LILO displays error messages when it can’t function properly. These error messages should be sufficient to identify the problem. The most common error messages and their solutions are shown in Table 4.1.

Table 4.1 Common LILO error messages.

Message Solution
Can’t put the boot sector on logical partition X LILO attempted to put the boot sector on the correct root filesystem on a logical partition. MBRs can only boot primary partitions by default. Override with the -b option and an explicit boot partition value, or use the configuration variable boot=device.
Got bad geometry The disk controller (mostly SCSI) doesn’t support automatic geometry detection. Use the file /etc/lilo/disktab to provide the disk parameters.
Invalid partition table, entry X The sector/head/cylinder and linear addresses of the first sector of the partition don’t match. This error usually occurs when an operating system creates partitions not aligned to tracks. Try the fix-table option.
First sector doesn’t have a valid boot signature The first sector of the device doesn’t seem to be a valid boot sector. Check the device name or rerun LILO to install the boot sector.
Cylinder number is too big A file is located beyond the 1024th cylinder, which LILO can’t access because of BIOS limitations. In most cases, the extra disk space is lost.
XXX doesn’t have a valid LILO signature XXX was located but it isn’t a valid LILO entry. If XXX is the boot sector, use the -i option or the install option to install the LILO boot sector.
XXX has an invalid stage code The entry at XXX is corrupted. Rerun LILO.
Kernel XXX is too big. The kernel is larger than 512K, which LILO can’t handle. Remove some unused drivers and recompile the kernel.
Partition entry not found The partition is not in the partition table. Use fdisk to enter the partition number in the partition table.
Sorry, don’t know how to handle device XXX LILO can’t determine the disk parameters. Use the file /etc/lilo/disktab to specify them.

To prevent LILO from booting the system, you must disable the boot sector (by using fdisk to change the active partition) or remove it completely. Most versions of LILO can be quickly disabled with the command

/etc/lilo/lilo -u

If you are using the newer directory structure, substitute the pathname as necessary. Some later versions of LILO use the menu-driven setup utility instead of a lilo binary. In most cases, there is a file called /etc/lilo/install that can accomplish the same procedures.

When removing a LILO boot sector in the Master Boot Record of the disk, you must replace the MBR with another record. If you want to replace the MBR with a DOS MBR, from a booted DOS floppy disk, enter the command

fdisk /mbr

Because backup copies of the boot sector are created whenever LILO creates a new version, you can copy the older versions of the boot sector back in place (assuming they are still available). For example, to restore the Master Boot Record saved in a file called boot.0800 (800 is the device number of a SCSI drive), issue the command

dd if=/etc/lilo/boot.0800 of=/dev/sda bs=446 count=1

If you are using another device, substitute the name of the saved boot file and the device name.

LILO is usually able to obtain information about the hard disks and floppy disks on the system by reading the kernel. On some systems(especially some SCSI adapters and adapters that do not behave as IDE or SCSI normal devices), though, this isn’t possible. When LILO can’t obtain the disk parameter information, it generates an error message about “bad geometry.”

The disk parameters can be physically read by LILO from the file /etc/lilo/disktab, which exists only with some versions of Linux. When the disktab file exists, it takes precedence over any auto-detected values. The disktab file contains the device number (hexadecimal), its BIOS code, and the disk geometry. A sample disktab file could have the following entries:

# /etc/lilo/disktab - LILO disk paramter table

#

# Dev. num BIOS code Secs/track Heads/cyl Cyls Part. Offset

#

0×800 0×80 32 64 1714 0

0×801 0×80 32 64 1714 1001

This example shows a SCSI disk with two partitions. The first partition /dev/sda1 has a device number 800, and the second partition /dev/sda2 has the device number 801. Both partitions have the BIOS code 80. Both the device number and BIOS code have to be given in hex format, which accounts for the leading 0x. The disk has 32 sectors per track, 64 heads per cylinder, and 1714 cylinders. Because both partitions are on the same disk, these parameters will be the same.

