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<head><title>LILO
Generic boot loader for Linux
Version 21.5
Technical overview</title>
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<div class="maketitle">
<h2 class="titleHead">LILO<br />
Generic boot loader for Linux<br />
Version 21.5<br />
 <br />
Technical overview</h2>
<div class="author" ><span
class="cmr-12">Werner Almesberger</span>
<br /> <span
class="cmr-12">John R. Coffman</span>
<br /><span
class="cmti-12">(reconstructed by Joachim Wiedorn)</span></div>
<br />
<div class="date" ><span
class="cmr-12">July 17, 2000</span></div>
</div>
<h3 class="likesectionHead"><a
id="x1-1000"></a>Contents</h3>
<div class="tableofcontents">
<span class="sectionToc" >1 <a
href="#x1-20001" id="QQ2-1-2">Load sequence</a></span>
<br /><span class="sectionToc" >2 <a
href="#x1-30002" id="QQ2-1-3">File references</a></span>
<br /><span class="sectionToc" >3 <a
href="#x1-40003" id="QQ2-1-4">Configuration parameters</a></span>
<br /><span class="sectionToc" >4 <a
href="#x1-50004" id="QQ2-1-5">Parameter line interface</a></span>
<br /><span class="sectionToc" >5 <a
href="#x1-60005" id="QQ2-1-6">External interface</a></span>
<br /><span class="sectionToc" >6 <a
href="#x1-70006" id="QQ2-1-7">Default command line in map file</a></span>
</div>
<!--l. 72--><p class="noindent" >This document describes internals of LILO and related parts of its environment (kernel, etc.). It is not
necessary to read or understand this document in order to install or use LILO. A general introduction and
installation instructions can be found in the user’s guide.
</p><!--l. 77--><p class="noindent" ><span
class="cmbx-10x-x-109">This document has only been partially updated and does reflect the status of Linux (until</span>
<span
class="cmbx-10x-x-109">2.2.16) and of LILO</span><span
class="cmbx-10x-x-109"> (until 21.5) in the year 2000.</span>
</p><!--l. 80--><p class="noindent" >
</p>
<h3 class="sectionHead"><span class="titlemark">1 </span> <a
id="x1-20001"></a>Load sequence</h3>
<!--l. 87--><p class="noindent" >The boot sector is loaded by the ROM-BIOS at address 0x07C00. It moves itself to address 0x96A00, sets
up the stack (growing downwards from 0x96A00 to 0x96800), loads the secondary boot loader at address
0x96C00 and transfers control to it. It displays an “L” after moving itself and an “I” before starting the
secondary boot loader. If a read error occurs when loading the secondary boot loader, a two-digit hex
code is displayed after the “L”. This results in an endless stream of error codes if the problem
is permanent. Displaying these error codes is disabled if the build-time option <span
class="cmtt-10x-x-109">NO1STDIAG </span>is
set.
</p><!--l. 97--><p class="noindent" >The secondary boot loader loads the descriptor table at 0x98800 and the sector containing the
default command line at 0x98C00. If the default command line is enabled, its magic number is
invalidated and the sector is written back to disk. This potentially dangerous operation can be
disabled by defining <span
class="cmtt-10x-x-109">LCF</span><span
class="cmtt-10x-x-109">_READONLY </span>when passing <span
class="cmtt-10x-x-109">second.S </span>through <span
class="cmss-10x-x-109">cpp</span>. Next, the secondary
boot loader checks for user input. If either the default is used or if the user has specified an
alternate image, the options sector is loaded at 0x98C00 and the parameter line is constructed at
0x99000. If the resulting line contains the option <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">lock</span></span></span>, the command line as entered by the
user (it is saved before the final line is constructed) is written to the disk as the new default
command line. Also, if a fallback command line is set, it is copied to the default command line
sector.
</p><!--l. 111--><p class="noindent" >If the user has supplied an initial RAM disk image, this file is loaded below the end of physical memory or
16 MB, whichever is lower. The start address is lowered to the next page boundary so that the memory area
occupied by the initial RAM disk can later be easily returned to the system’s free memory pool. The 16 MB
limit exists because the BIOS functions used to transfer data in memory are only specified for an 24 bit
address space.
