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.
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 command:
- syslinux /dev/fd0
This will alter the boot sector on the disk and copy a file named LDLINUX.SYS into its root directory.
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 configuration 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(8) -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.
Syslinux supports the loading of initial ramdisks (initrd) and the bzImage kernel format.
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).
In the configuration file blank lines and comment lines beginning with a hash mark (#) are ignored.
- 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.
- If no configuration file is present, or no "default" entry is present in the configuration file, the default is "linux auto".
- NOTE: Earlier versions of SYSLINUX used to automatically
- append the string "auto" to whatever the user specified using the DEFAULT command. As of version 1.54, this is no longer true, as it caused problems when using a shell as a substitute for "init." You may want to include this option manually.
- 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(8) "append" option.
- kernel image append options ...
- Indicates that if label is entered as the kernel to boot, syslinux should instead boot image, and the specified "append" 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). Use "append -" to use no options at all. Up to 128 "label" entries are permitted.
- Labels are mangled as if they were DOS filenames, and must be unique after mangling. For example, two labels "v2.1.30" and "v2.1.31" will not be distinguishable.
- The "image" doesn't have to be a Linux kernel; it can be a boot sector or a COMBOOT file (see below.)
- 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. 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 keytab-lilo.pl(8) from the lilo(8) distribution can be used to create such keymaps.
- display filename
- Displays the indicated file on the screen at boot time (before the boot: prompt, if displayed). Please see the section below on DISPLAY files. 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.
f1 filename f2 filename ... f9 filename f10 filename f11 filename f12 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.)
- 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. For f10-f12, hit <Ctrl-F>A, <Ctrl-F>B, <Ctrl-F>C. For compatiblity with earlier versions, f10 can also be entered as <Ctrl-F>0.
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> = <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-U> = 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> = <Ctrl-Z> = ASCII 26
- End of file (DOS convention).
Comboot Images and 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 recognised:
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.
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:
- The program must not execute any DOS calls (since there is no DOS), although it may call the BIOS. The only exception is that the program may execute INT 20h (Terminate Program) to return to the syslinux prompt. Note especially that INT 21h AH=4Ch, INT 21h AH=31h or INT 27h are not supported.
- Only the fields pspInt20 at offset 00h, pspNextParagraph at offset 02h and pspCommandTail at offset 80h (contains the arguments from the syslinux command line) in the PSP are supported. All other fields will contain zero.
- The program must not modify any main memory outside its 64K segment if it returns to syslinux via INT 20h.
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.
Novice protection 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.
Bootable CD-ROMs 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:
- Use the -s (safe, slow and stupid) option to SYSLINUX
- Put the boot image as close to the beginning of the ISO 9660 filesystem as possible.
A CD-ROM is so much faster than a floppy that the -s option shouldn't matter from a speed perspective.
Of course, you probably want to use ISOLINUX instead. See the documentation file isolinux.doc.
Booting from a FAT partition on a hard disk SYSLINUX
can boot from a FAT filesystem partition on a hard disk (including FAT32). 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. A sample master boot sector (mbr.bin
) is included with SYSLINUX
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, send a message to firstname.lastname@example.org with the line:
in the body of the message. The submission address is email@example.com.