The partition offset is an optional field. The first partition starts at offset 0, and the second starts at offset 1001 (the number of sectors from the start of the disk). The partition offsets need only be explicitly given when the kernel cannot obtain that information. Most hard disks (including removable and optical disks) don’t need the partition offsets, but CD-ROMs sometimes do.

When filling in the /etc/lilo/disktab file you don’t have to have the details exactly right. Most systems will remap the drive parameters to 32 sectors per track and 64 heads, whether those numbers are correct or not. (This is a BIOS action.) The number of cylinders must be at least equal to or higher than the number of actual cylinders to avoid truncation of the disk space the operating system recognizes.

Some BIOS versions will not allow disks exceeding certain values. This problem usually occurs with IDE and SCSI controller cards that are designed for DOS systems with relatively small (<1G) disk drives and older BIOSs. Device drivers allow some high-capacity drives to be used, although some systems will have a problem accessing files beyond the 1G limit.

LILO can boot a kernel image from several locations, such as a regular file on the root filesystem or any other mounted filesystem, a block device such as a floppy disk, or the boot sector of another partition or disk. The type of boot is dictated by entries in a configuration file. Boot image configuration files can have several variables defined, all of which have been mentioned in the preceding sections. Valid configuration file variables include alias, label, optional, password, ramdisk, read-only, read-write, restricted, root, and vga.

To boot a kernel image from a file, all that is necessary in the configuration file is the name of the image. For example, the line

image=/linux_main

boots the image called linux_main.

To boot an image from another device, the sectors that must be read on that device have to be specified. Several methods of providing the sector information exist. The starting sector must be provided, but you can then either specify a number of sectors to be read (start+length) or the end sector number (start-finish). If only one number is provided (the start sector), only that sector is read.

For example, the contents of this configuration file

image=/dev/fd0

range=1+512

will boot the kernel from the floppy disk, starting at sector 1 and reading the next 512 sectors.

You can specify more than one configuration for an image because LILO stores values in an image descriptor file and not in the image itself. For example, a configuration file can contain the following entries:

image=/linux_main

label=linux-hda1

root=/dev/hda1

image=/linux_main

label=linux-hda3

root=/dev/hda3

image=/linux_main

label=linux-flop

root=/dev/fd0

This code has three configurations for the same Linux kernel (linux_main), but it also has different root devices with three different alternate names. The boot devices for the image are /dev/hda1, /dev/hda3, and /dev/fd0 respectively. Whitespace in the configuration file is ignored, so the indentations are for ease of reading only.

You can store configuration options for the map installer in the file /etc/lilo/config or /etc/lilo.conf (depending on the version of Linux). The file consists of sets of parameter-value pairs, although some options do not need a value. You can use whitespace between the parameter and the equal sign and between the equal sign and the value. You can include comments by starting the line with a pound sign. A new line character terminates the comment. As a rule, variable names are case insensitive, while values are usually case sensitive. It is good practice, though, to keep all entries lowercase (as is UNIX convention).

You can put the following options into the map installer configuration file /etc/lilo/config or /etc/lilo.conf:

* The alias=name option allows an image to be called by the string name as well as its normal filename.

* The append=string option appends string to the command line passed to the kernel. This option is mostly used to pass boot parameters for hardware devices that are not automatically detected by the kernel.

* The backup=file option copies the original boot sector to file instead of /etc/lilo/boot.hex_num. You also can specify a device (like /dev/null) instead of a file.

* The boot=dev option specifies the device that contains the boot sector. If no name is specified, the currently mounted root partition is used.

* The compact option merges read requests for adjacent sectors into a single read request, reducing the load time and file size. This option is commonly used with floppy disks.

* The delay=secs option gives the time in tenths of a second that the system should wait before booting the image. If no delay is provided, the boot is immediate.

* The disktab=file option gives the name of the disk parameter table. If no filename is given, /etc/lilo/disktab is used.

* The fix-table option lets LILO adjust sector/head/cylinder addresses. This option is usually used with operating systems that may change these addresses. LILO readjusts incorrect entries if fix-table is specified.

* The force-backup=file option is similar to backup, but it overwrites any existing file. If force-backup is used in the configuration options, any other backup option is ignored.