</p><!--l. 119--><p class="noindent" >Next, the floppy boot sector of that image is loaded at
0x90000<span class="footnote-mark"><a
href="tech_21-52.html#fn1x0"><sup class="textsuperscript">1</sup></a></span><a
id="x1-2001f1"></a>,
the setup part is loaded at 0x90200 and the kernel part is loaded at 0x10000, or, if the kernel has been
compiled for being loaded “high” (i.e. with <span
class="cmtt-10x-x-109">make bzImage</span>), it is loaded at 0x100000 instead. During the
load operations, the sectors of the map file are loaded at 0x98600.
</p><!--l. 126--><p class="noindent" >If the loaded image is a kernel image, control is transferred to its setup code. If a different operating system
is booted, things are a bit more difficult: the chain loader is loaded at 0x90200 and the boot sector of the
other OS is loaded at 0x90400. The chain loader moves the partition table (loaded at 0x903BE as part of
the chain loader) to 0x00600 and the boot sector to 0x07C00. After that, it passes control to the boot
sector.
</p><!--l. 134--><p class="noindent" >Chain loaders that allow booting from a second drive (either floppy or hard disk) also install a
small function to intercept BIOS calls and to swap the drive numbers at the top of available
memory.
</p><!--l. 138--><p class="noindent" >The secondary boot loader displays an “L” after being started and an “O” after loading the descriptor table
and the default command line. Before loading the descriptor table, it checks, whether it has been loaded at
the correct location and displays a question mark if it hasn’t. If the descriptor table has an incorrect
checksum, a minus sign is displayed.
</p><!--l. 144--><p class="noindent" >
</p>
<center class="math-display" >
<img
src="tech_21-50x.png" alt="0x00000 |-------------------------------|1982 bytes
|-------------------------------|
0x007BE |--------Partition-table---------|64 bytes
0x007FE |-------------------------------|29 kB
0x07C00 |--------Boot-load-area---------|512 bytes
0x07E00 |-------------------------------|32.5 kB
0x10000 | |448 kB
| |
| Kernel |
| |
| |
|-------------------------------|
0x90000 |------Floppy-boot-sector-------|512 bytes
0x90200 |---------Setup-(kernel)---------|39.5 kB (2 kB used)
0x9A000 |------Primary--boot-loader-------|512 bytes
0x9A200 | Stack |3.5 kB
0x9B000 |-----Secondary-boot-loader------|8 kB (3.5 kB used)
0x9D000 |--------Map--load-area---------|512 bytes
|-------------------------------|
0x9D200 |--------Descriptor-table---------|1 kB
0x9D600 |---Default command--line, etc.--|512 bytes
0x9D800 |---Keyboard-translation-table----|512 bytes
0x9DA00 |Parameter-line-construction-area-|1 kB
0x9DC00 | |7.5 kB
| |
|--------Drive-swapper----------|1 kB
---------------------------------
0xA0000
" class="math-display" /></center>
<!--l. 192--><p class="noindent" >The area 0x90020-0x90023 is overlaid by a command-line descriptor while the secondary boot loader is
running.
</p><!--l. 195--><p class="noindent" >If the build-time configuration option <span
class="cmtt-10x-x-109">LARGE</span><span
class="cmtt-10x-x-109">_EBDA </span>is set, all the addresses in the area 0x90000-0x9FFFF are
changed to 0x80000-0x8FFFF, with the exception of the location of the driver swapper, which automatically
follows the end of the available memory.
</p><!--l. 200--><p class="noindent" >
</p>
<h3 class="sectionHead"><span class="titlemark">2 </span> <a
id="x1-30002"></a>File references</h3>
<!--l. 206--><p class="noindent" >This section describes the references among files involved in the boot procedures.
</p><!--l. 209--><p class="noindent" >
</p>
<center class="math-display" >
<img
src="tech_21-51x.png" alt=" --------------|
| Default |
/| |
// -command--line---
/ | Image |
/ // | |
/ / |-descriptors-- |
/ / | |
// / | |
-------------/-/ | map |
|Boot sector /\ |
| \\|\
-------------\\\\ \ \ \ |-------------|
\ \\ \ \ Primary |
\ \ \ | boot loader |
\\\ \ \ ---------------
\\ \ | |
\ \\ | |
\ \ |- --- --- -- |
\ \| Secondary |
\\ | boot loader |
\ |- --- --- -- |
\\| |
| |
---------------
boot.b
" class="math-display" /></center>
<!--l. 213--><p class="noindent" >The boot sector contains the primary boot loader, the address of the default command line sector, the
address of both descriptor table sectors and the addresses of the sectors of the secondary boot loader. The
generic boot sector is copied from <span
class="cmtt-10x-x-109">boot.b</span>.
