by H. Peter Anvin

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{ SYSLINUX Questions } { What is SYSLINUX? }
SYSLINUX is a boot loader for the Linux operating system which operates off an MS-DOS/Windows FAT filesystem. It is intended to simplify first-time installation of Linux, and for creation of rescue- and other special-purpose boot disks.

SYSLINUX can be used, when properly set up, to completely eliminate the need for distribution of raw diskette images for boot floppies. A SYSLINUX floppy can be manipulated using standard MS-DOS (or any other OS that can access an MS-DOS filesystem) tools once it has been created.

{ What SYSLINUX is Not... }
SYSLINUX is probably not suitable as a general purpose boot loader. It can only boot Linux from a FAT filesystem, and not, for example, ext2. Since a native Linux implementation will typically use ext2, another boot loader (e.g. LILO) is probably more suitable. In a system which actually contains DOS or Windows, LOADLIN may be simpler to use.

However, SYSLINUX has shown itself to be quite useful in a number of special-purpose applications.

{ How do I Create a Bootable Floppy? }
In order to create a bootable Linux floppy using SYSLINUX, prepare a normal MS-DOS formatted floppy. Copy one or more Linux kernel files to it, then execute the DOS command:
syslinux [-s] a:
(or whichever drive letter is appropriate; the [] meaning -s is optional)

If you're running in a Win95/98/ME DOS box, you should execute the command lock a: first. If you're running in a WinNT/2K DOS box, you will probably get a dialog box about not getting exclusive access and with Abort/Retry/Ignore buttons; people have reported that selecting "Ignore" makes the command complete correctly.

Under Linux, execute the command:

syslinux [-s] [-o offset] /dev/fd0
(or, again, whichever device is the correct one.)

This will alter the boot sector on the disk and copy a file named LDLINUX.SYS into its root directory.

The -s option, if given, will install a "safe, slow and stupid" version of SYSLINUX. This version may work on some very buggy BIOSes on which SYSLINUX would otherwise fail. If you find a machine on which the -soption is required to make it boot reliably, please send as much info about your machine as you can, and include the failure mode.

The -o option is used with a disk image file and specifies the byte offset of the filesystem image in the file.

On boot time, by default, the kernel will be loaded from the image named LINUX on the boot floppy. This default can be changed, see the section on the SYSLINUX config file.

If the Shift or Alt keys are held down during boot, or the Caps or Scroll locks are set, SYSLINUX will display a LILO-style "boot:" prompt. The user can then type a kernel file name followed by any kernel parameters. The SYSLINUX loader does not need to know about the kernel file in advance; all that is required is that it is a file located in the root directory on the disk.

{ How do I Configure SYSLINUX? }
All the configurable defaults in SYSLINUX can be changed by putting a file called SYSLINUX.CFG in the root directory of the boot floppy. This is a text file in either UNIX or DOS format, containing one or more of the following items (case is insensitive for keywords; upper case is used here to indicate that a word should be typed verbatim):

All options here applies to PXELINUX as well as SYSLINUX unless otherwise noted. See pxelinux.doc for additional information on PXELINUX.

DEFAULT kernel options...

Sets the default command line. If SYSLINUX boots automatically, it will act just as if the entries after DEFAULT had been typed in at the "boot:" prompt, except that the option "auto" is automatically added, indicating an automatic boot.

If no configuration file is present, or no DEFAULT entry is present in the config file, the default is kernel name "linux", with no options.

APPEND options...
Add one or more options to the kernel command line. These are added both for automatic and manual boots. The options are added at the very beginning of the kernel command line, usually permitting explicitly entered kernel options to override them. This is the equivalent of the LILO "append" option.
IPAPPEND flag_val [PXELINUX only]
The IPAPPEND option is available only on PXELINUX, and indicates (if the flag value is 1) that an option of the following format should be generated and added:
... based on the input from the DHCP/BOOTP or PXE boot server.
LABEL label
KERNEL image
APPEND options...
IPAPPEND flag_val [PXELINUX only]

Indicates that if "label" is entered as the kernel to boot, SYSLINUX should instead boot "image", and the specified APPEND and IPAPPEND options should be used instead of the ones specified in the global section of the file (before the first LABEL command.) The default for "image" is the same as "label", and if no APPEND is given the default is to use the global entry (if any). Up to 128 LABEL entries are permitted. (for ISOLINUX, 64 LABEL entries.)