* The install=sector option installs the image in the specified boot sector. If no value is given, /etc/lilo/boot.b is used.

* The label=name option renames an image to the alternate string name.

* The linear option generates linear sector addresses instead of sector/head/cylinder addresses. Linear addresses are independent of disk geometry and are translated in real time. Linear boot disks may not be portable.

* The literal=string option is similar to the append variable but it removes any other options, using only those specified in string.

* In the map=file option, file is the map file location. If no value is given, /etc/lilo/map is used.

* The message=file option uses the contents of file as a message displayed before the boot prompt. The message cannot be larger than 64K. If the message is changed or moved, the map file must be rebuilt.

* The optional option makes an image optional. If the image’s file can’t be located, it is not booted. This option is useful for testing new kernels.

* The password=password option sets a password for all images. If the restricted option exists, a password is required only to boot the image to which the configuration file refers.

* The prompt option forces the boot prompt without checking for any keypresses. This option is usually combined with the timeout option to force unattended reboots.

* The ramdisk=size option sets the optional RAM disk to size. A setting equal to zero suppresses the RAM disk.

* The read-only option mounts the root filesystem as read-only.

* The read-write option mounts the root filesystem as read-write.

* The restricted option relaxes password protection.

* The root=dev option specifies the device to be mounted as the root filesystem. If the value current is used, the root device is the device on which the root filesystem is currently mounted (unless changed with the -r command-line option).

* The serial=parms option sets a serial line for control, initializing the line and accepting input from it (as well as the console). The format of the parameters is port, baud_rate, parity, bits. When serial is set, the delay value is set to 20 automatically, unless this value is overridden.

* The timeout=secs option sets the number of tenths of a second that system waits for keyboard input before loading the image. this option is also used to specify password input timeouts. The default value is infinite.

* The verbose=level option displays progress messages. The higher the level, the more messages are displayed. If the -v command-line option is also included, the highest level specified in either variable is used.

* The vga=mode option sets the VGA text mode for use during booting. Valid values include normal (80×25 text mode), extended or ext (80×50 text mode), and ask (prompt for the mode during boot). To obtain a list of available modes, boot with the parameter vga=ask and press Enter when asked for a value. Case is not important in the values of the vga option.

If any parameter is not specified either on the command line or in the configuration file, default values are used. Some values are also maintained within the kernel image (such as ramdisk, root, and vga).

The LILO map installer utility accepts a number of options on the command line. Many of the command-line options are mirrored by configuration variables, discussed in the next section. The following list describes the command-line options:

* The -b dev option uses dev as the boot device. If no value is specified, the default device given by the boot configuration variable is used.

* The -c option turns on compact, merging read requests for adjacent sectors into one request to reduce load time. This option is often used for boot floppy disks. You also can use the compact configuration variable to specify this option.

* The -C file option uses file as the configuration file. If no file is specified, /etc/lilo/config is used as the default.

* The -d secs option specifies the number of tenths of a second to wait before booting the first image. You also can specify this number in the configuration variable delay.

* The -f file option uses file as the name of the disk parameter table (called disktab). If a filename is omitted, the file /etc/lilo/disktab is used.

* The -i sector option installs the kernel as the new boot sector. The argument to be used with this option can be read from the install configuration variable.

* The -I name option displays the path of the named kernel image file. If no matching name is found, an error is generated. You can add the v option after the name to verify the existence of the image file. This option uses the BOOT_IMAGE environment variable.

* The -l option generates linear sector addresses instead of the default sector/head/cylinder addresses. You also can specify this option with the configuration variable linear.

* The -m file option uses file as the location of the map file. If no filename is given, /etc/lilo/map is used.

* The -P fix option allows LILO to adjust sector/head/cylinder addresses using the table file. You also can specify this option with the fix-table configuration variable.

* The -P ignore option overrides correction of sector/head/cylinder addresses. You also can specify this option with the ignore-table configuration variable.

* The -q option displays the currently mapped files.

* The -r dir option performs a chroot command on dir before continuing. This option is necessary if the root filesystem is mounted in a different location from the map installer command. Because the current directory is changed with this command, use absolute pathnames for all files.