</p><!--l. 218--><p class="noindent" >The primary boot loader can store up to eight sector addresses of the secondary boot loader.
</p><!--l. 221--><p class="noindent" >
</p>
<center class="math-display" >
<img
src="tech_21-52x.png" alt="|-------------|
| Default |
| |
--command--line-|
| |
| Image |
- --- --- --- -
| |
| descriptors |
--------------|
| |
| Zero sector |
--------------|
| Keyboard |
| |
--trans. table-|
| First |
| |
----section----|
| |
| Second |
- --- --/ --- -
| // |
| section |
--------------|
| Third |
| |
----section----|
" class="math-display" /></center>
<!--l. 225--><p class="noindent" >The map file consists of so-called sections and of special data sectors. Each section spans an integral number
of disk sectors and contains addresses of sectors of other files.
</p><!--l. 230--><p class="noindent" >There are three exceptions: 1. If a “hole” is being covered or if the floppy boot sector of an unstripped
kernel has been omitted, the address of the zero sector is used. This sector is part of the map file. 2. When
booting a different operating system, the first sector is the merged chain loader that has been written to the
map file before that section. 3. Each map section describing an image is followed by a sector containing the
options line of that image.
</p><!--l. 238--><p class="noindent" >The last address slot of each map sector is either unused (if the map section ends in this sector) or contains
the address of the next map sector in the section.
</p><!--l. 242--><p class="noindent" >The ifive sectors at the beginning of the map file are special: the first sector contains the default command
line, the next two sectors contain the boot image descriptor table and the fourth sector is filled with zero
bytes. This sector is mapped whenever a file contains a “hole”. The fifth sector contains the keyboard
translation table.
</p><!--l. 248--><p class="noindent" >
</p>
<center class="math-display" >
<img
src="tech_21-53x.png" alt="|-----------| --------------|
|Descriptor | | |
------------ | |
| | \ |-------------| //-- -- --- -- -
|| || \\ | | / | |
| | | Fallback | / | |
| | | | / /| |
| | -- --- -- --- - / / --- -- --- -- -
| | | Options |////// | |
|| || | // / | |
| | |- --- -- ---/// --- -- --- --
| ||| // | |
| | /| | |
| -- -// /-/--- -
|| | | | |
| | | | |
| -------------\-\ --- -- --- -- -
| \ \\ | |
| Map section | |
|| ---------------
|
optional | Kernel image
|
|
||----------------------------------|
| | | |
| | | |
-- --/ /-/--- - --- -- --- -- -
| / | | |
| | | |
-------------\-
\ | |
Map section \ | |
\ --- -- --- -- -
\\ | |
| |
| |
---------------
Initial RAM disk
" class="math-display" /></center>
<!--l. 252--><p class="noindent" >A kernel image consists simply of a sequence of sectors being loaded. The map section also contains a sector
with a fallback command line and a sector with parameter line options. Optionally, a RAM disk image,
specified by a second map section, can be loaded.
</p><!--l. 257--><p class="noindent" >
</p>
<center class="math-display" >
<img
src="tech_21-54x.png" alt=" |--------------| |-------------|
| | | Original |
| chain.b | | partition |
|-----------| -----/---------| /----table------
| | / /
-Descriptor-- / / /
\ // / /
\\ |--------------| / /
| | / //
| Fallback |/ /
- --- -- --- --/ / /
| //| / /
| (Options)/ /
| --- -- --/// /-|
| \
| Chain loader |\ |
| --- -- --- --cccccccccccmmmmmmmmmmmcmcmcmcmcmcmcmccccrmrmrmrmrmrrrrrrrrrrrrrmmmmnnnnnmmmmmmmmmmmmmrmrmrmrm.....n.nnnnnnnnnnnnnnnn................
| cccccccmmcmcmcmcccccc/mmmmmcmcmcmcmcmcmcmcrrrrrmmmmmmcmcmcmccccccrrrrrrrrrrrrmmmmmmrmrmrmrrrrrrmmmmmmcmcmcmcmccmmmmmmmmmmmmrmnnnnnnnnmmmmmmrmrmrmrrrrrrrmmnnnnnnnnnnnnn........n.nnnnnnnnmmmmmmmrmrm.....................n.nnnnnnnn........ |-------------|
| /-|----------|Boot sector |
---------------| | |
---------------
Map section
" class="math-display" /></center>
<!--l. 261--><p class="noindent" >When booting another operating system, the chain loader (<span
class="cmtt-10x-x-109">chain.b</span>) is merged with the patched partition
table<span class="footnote-mark"><a
href="tech_21-53.html#fn2x0"><sup class="textsuperscript">2</sup></a></span><a
id="x1-3001f2"></a>
and written into the map file. The map section of this boot image starts after that sector and contains only
the address of a dummy floppy boot sector (the zero sector, but its contents are irrelevant), the loader
sector and the boot sector of the other operating system. Not that the map section also contains the
fallback sector and a (useless) sector for options.