Note that LILO uses the syntax:

image = mykernel
  label = mylabel
  append = "myoptions"

... whereas SYSLINUX uses the syntax:
label mylabel
  kernel mykernel
  append myoptions

Notes: Labels are mangled as if they were filenames, and must be unique after mangling. For example, two labels "v2.1.30" and "v2.1.31" will not be distinguishable under SYSLINUX, since both mangle to the same DOS filename.

The "kernel" doesn't have to be a Linux kernel; it can be a boot sector or a COMBOOT file


Append nothing. APPEND with a single hyphen as argument in a LABEL section can be used to override a global APPEND.
On PXELINUX, specifying "LOCALBOOT 0" instead of a "KERNEL" option means invoking this particular label will cause a local disk boot instead of booting a kernel.

The argument 0 means perform a normal boot. The argument 4 will perform a local boot with the Universal Network Driver Interface (UNDI) driver still resident in memory. Finally, the argument 5 will perform a local boot with the entire PXE stack, including the UNDI driver, still resident in memory. All other values are undefined. If you don't know what the UNDI or PXE stacks are, don't worry -- you don't want them, just specify 0.

On ISOLINUX, the "type" specifies the local drive number to boot from; 0x00 is the primary floppy drive and 0x80 is the primary hard drive. The special value -1 causes ISOLINUX to report failure to the BIOS, which, on recent BIOSes, should mean that the next boot device in the boot sequence should be activated.

IMPLICIT flag_val
If flag_val is 0, do not load a kernel image unless it has been explicitly named in a LABEL statement. The default is 1.
TIMEOUT timeout
Indicates how long to wait at the boot: prompt until booting automatically, in units of 1/10 s. The timeout is cancelled as soon as the user types anything on the keyboard, the assumption being that the user will complete the command line already begun. A timeout of zero will disable the timeout completely, this is also the default.

NOTE: The maximum possible timeout value is 35996; corresponding to just below one hour.

SERIAL port [baudrate]
Enables a serial port to act as the console. "port" is a number (0 = /dev/ttyS0 = COM1, etc.); if "baudrate" is omitted, the baud rate defaults to 9600 bps. The serial parameters are hardcoded to be 8 bits, no parity, 1 stop bit.

For this directive to be guaranteed to work properly, it should be the first directive in the configuration file.

FONT filename
Load a font in .psf format before displaying any output (except the copyright line, which is output as ldlinux.sys itself is loaded.) SYSLINUX only loads the font onto the video card; if the .psf file contains a Unicode table it is ignored. This only works on EGA and VGA cards; hopefully it should do nothing on others.
KBDMAP keymap
Install a simple keyboard map. The keyboard remapper used is *very* simplistic (it simply remaps the keycodes received from the BIOS, which means that only the key combinations relevant in the default layout -- usually U.S. English -- can be mapped) but should at least help people with AZERTY keyboard layout and the locations of = and , (two special characters used heavily on the Linux kernel command line.)

The included program from the LILO distribution can be used to create such keymaps. The file keytab-lilo.doc contains the documentation for this program.

DISPLAY filename
Displays the indicated file on the screen at boot time (before the boot: prompt, if displayed). This option takes the place of the LINUXMSG.TXT and BOOTMSG.TXT files in SYSLINUX 1.0. Please see the section below on DISPLAY files.

NOTE: If the file is missing, this option is simply ignored.