* The -R words option stores words in the map file for use by the boot loader. The words are parameters that the boot process uses as part of the default command line. The first word must be the name of the boot image.

* The -s file option copies the original boot sector to file instead of /etc/lilo/boot.hex_num.

* The -S file option is the same as -s, but it overwrites the old file if one exists.

* The -t option performs a test by executing the entire installation process except writing the map file and boot sector. Ideally, you use this option with the -v option to verify accurate behavior of the map installer.

* The -u dev option restores the backup copy of the boot sector for dev. If no device is specified, the default value is used. If the default is not a valid value, the current root device is used. The backup copy is checked for a time stamp before the write is completed.

* The -U dev option is the same as -u except that it doesn’t check for the time stamp.

* The -v level option uses the verbose output level specified to display messages.

* The -V option displays the version number of the map installer, and then exits.

The map installer is the program that updates the boot sector and creates the map file. The map installer is usually the file /etc/lilo/lilo. Whenever the map installer is running, it checks for errors in the boot sector. If an error is detected, no changes to the boot sector are written, and the installer terminates.

When a boot sector is successfully updated by the map installer, the old boot sector contents are copied into the directory /etc/lilo with the name boot.hex_num where hex_num is the hexadecimal device number of the partition that was rewritten. The hexadecimal device numbers were mentioned in the previous section. When the map installer writes to a partition’s boot sector, the old copy of the boot sector is stored in a file with the name part.hex_num. Again, hex_num is the number of the device.

You can modify the map installer’s behavior by supplying command line parameters when the installer is invoked or by making entries in the configuration file /etc/lilo/config. On later versions of Linux, including the version supplied on the CD-ROM with this book, the file is /etc/lilo.conf. A sample /etc/lilo.conf looks like the following:

# LILO configuration file

# generated by ‘liloconfig’

#

# Start LILO global section

boot = /dev/fd0

#compact # faster, but won’t work on all systems.

delay = 50

vga = normal # force sane state

ramdisk = 0 # paranoia setting

# End LILO global section

# Linux bootable partition config begins

image = /vmlinuz

root = /dev/sda3

label = Linux

read-only # Non-UMSDOS filesystems should be mounted read-only for checking

# Linux bootable partition config ends

# DOS bootable partition config begins

other = /dev/sda1

label = DOS

table = /dev/sda

# DOS bootable partition config ends

As you can see, this lilo.conf file uses a boot floppy disk to hold the LILO image (/dev/fd0) and avoids overwriting the MBR or superblock of a partition. The Linux partition is /dev/sda3 and is called Linux. A DOS partition, /dev/sda1, called DOS also exists on the hard drive. You can use many of the available options from either the LILO command line or the configuration file. The following sections discuss the configuration options.

Regardless of which boot process you use, LILO pauses momentarily when Linux is booting to check that the Shift, Ctrl, and Alt keys are not pressed and that Caps Lock and Scroll Lock are set. If none of the keys are pressed and the locks are on, LILO displays the boot prompt. At this point, LILO is waiting for the name of the boot image to use to be entered. If a timeout occurs or Enter is pressed, the default boot image is loaded. The boot image is the kernel of whichever operating system is to be loaded, including DOS.

If you want to boot an image other than the default, you can enter its name at the boot prompt. To obtain a list of all known boot images, enter a question mark or hit the Tab key (depending on the image and keyboard setting). Recent versions of Linux set the boot image name in a LILO file, as described in the next section. With some earlier versions of Linux, the name of the default boot image is located in the file /etc/rc (or in a file in the subdirectory /etc/rc.d) on a line with the keyword

BOOT_IMAGE=

You also can specify parameters for the kernel by entering them at the boot prompt. Unless overridden, the parameters will be used by the default image when it boots. Valid parameters differ a little depending on the version of Linux, but most versions support the following:

* The no387 parameter disables any on-board floating-point unit (FPU).

* The root parameter boots from a device specified after the root parameter, such as root=/dev/sda1. The root command takes either a hexadecimal device number or the full path name of the device.

* The ro parameter mounts the root filesystem as read-only.