</p>
<h3 class="sectionHead"><span class="titlemark">3 </span> <a
id="x1-40003"></a>Configuration parameters</h3>
<!--l. 275--><p class="noindent" >The boot sector of each kernel contains a set of configuration parameters that have to be available at boot
time before the kernel can access file systems. These parameters can be set when the kernel is
compiled and later be changed with programs like <span
class="cmss-10x-x-109">rdev</span>. LILO can supersede the parameters (in
memory) at boot time by placing the corresponding items on the parameter line passed to the
kernel.
</p><!--l. 282--><p class="noindent" >The parameters are stored at the following (decimal) offsets:
</p><!--l. 284--><p class="noindent" >
</p><dl class="description"><dt class="description">
<span
class="cmbx-10x-x-109">497</span> </dt><dd
class="description">the size of the setup code in sectors (512 bytes). Older kernels may put a zero at this place.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">498-499</span> </dt><dd
class="description">is a flag specifying whether the root file system should be mounted read-only (if non-zero)
or read-write (if zero).
</dd><dt class="description">
<span
class="cmbx-10x-x-109">500-501</span> </dt><dd
class="description">the size of the kernel, counted in paragraphs (16 bytes).
</dd><dt class="description">
<span
class="cmbx-10x-x-109">502-503</span> </dt><dd
class="description">this parameter is currently unused.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">504-505</span> </dt><dd
class="description">the size of the RAM disk in kilobytes. No RAM disk is created if this parameter is set to
zero.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">506-507</span> </dt><dd
class="description">the text mode the VGA is set to.
<dl class="description"><dt class="description">
<span
class="cmbx-10x-x-109">0xFFFD</span> </dt><dd
class="description">the user is asked to specify the VGA mode at boot time.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">0xFFFE</span> </dt><dd
class="description">uses 80x50 (“extended”) mode.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">0xFFFF</span> </dt><dd
class="description">uses 80x25 (“normal”) mode.</dd></dl>
<!--l. 299--><p class="noindent" >Any other value selects the corresponding mode as displayed in the interactive VGA mode selection
menu. This is the only option that is set by LILO by patching the boot sector instead of passing it on
the parameter line.
</p></dd><dt class="description">
<span
class="cmbx-10x-x-109">508</span> </dt><dd
class="description">the minor number of the device that should be mounted as root.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">509</span> </dt><dd
class="description">the major number of the device that should be mounted as root.</dd></dl>
<!--l. 309--><p class="noindent" >
</p>
<h3 class="sectionHead"><span class="titlemark">4 </span> <a
id="x1-50004"></a>Parameter line interface</h3>
<!--l. 311--><p class="noindent" >The kernel supports processing of parameters that are provided by the boot loader. The parameter string is
a NUL-terminated ASCII string that contains space-separated words or <span
class="cmti-10x-x-109">variable</span><span
class="cmtt-10x-x-109">=</span><span
class="cmti-10x-x-109">value </span>pairs. A description
of how they are interpreted can be found in the section of the user’s guide labeled “The boot
prompt”.