PROMPT flag_val
If flag_val is 0, display the boot: prompt only if the Shift or Alt key is pressed, or Caps Lock or Scroll lock is set (this is the default). If flag_val is 1, always display the boot: prompt. This option takes the place of testing for the LINUXMSG.TXT file in SYSLINUX 1.0.
F1 filename
F2 filename
F9 filename
F0 filename
Displays the indicated file on the screen when a function key is pressed at the boot: prompt. This can be used to implement pre-boot online help (presumably for the kernel command line options.) Note that F10 MUST be entered in the config file as "F0", not "F10", and that there is currently no way to bind file names to F11 and F12. Please see the section below on DISPLAY files.

When using the serial console, press <Ctrl-F><digit> to get to the help screens, e.g. <Ctrl-F><2> to get to the F2 screen, and <Ctrl-F><0> for the F10 one.

In the configuration file blank lines and comment lines beginning with a hash mark (#) are ignored.

Note that the configuration file is not completely decoded. Syntax different from the one described above may still work correctly in this version of SYSLINUX, but may break in a future one.

{ Can SYSLINUX Handle Large Kernels? }
This version of SYSLINUX supports large kernels (bzImage format), eliminating the 500K size limit of the zImage kernel format. bzImage format kernels are detected automatically and handled transparently to the user.

This version of SYSLINUX also supports a boot-time-loaded ramdisk (initrd). An initrd is loaded from a DOS file if the option "initrd=filename" (where filename is the filename of the initrd image; the file must be located in the root directory on the boot floppy) is present on the processed command line (after APPEND's have been added, etc.). If several initrd options are present, the last one has precedence; this permits user-entered options to override a config file APPEND. Specifying "initrd=" without a filename inhibits initrd loading. The file specified by the initrd= option will typically be a gzipped filesystem image.

NOTE: One of the main advantages with SYSLINUX is that it makes it very easy to support users with new or unexpected configurations, especially in a distribution setting. If initrd is used to extensively modularize the distribution kernel, it is strongly recommended that a simple way of adding drivers to the boot floppy be provided. The suggested manner is to let the initrd system mount the boot floppy and look for additional drivers in a predetermined location.

To bzImage and recent zImage kernels, SYSLINUX 1.30 and higher will identify using the ID byte 0x31. PXELINUX identifies using the ID byte 0x32, and ISOLINUX 0x33. The ID range 0x34-0x3f is reserved for future versions of derivatives of SYSLINUX.

{ What is the DISPLAY File Format? }
DISPLAY and function-key help files are text files in either DOS or UNIX format (with or without <CR>). In addition, the following special codes are interpreted:

<FF> <FF> = <Ctrl-L> = ASCII 12
Clear the screen, home the cursor. Note that the screen is filled with the current display color.
<SI><bg><fg> <SI> = <Ctrl-O> = ASCII 15
Set the display colors to the specified background and foreground colors, where <bg> and <fg> are hex digits, corresponding to the standard PC display attributes
0 = black               8 = dark grey
1 = dark blue           9 = bright blue
2 = dark green          a = bright green
3 = dark cyan           b = bright cyan
4 = dark red            c = bright red
5 = dark purple         d = bright purple
6 = brown               e = yellow
7 = light grey          f = white

Picking a bright color (8-f) for the background results in the corresponding dark color (0-7), with the foreground flashing.

Colors are not visible over the serial console.

<CAN>filename<newline> <CAN> = <Ctrl-X> = ASCII 24
If a VGA display is present, enter graphics mode and display the graphic included in the specified file. The file format is an ad hoc format called LSS16; the included Perl program "ppmtolss16" can be used to produce these images. This Perl program also includes the file format specification.

The image is displayed in 640x480 16-color mode. Once in graphics mode, the display attributes (set by <SI> code sequences) work slightly differently: the background color is ignored, and the foreground colors are the 16 colors specified in the image file. For that reason, ppmtolss16 allows you to specify that certain colors should be assigned to specific color indicies.