* The rw parameter mounts the root filesystem as read-write.

* The single parameter boots the Linux system into single-user (system administrator) mode.

As mentioned in the preceding list, you can use a hexadecimal device number in the root parameter. The device numbers are assigned depending on the type of device. For floppy disk drives, the numbers are assigned starting with 200: /dev/fd0 is 200, /dev/fd1 is 201, /dev/fd2 is 202, and so on. Hard disks have numbers assigned depending on the type of device. For most hard disks, the numbers start at 301 (not 300 because there is no /dev/hd0): /dev/hda1 is 301, /dev/hda2 is 302, and so on. When a second hard drive is used, the numbers jump to 340: /dev/hdb1 is 341, /dev/hdb2 is 342, and so on. For /dev/sda devices, numbering starts at 801: /dev/sda1 is 801, /dev/sda2 is 802, and so on. The second hard drive starts at 811: /dev/sdb1 is 811, /dev/sdb2 is 812, and so on. Because floppy disk and hard disk drives are usually the only devices that can act as a boot devices, these numbers should suffice for all occurrences except removable media.

You can combine parameters if you separate them with a space. At the boot prompt, the line

image5 root=/dev/hda2 single ro

will boot the kernel called image5 located on the device /dev/hda2, for example. The filesystem will be mounted as a read-only device, and single-user mode will be invoked.

Most recent versions of Linux, including the one supplied on this book’s CD-ROM, let you manage LILO through a menu-driven routine usually started through setup. Typically, when you have made any changes to the installation (such as adding new software), the last component of the setup program asks you whether you want to work with LILO. Figure 4.3 shows this screen.

Figure 4.3.
Whenever you make changes to Linux through the setup utility, it asks whether you want to use LILO.

From the menu-driven system shown in Figure 4.3, select the Begin option. Usually, the first thing the program asks for is any boot-time instructions that need to be specified when Linux starts, as shown in Figure 4.4. Normally, there are no boot-time instructions, but if you had to enter any parameters to start the Linux installation process, specify the same options here.

Figure 4.4.
The LILO installation process starts by asking whether you need any special boot-time instructions.

The next prompt, shown in Figure 4.5, asks where you want your LILO instructions written. The primary options were discussed earlier in the chapter. This version of LILO lets you select the Master Boot Record (MBR), the master sector of the primary Linux partition (called a superblock on this screen), or a floppy disk. The method you select depends on how you want Linux to start and whether it coexists with another operating system. For a minimal impact on your system, select the floppy disk boot option. This option requires you to place the boot floppy disk in the floppy disk drive when starting the machine, but then it boots cleanly into Linux. If the boot floppy disk is left out of the floppy disk drive, any other active partition (such as DOS or OS/2) is booted. Because the LILO instructions are only written to floppy disk with this approach, existing MBRs or partition boot sectors don’t change at all. This approach makes it very easy to remove Linux and prevent problems with other operating systems that exist on your drives.

Figure 4.5.
You are asked where you want to install LILO.

If you are installing only Linux, or plan to have very rare use of existing DOS or OS/2 partitions, use the Master Boot Record of partition boot sector. The only drawback to this approach is that, you must low-level format your hard drive if you want to remove Linux in the future. The Linux MBR is not compatible with DOS’.

A compromise is to use the partition boot sector (superblock). This approach lets you boot straight into Linux when the partition is active or use fdisk to activate any other partition. Although this option has more impact than the floppy disk boot option, yo can easily remove it by reformatting the partition and installing a new operating system.

If you are installing Linux to experiment with it and you are not sure whether you will leave it on your hard drive, use the boot floppy disk. This option provides the most hassle-free approach for backing out of Linux in the future. You can always rerun LILO and write the boot instructions to the MBR or superblock later.

The LILO system asks you whether you want a delay before Linux starts to boot, as shown in Figure 4.6. You can decide to not pause, to wait either 5 or 30 seconds, or to wait until LILO is told explicitly to boot. This last option is useful if you have two operating systems on your hard drive because it saves you from fooling around with fdisk to alter the active partition. A good compromise is either 5 or 30 seconds.