</p><!--l. 318--><p class="noindent" >The following descriptor has to be set up to pass a parameter string to the kernel:
</p><!--l. 321--><p class="noindent" >
</p><dl class="description"><dt class="description">
<span
class="cmbx-10x-x-109">0x90020</span> </dt><dd
class="description">the magic number 0xA33F.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">0x90022</span> </dt><dd
class="description">the offset of the first byte of the parameter line relative to 0x90000.</dd></dl>
<!--l. 327--><p class="noindent" >The boot loader composes the parameter line from the command line, from the options sector and from
some internally generated prefixes (typically <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">auto</span></span></span> and <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">BOOT_IMAGE=</span></span></span>), as follows:
</p><!--l. 331--><p class="noindent" >
</p>
<center class="math-display" >
<img
src="tech_21-55x.png" alt=" |---------------------------------------|
| Image | |
Command line | name | Command -line options |
---------------------------\------------
| \ \ \
| \ \
| \ \
| \ \ \
|-----------|----------------------------------|-------\\-------------------|
| Pre- | Image | | |
|fixes | name | Static options | Command -line options |
--------------------------------|--------------------------------------------
Parameter string |
passed to the kernel |
|
|
|----------------------------|
Options sector | Static options |
| |
------------------------------
" class="math-display" /></center>
<!--l. 335--><p class="noindent" >Example: <br
class="newline" />Command line: <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">vmlinuz</span><span
class="cmtt-10x-x-109"> root=802</span></span></span><br
class="newline" />Options sector: <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">root=801</span><span
class="cmtt-10x-x-109"> ro</span></span></span>
</p><!--l. 339--><p class="noindent" >yields <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">BOOT_IMAGE=vmlinuz</span><span
class="cmtt-10x-x-109"> root=801</span><span
class="cmtt-10x-x-109"> ro</span><span
class="cmtt-10x-x-109"> root=802</span></span></span>
</p><!--l. 341--><p class="noindent" >Because parameter line options can typically be overridden, the first <span class="obeylines-h"><span class="verb"><span
class="cmtt-10x-x-109">root</span></span></span> option is ignored by the
kernel.
</p><!--l. 346--><p class="noindent" >
</p>
<h3 class="sectionHead"><span class="titlemark">5 </span> <a
id="x1-60005"></a>External interface</h3>
<!--l. 348--><p class="noindent" >LILO is able to receive its command line from a program that is booted before it. This externally provided
command line is only used if the user does not use the normal mechanism to invoke the boot
prompt.
</p><!--l. 352--><p class="noindent" >The following register contents are expected:
</p><!--l. 354--><p class="noindent" >
</p><dl class="description"><dt class="description">
<span
class="cmbx-10x-x-109">DL</span> </dt><dd
class="description">contains the value 0xFE.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">ES:SI</span> </dt><dd
class="description">points to the string “LILO”. The string must be in upper case and no terminating character
is needed. The string must not cross segment boundaries, i.e. <span
class="cmbx-10x-x-109">SI </span>must be below 0xFFFD.
</dd><dt class="description">
<span
class="cmbx-10x-x-109">ES:BX</span> </dt><dd
class="description">points to a NUL-terminated string that is used as the command line. This string has a
maximum length of 78 characters (not including the terminating NUL) and must not cross
segment boundaries.</dd></dl>
<!--l. 364--><p class="noindent" >There are two values of the externally provided command line that have a special meaning:
</p>
<ul class="itemize1">
<li class="itemize">an empty string (<span
class="cmbx-10x-x-109">ES:BX </span>points to a NUL byte) is interpreted as a request to enter the boot
prompt and to accept keyboard input.
</li>
<li class="itemize">a string that consists only of blanks is interpreted as a request to boot the default boot image.</li></ul>
<!--l. 374--><p class="noindent" >LILO can also obtain the default command line from the map file. It is only used if no externally provided
command line is available.
</p><!--l. 379--><p class="noindent" >
</p>
<h3 class="sectionHead"><span class="titlemark">6 </span> <a
id="x1-70006"></a>Default command line in map file</h3>
<!--l. 381--><p class="noindent" >The first sector of the map file is reserved for a default command line. Unless the user invokes the boot
prompt by pressing a shift key or unless an externally provided command line is present, the command line
in the map file is interpreted as if it had been typed on the keyboard.
</p><!--l. 386--><p class="noindent" >The first two bytes of the first sector of the map file have to contain the magic number <span
class="cmtt-10x-x-109">DC</span><span
class="cmtt-10x-x-109">_MAGIC </span>(0xF4F2)
in little-endian byte order. They are followed by a NUL-terminated string with a maximum length of 510
bytes, including the NUL. Note that the boot loader limits command lines to 78 characters after removing
duplicate spaces.
</p><!--l. 392--><p class="noindent" >The command line is disabled by either clobbering the magic number or by using an empty string (i.e. only
a NUL byte) as the command line.
</p>
<div class="center"
>
<!--l. 397--><p class="noindent" >
</p><!--l. 398--><p class="noindent" ><span
class="cmti-10x-x-109">Last updated by John R. Coffman on July 17, 2000</span><br />
<span
class="cmti-10x-x-109">reconstructed by Joachim Wiedorn on January 29, 2011</span></p></div>
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