on: bochs (with EDD turned on or off by patching the BIOS), PCs
use the BIOS Equipment List to determine how many hard drives are
installed and if the drive number we received in %dl is valid.
- Don't bother to disable interrupts when setting up the stack. The 8086
and beyond implicitly disable interrupts after an instruction that sets
%ss (for example, a pop or a mov) so that you can safely set %ss and %sp
in two consecutive instructions. An exception to this is the lss
instruction, which can set both registers simultaneously and thus doesn't
need this hack.
- Add support for EDD BIOS extensions to support booting off of hard drives
of nearly arbitrary length.