Figure 4.6.
You can set a delay before booting into Linux with one of the options shown in this menu.

After you set the delay, LILO returns you to the main LILO menu (shown in Figure 4.3). The next step is to identify the partitions that LILO will know about. The menu lists three operating systems: DOS, OS/2, and Linux. If you have either DOS or OS/2, you can give the partition information to LILO to allow it to boot this operating system instead of Linux (assuming you have a delay in the boot process). Begin with the Linux operating system, though. Select Linux on the menu (the menu choices are in order of most common use from top to bottom, generally) to display the Linux partition information screen shown in Figure 4.7.

Figure 4.7.
Use this screen to identify the boot Linux partition. In this example, only a single Linux data partition exists.

The example shown in Figure 4.7 shows a single Linux data partition on the hard drive, called /dev/sda3. The Linux swap partitions never show up on these lists (unless they were misidentified when they were installed). Enter the name of the Linux partition that is to be used for the boot partition (in the example, /dev/sda3 would be typed in the entry field). The next screen, shown in Figure 4.8, asks for a name for this partition. This name is used to identify the partition at boot time. For Linux partitions, just enter the word Linux to provide an unambiguous name for the partition.

Figure 4.8.
You need to assign a name to the Linux partition.

After entering the partition name, you are back to the main LILO menu. Now, add any other operating systems, such as DOS or OS/2. To install a DOS partition, for example, select DOS from the menu. A screen very similar to the one for selecting the Linux partition appears, as shown in Figure 4.9. Enter the name of the DOS partition (in this case, /dev/sda1).

Figure 4.9.
When you add another operating system to LILO, the partitions that apply are displayed. You can add this partition to the LILO boot table.

After the partition has been identified, name the partition as shown in Figure 4.10. Again, a simple name is best, so enter DOS or OS/2 as applicable. Don’t bother with fancy names; they are more trouble than they are worth!

Figure 4.10.
As with the Linux partition, you need to assign a name to the other partitions LILO will know about.

After you name all the partitions, you are brought back yet again to the LILO menu. The next step is to install the LILO information. Select the Install option from the menu. The system writes all the partition details you’ve supplied, and then you can exit LILO. The other options on the LILO menu allow you to modify an existing LILO table. In most cases, it’s easier to just start again. You can use this automated LILO installation any time you want. Take care that you enter the proper information, though, as an error can make it impossible to access a partition. Keep an emergency boot disk handy!

A slight modification of the last boot process is replacing the normal MBR with a utility called BOOTACTV, which prompts for the partition to boot from. This utility requires that a non-DOS-compatible copy of the boot sector be written, so you should use it only when Linux will be the dominant operating system and LILO is not booting the other operating systems properly.

When in place, the Master Boot Record holds a copy of BOOTACTV. When booted, BOOTACTV enables you to choose which operating system to boot. BOOTACTV can then read a boot sector from a partition to load that operating system. When the MBR holds BOOTACTV, you can’t use the MBR as you normally would with other operating systems, such as DOS or OS/2. You can, though, replace BOOTACT with a normal MBR.

To install BOOTACTV, follow these steps:

1. Boot Linux as usual. Make sure you have a boot floppy disk in case of problems.

2. Copy your existing Master Boot Record to a floppy disk in case of problems. The command to copy the MBR from the main drive (/dev/hda) to a floppy disk using 512 character blocks (the default) is
dd if=/dev/hda of=/fd/MBR bs=512 count=1
3. Use the setup or LILO installation program to copy LILO into the boot sector of the Linux partition (not the Master Boot Record).

4. Install BOOTACTV into the Master Boot Record. The BOOTACTV utility is usually called bootactv.bin and should be in the current directory when you install it into the MBR with the command
dd if=bootactv.bin of=/dev/hda bs=446 count=1
5. Reboot the machine to boot BOOTACTV from the Master Boot Record.