Color indicies 0 and 7, in particular, should be chosen with care: 0 is the background color, and 7 is the color used for the text printed by SYSLINUX itself.

<EM> <EM> = <Ctrl-Y> = ASCII 25
If we are currently in graphics mode, return to text mode.
<DLE>..<ETB> <Ctrl-P>..<Ctrl-W> = ASCII 16-23
These codes can be used to select which modes to print a certain part of the message file in. Each of these control characters select a specific set of modes (text screen, graphics screen, serial port) for which the output is actually displayed:
Character			Text	Graph	Serial
<DLE> = <Ctrl-P> = ASCII 16	No	No	No
<DC1> = <Ctrl-Q> = ASCII 17	Yes	No	No
<DC2> = <Ctrl-R> = ASCII 18	No	Yes	No
<DC3> = <Ctrl-S> = ASCII 19	Yes	Yes	No
<DC4> = <Ctrl-T> = ASCII 20	No	No	Yes
<NAK> = <Ctrl-U> = ASCII 21	Yes	No	Yes
<SYN> = <Ctrl-V> = ASCII 22	No	Yes	Yes
<ETB> = <Ctrl-W> = ASCII 23	Yes	Yes	Yes
For example:

<DC1>Text mode<DC2>Graphics mode<DC4>Serial port<ETB>

... will actually print out which mode the console is in!

<SUB> <SUB> = <Ctrl-Z> = ASCII 26
End of file (DOS convention).

{ Can SYSLINUX Boot Other Operating Systems? }
This version of SYSLINUX supports chain loading of other operating systems (such as MS-DOS and its derivatives, including Windows 95/98), as well as COMBOOT-style standalone executables (a subset of DOS .COM files; see separate section below.)

Chain loading requires the boot sector of the foreign operating system to be stored in a file in the root directory of the filesystem. Because neither Linux kernels, boot sector images, nor COMBOOT files have reliable magic numbers, SYSLINUX will look at the file extension. The following extensions are recognized:

none or other Linux kernel image
.cbt COMBOOT image (not runnable from DOS)
.bss Boot sector (DOS superblock will be patched in)
.bs Boot sector
.com COMBOOT image (runnable from DOS)
For filenames given on the command line, SYSLINUX will search for the file by adding extensions in the order listed above if the plain filename is not found. Filenames in KERNEL statements must be fully qualified.

Can I have an example?

Yes! Here's how you can make a SYSLINUX disk that can boot either DOS or Linux. The example assumes that the drive is A: in DOS and /dev/fd0 in Linux; for other drives, substitute the appropriate drive designator.

Method I: Using both DOS & Linux to make the floppy

  1. Make a DOS bootable disk. This can be done either by specifying the /s option when formatting the disk in DOS, or by running the DOS command SYS (this can be done under DOSEMU if DOSEMU has direct device access to the relevant drive):
    format a: /s
    sys a:
  2. Boot Linux. Copy the DOS boot sector from the disk into a file:
    dd if=/dev/fd0 of=dos.bss bs=512 count=1
  3. Run SYSLINUX on the disk:
    [lock a: if you're running under Win95/98/ME]
    syslinux /dev/fd0
  4. Mount the disk and copy the DOS boot sector file to it. The file *must* have extension .bss:
    mount -t msdos /dev/fd0 /mnt
    cp dos.bss /mnt
  5. Copy the Linux kernel image(s), initrd(s), etc to the disk, and create/edit syslinux.cfg and help files if desired:
    cp vmlinux /mnt
    cp initrd.gz /mnt
  6. Unmount the disk (if applicable.)
    umount /mnt
Method II: Using only DOS to make the floppy

To make this installation in DOS only, you need the utility (included with SYSLINUX) as well as the installer.

