NOTE: This was the annoncement on linux-kernel of bzImage+initrd
      Starting with kernel version 1.3.73 bzImage+initrd is part of the
      official kernel.
      YOU NEED NOT TO APPLY THE PATCH
-------------------------------------------------------------------------

From lermen@elserv.ffm.fgan.deWed Mar  6 11:10:17 1996
Date: Wed, 6 Mar 1996 11:09:46 +0100 (MET)
From: Hans Lermen <lermen@elserv.ffm.fgan.de>
To: linux-kernel@vger.rutgers.edu
Cc: Werner Almesberger <almesber@lrc.epfl.ch>
Subject: PATCH: Linux high in the sky and more

Hi,

this is a patch that adds direct loading of kernels above 1 MB, support for
an initial RAM disk that is loaded by the boot loader and that can be
mounted as root before the "real" root is mounted, plus a few minor fixes
to make LOADLIN work better, particularly in W95 environments. The initial
RAM disk feature (we call it simply "initrd") should help to confine the
number of prebuilt kernels a distribution has to come with to something
reasonable, i.e. one.

All these features will be supported in the next versions of LILO and
LOADLIN, which both are essentially ready to be released.

The features are in detail:
 - kernels can be loaded directly above 1 MB, which removes the 0.5 MB
   compressed size restriction. This also works with dd'ed floppy boot.
 - a file (e.g. a RAM disk image) can be loaded by the boot loader and is
   then available to the kernel, e.g. for mounting
 - when such a RAM disk is mounted on /, a special initialization program
   /linuxrc is executed. When it terminates, the RAM disk can be unmounted
   (or "moved" to a different mount point) and the "real" root FS is mounted.
   The root FS device can be altered from within that environment.
 - no loading restrictions when using LOADLIN under Windows 95 (the 0x90000
   segment is mostly occupied by W'95 )

Attached is a detailed description of what the patch does. The patch itself
(for 1.3.71) can be found at
ftp://lrcftp.epfl.ch/pub/people/almesber/lilo/bzImage+initrd-1.3.72.patch.gz
and
ftp://elserv.ffm.fgan.de/pub/linux/loadlin-1.6/bzImage+initrd-1.3.72.patch.gz

Since the first announce of this patch in mid-February, some additional
features have been included, bugs have been fixed and documentation has been
added. We think it is stable enough to be included in the mainstream kernel.


Cheers, Werner & Hans

---------------------------------- cut here -----------------------------------

Loading of big kernels (1 MB compressed)
----------------------------------------

We call the new kernel format bzImage (big-zImage), and in fact it's the
the old zImage kernel with the following changes:

1. The gunzip code (arch/i386/boot/compressed/misc.c) will now be linked at
   0x100000. Because it would overwrite itself while decompressing, the
   first part of the decompressed data is written to low mem. When low mem
   becomes exhausted, then the rest is put behind __end. When decompression
   has completed, then a small move routine is copied down to low mem,
   which then moves the kernel at its proper place. This technique has the
   advantage that it needs the fewest changes to the old zImage.

2. The arch/i386/boot/setup.S has gotten a header, which from the kernel's
   point of view is always located at 0x90202. This header contains
   information used to coordinate the operation of loaders and the kernel
   (e.g. to detect inconsistencies).

3. Because the kernel size is given as a 16 bit value (segment format) in
   the bootsector, the compressed size of the bzImage is limited to 0xffff0
   Bytes (~1MB). Given a compression ratio of 1:2, which is quite
   realistic, the decompressed kernel can be about 2 Meg.


Loading of ramdisk directly by the loaders
------------------------------------------

With the same technique used for the kernel, also a ramdisk image can be
loaded high. The image is loaded directly below mem_end. The start address
and the size are put into the setup.S header so the kernel can find the
data. The pages used by the image are marked "in use" and are freed later,
when no longer needed.

We've extended the RAM disk code to allow loading of a RAM disk from that
data. (Typically, the loaded data will be compressed.) Then, that initrd
can be mounted as root and the executable /linuxrc will be run. /linuxrc
can now access additional media and load modules. (Actually, /linuxrc can
do most things init can do.)

When done, /linuxrc terminates, the "real" root file system is mounted
and the initrd is either unmounted or it is "moved" to the mount point
/initrd. Note that, in the latter case, the initrd is not unmounted, so
any open files do not need to be closed, i.e. processes can survive the
root change.

This avoids the need to either have a large collection of precompiled
kernels, to re-compile the kernel, or to have a "kernel link set". In
particular, the boot floppy could be self-contained, i.e. no additional
media would be necessary to create a bootable hard disk based system.

There are a few other options, like
 - if root=/dev/ram, we boot directly into the RAM disk, using init
 - from within the initrd environment, the root FS device can be changed
   (using /proc/sys/kernel/real-root-dev)
 - if root=/dev/ram _after_ initrd, we simply start init in initrd

A detailed description of the initrd technique can be found within
the patch in the file linux/Documentation/initrd.txt.


Better support for LOADLIN
--------------------------

The fact that LOADLIN operates in a DOS/Windows environment adds two
complications:

 - the kernel cannot be loaded in physically contiguous memory. Instead,
   LOADLIN has to re-arrange the pages after executing setup (which may have
   to be done in VM86 mode), but before booting the actual kernel
 - segment 0x9000 may be occupied (i.e. under Windows 95), so that the
   setup.S code must run in a different segment

For those two reasons, the code32_start has been added to the setup.S
header and setup.S itself has been made position independent. Since
earlier versions of LOADLIN already used a setup header similar to the
one we need now anyway, we've retained compatibility by basing the new
header on the old one.


Final remarks
-------------

Given that many distributions will be based exclusively on the next major
release of Linux for the next year, we think that it is important that
this patch be integrated into 1.4/2.0, although the feature freeze is
already in place.

The next versions of LOADLIN and LILO will support the features described
above. Some preliminary test versions are available on
ftp://elserv.ffm.fgan.de/pub/linux/loadlin-1.6/loadlin-1.6-pre11-bin.tgz
and
ftp://lrcftp.epfl.ch/pub/people/almesber/lilo/lilo.18dev3.tar.gz

This patch was jointly developed by Werner Almesberger and Hans Lermen.

-- 
  _________________________________________________________________________
 / Werner Almesberger, DI-LRC,EPFL,CH   werner.almesberger@lrc.di.epfl.ch /
/_IN_R_133__Tel_+41_21_693_6621__Fax_+41_21_693_6610_____________________/

--

Hans Lermen
<lermen@elserv.ffm.fgan.de>
