freebsd-src/sys/i386/boot/dosboot

Hi Everybody!

This is version 1.5 of "fbsdboot", a program that allows you to boot a kernel
from a MS-DOS partition or a FreeBSD partition. This program runs using DOS.
It works with various memory managers (like  EMM386, 386MAX) under certain
circumstances.

First, a FreeBSD kernel is always loaded to memory starting at 0x100000. To
assure that loading the kernel *does not* overwrite memory used by memory
managers, high memory for the kernel is allocated and after loading the kernel
it's moved to 0x100000.

Second, there are many ways to switch to protected mode which is necessary to
start the kernel. Each BIOS gives you the possibility to use INT15H (AH=89H)
to do that. But some memory-managers like 386max does not allow you to use
this method.

An other way to do the switch is to use DPMI services, but they do not
guarantee, that the protected mode application is executed with privilege
level 0. Therefore this method is *not* used.

VCPI services offer another way to switch to protected mode, and VCPI servers
are built into "emm386.exe", "386max" and "qemm". That's why, this method is
implemented in fbsdboot.exe.

Fbsdboot.exe tries to switch to protected mode using VCPI services. If they're
not available INT15H is used to do the switch. If that fails, it's not possible
for this version of fbsdboot.exe to boot a kernel :-(.

You can get commandline options of fbsdboot if you start it with "-?" as option!

I don't know, if fbsdboot works with QEMM, as I don't have the possibility to
test it.

Enjoy and have fun!

Christian.
cg@fimp01.fim.uni-linz.ac.at


PS: Many thanks to Bruce Evans for his assistance!