Baslinux mailing list

[kenm AT surfbest.net]

From: Ken Martwick <kenm AT surfbest.net>
To: baslinux AT lists.ibiblio.org
Subject: BL3 - helping Xvesa
X-Mailer: GyikSoft Mailer for UNIX v3.2 by Arpi/ESP-team 
(http://esp-team.scene.hu)

When a PC first boots, the BIOS provides (real-mode) interrupt service
via an array of 256 "interrupt vectors" at the bottom of addressable
memory.  These do not normally concern Linux, but they do affect the
booting process if loadlin is used and have a major impact on the
compact Xvesa server.  Assuming the BIOS interrupt vectors are good
(many are buggy), the problems begin when DOS boots and adds its
changes to the vectors; these won't be usable after Linux boots.
My first idea to correct the Xvesa problems was to write a "clean"
set of BIOS interrupt vectors into memory after BL3 boots.  This
is still a good idea, but I haven't figured out exactly how to
do it, especially at physical address zero!
It is easier to make such a correction while still in DOS, but
DOS must still be able to allow loadlin to boot Linux, so my
idea was to only "correct" some of the altered vectors.  In
fact, I wrote a quick program to change the first 32.
On the basis of tests with a 486, a Pentium, and a Pentium II,
this seems to work flawlessly and to allow booting from a
"normal" DOS environment with XMS ("himem.sys" or equal).  I
do not use emm386, so I cannot say if it would work.  Tests
were conducted using PTS-DOS and FreeDOS.
In practice, the messiest part of this procedure is getting a
clean set of interrupt vectors.  I have found a "multiboot"
program which comes with utilities to do this with the use
of a temporary boot floppy.  After that, all that is needed is
to run my "fix1f.exe" program with the vectors "vector.bin" in
the same directory.  If someone can provide a site for this
(<200K) package, I believe I could email it as an attachment.
Ken Martwick