  1. Make a DOS bootable disk. This can be done either by specifying the /s option when formatting the disk in DOS, or by running the DOS command SYS:
    format a: /s
    sys a:
  2. Copy the DOS boot sector from the disk into a file. The file *must* have extension .bss:
    copybs a: a:dos.bss
  3. Run SYSLINUX on the disk:
    syslinux a:
  4. Copy the Linux kernel image(s), initrd(s), etc to the disk, and create/edit syslinux.cfg and help files if desired:
    copy vmlinux a:
    copy initrd.gz a:
{ What Are COMBOOT Files? }
A COMBOOT file is a standalone executable in DOS .COM format. They can, among other things, be produced by the Etherboot package by Markus Gutschke and Ken Yap. The following requirements apply for these files to be sufficiently "standalone" for SYSLINUX to be able to load and run them: SYSLINUX requires that COMBOOT files end in ".COM" or ".CBT". Files ending in .COM can be run from the DOS command line, files ending in .CBT cannot, otherwise there is no difference. SYSLINUX will prefer a .CBT file over a similarly named .COM.

SYSLINUX currently doesn't provide any form of API for the use of COMBOOT files. If there is need, a future version may contain an INT interface to some SYSLINUX functions; please contact me if you have a need or ideas for such an API.

{ What Protections Exist In SYSLINUX? }
SYSLINUX will attempt to detect if the user is trying to boot on a 286 or lower class machine, or a machine with less than 608K of low ("DOS") RAM (which means the Linux boot sequence cannot complete). If so, a message is displayed and the boot sequence aborted. Holding down the Ctrl key while booting disables this feature.

The compile time and date of a specific SYSLINUX version can be obtained by the DOS command "type ldlinux.sys". This is also used as the signature for the LDLINUX.SYS file, which must match the boot sector.

Any file that SYSLINUX uses can be marked hidden, system or readonly if so is convenient; SYSLINUX ignores all file attributes. The SYSLINUX installed automatically sets the readonly attribute on LDLINUX.SYS.

{ Does SYSLINUX Support Bootable CDs? }
SYSLINUX can be used to create bootdisk images for El Torito-compatible bootable CD-ROMs. However, it appears that many BIOSes are very buggy when it comes to booting CD-ROMs. Some users have reported that the following steps are helpful in making a CD-ROM that is bootable on the largest possible number of machines: A CD-ROM is so much faster than a floppy that the -s option shouldn't matter from a speed perspective.

Of course, you want to use ISOLINUX instead. See isolinux.doc.

{ Can I Use SYSLINUX on a Hard Drive? }
SYSLINUX can boot from a FAT12 or FAT16 filesystem partition on a hard disk (FAT32, introduced in Windows 95 OSR-2, is not supported, however.) The installation procedure is identical to the procedure for installing it on a floppy, and should work under either DOS or Linux. To boot from a partition, SYSLINUX needs to be launched from a Master Boot Record or another boot loader, just like DOS itself would.

Under DOS, you can install a standard simple MBR on the primary hard disk by running the command:

Then use the FDISK command to mark the appropriate partition active.

A simple MBR, roughly on par with the one installed by DOS (but unencumbered), is included in the SYSLINUX distribution

{ What Bugs are Still in SYSLINUX? }
SYSLINUX is unsafe to use on any filesystem that extends past cylinder 1024. This is a fundamental limitation of the standard BIOS API.

SYSLINUX will not work (and will refuse to install) on filesystems with a cluster size of more than 16K (typically means a filesystem of more than 1 GB.)

{ How Can I Give Feedback? }
I would appreciate hearing of any problems you have with SYSLINUX. I would also like to hear from you if you have successfully used SYSLINUX, *especially* if you are using it for a distribution.

If you are reporting problems, please include all possible information about your system and your BIOS; the vast majority of all problems reported turn out to be BIOS or hardware bugs, and I need as much information as possible in order to diagnose the problems.

There is a mailing list for discussion among SYSLINUX users and for announcements of new and test versions. To join, please visit; you can also examine the list archives there.

Any feedback on these instructions or on the HTML formatting should be cc:'ed to David Weekly.