Your machine should load BOOTACTV and allow you to boot any other operating system on a partition. If Linux or another operating system does not boot, use your boot floppy disk to start Linux. If only Linux doesn’t boot, the boot sector LILO for the Linux partition is not working and can be rewritten using the setup or LILO configuration utilities. If none of the partitions boot, remove BOOTACTV by replacing the old Master Boot Record with the command

dd if=/fd/MBR of=/dev/hda bs=446 count=1

You can also reinstall the Master Boot Record from within DOS, if you have a DOS boot floppy disk. When in DOS, issue the command

fdisk /mbr

If you don’t want to alter your Master Boot Record but have more than one partition dedicated to Linux, you can install BOOTACTV on one of the partition’s boot sectors and use the fdisk utility to toggle the active partition. However, if the hard disk is repartitioned or the filesystems are altered in size, the boot sector will have to be rewritten. To write BOOTACTV to the fourth primary hard disk partition, for example, copy the existing MBR to the partition’s boot sector, and then install BOOTACTV with the following commands:
dd if=/dev/hda of=/dev/hda4 bs=512 count=1
dd if=bootactv.bin of=/dev/hda4 bs=446 count=1

Installing DOS and Linux

Most Linux installations coexist with DOS and use the DOS Master Boot Record. In this case, the MBR is read and the active partition (set by fdisk) is booted automatically. This installation method is one of the safest because no changes to the DOS-installed Master Boot Record are performed, and it is easy to remove or reconfigure partitions at any time without worrying about compatibility with the MBR.

Later versions of DOS (6.00 or higher) will overwrite an existing MBR if they are installed after Linux. If this happens, you won’t be able to boot Linux from the MBR, although DOS will boot. You can fix the problem by running LILO again or by making the Linux partition active.

You can change the active partition at any time using the fdisk utility or the Linux utility activate. The setup program within Linux can usually change the boot partition, too. Only one partition on a hard disk can be active at a time. Some operating systems, including Linux, let you change your mind about which operating system to boot after the active partition has been read, assuming a delay was built into the boot process. Linux, for example, can display the boot prompt and wait for a reply or a timeout to occur before starting to boot Linux.

To use this type of approach for Linux, install LILO into the boot sector of the Linux partition. To make it bootable, run fdisk and set that partition number as the active partition. Rebooting the machine will boot into the active partition. When Linux is replaced or removed, the boot sector of the new operating system will overwrite the Linux partition’s boot sector, requiring no changes to the MBR.

Using BOOTLIN

With the Linux BOOTLIN configuration, which is also a common method of installation, the Master Boot Record does not change. During the boot process, the Master Boot Record is read, and then a decision about which operating system to load is made. This decision is usually based on a user prompt. Essentially, this boot process is the same as a normal DOS boot except the program BOOTLIN is invoked in either the CONFIG.SYS or AUTOEXEC.BAT files. This program can then execute a program that lets you choose the operating system to load. The program BOOT.SYS, for example, may be used to present a menu that enables you to choose between a Linux and DOS boot.

To install BOOTLIN in your DOS partition, follow these steps:

1. Boot Linux. Make sure you have a boot floppy disk in case of problems.

2. Place a copy of the Linux kernel in your DOS partition either through DOS or with one of the Linux Mtools. You only have to copy the kernel file into the home directory (or any subdirectory) of the DOS partition. You can even do this step from the floppy disk.

3. Copy BOOT.SYS and BOOTLIN.SYS to the DOS partition, using the same process you used to copy the Linux kernel.

4. Add both the BOOT.SYS and BOOTLIN.SYS files to your CONFIG.SYS file.

5. Make sure DOS is the active partition, and reboot the machine.

When DOS starts, the BOOT device driver should give you the option of booting DOS or Linux. If you have problems, remove the BOOT.SYS and BOOTLIN.SYS files from the CONFIG.SYS file and you are back to normal.

Using the BOOT.SYS program has a useful advantage in that no boot sectors are altered to support several operating systems. As a result, loading and removing operating systems from a hard disk is easier. You can use both the Master Boot Record with active partition and BOOT.SYS approaches so that the hard disk starts to boot whichever operating system has the active flag, and then pauses and waits for confirmation from the user (or a timeout to occur). In this case, no changes to the Master Boot Record need to be made.