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376 lines
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<TITLE>The Linux Bootdisk HOWTO: FAQ</TITLE>
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<H2><A NAME="s5">5. FAQ</A></H2>
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<H2><A NAME="ss5.1">5.1 Q. How can I make a boot disk with a XXX driver?</A></H2>
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<P>The easiest way is to obtain a Slackware kernel from your nearest
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Slackware mirror site. Slackware kernels are generic kernels which atttempt
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to include drivers for as many devices as possible, so if you have a
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SCSI or IDE controller, chances are that a driver for it is included
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in the Slackware kernel.</P>
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<P>Go to the a1 directory and select either IDE or SCSI
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kernel depending on the type of controller you have. Check the xxxxkern.cfg
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file for the selected kernel to see the drivers which have been included
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in that kernel. If the device you want is in that list, then the
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corresponding kernel should boot your computer. Download the xxxxkern.tgz
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file and copy it to your boot diskette as described above in the section
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on making boot disks.</P>
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<P>You must then check the root device in the kernel, using the rdev
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command:
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<BLOCKQUOTE><CODE>
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<PRE>
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rdev zImage
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Rdev will then display the current root device in the kernel. If this
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is not the same as the root device you want, then use rdev to change it.
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For example, the kernel I tried was set to /dev/sda2, but my root
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scsi partition is /dev/sda8. To use a root diskette, you would have
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to use the command:
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<BLOCKQUOTE><CODE>
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<PRE>
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rdev zImage /dev/fd0
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>If you want to know how to set up a Slackware root disk as well,
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that's outside the scope of this HOWTO, so I suggest you check the
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Linux Install Guide or get the Slackware distribution. See the section
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in this HOWTO titled "References".</P>
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<H2><A NAME="ss5.2">5.2 Q. How do I update my boot floppy with a new kernel?</A></H2>
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<P>Just copy the kernel to your boot diskette using the dd command for
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a boot diskette without a filesystem, or the cp command for a
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boot/root disk. Refer to the section in this HOWTO titled "Boot"
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for details on creating a boot disk. The description applies equally
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to updating a kernel on a boot disk.</P>
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<H2><A NAME="ss5.3">5.3 Q. How do I remove LILO so that I can use DOS to boot again?</A></H2>
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<P>This is not really a Bootdisk topic, but it is asked so often, so: the
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answer is, use the DOS command:
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<BLOCKQUOTE><CODE>
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<PRE>
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FDISK /MBR
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>MBR stands for Master Boot Record, and it replaces the boot sector
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with a clean DOS one, without affecting the partition table. Some
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purists disagree with this, but even the author of LILO, Werner
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Almesberger, suggests it. It is easy, and it works.</P>
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<P>You can also use the dd command to copy the backup saved by LILO
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to the boot sector - refer to the LILO documentation if you wish
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to do this.</P>
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<H2><A NAME="ss5.4">5.4 Q. How can I boot if I've lost my kernel AND my boot disk?</A></H2>
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<P>If you don't have a boot disk standing by, then probably
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the easiest method is to obtain a Slackware kernel for your
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disk controller type (IDE or SCSI) as described above for "How do I
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make a boot disk with a XXX driver?". You can then boot your
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computer using this kernel, then repair whatever damage there is.</P>
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<P>The kernel you get may not have the root device set to the disk
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type and partition you want. For example, Slackware's generic
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scsi kernel has the root device set to /dev/sda2, whereas my
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root Linux partition happens to be /dev/sda8. In this case the
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root device in the kernel will have to be changed.</P>
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<P>You can still change the root device and ramdisk settings in the kernel
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even if all you have is a kernel, and some other operating system,
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such as DOS.</P>
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<P>Rdev changes kernel settings by changing the
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values at fixed offsets in the kernel file, so you can do the same
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if you have a hex editor available on whatever systems you do still
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have running - for example, Norton Utilities Disk Editor under DOS.
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You then need to check and if necessary change the values in the
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kernel at the following offsets:
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<BLOCKQUOTE><CODE>
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<PRE>
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0x01F8 Low byte of RAMDISK size
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0x01F9 High byte of RAMDISK size
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0x01FC Root minor device number - see below
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0X01FD Root major device number - see below
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>The ramdisk size is the number of blocks of ramdisk to create.
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If you want to boot from a root diskette then set this to decimal
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1440, which is 0x05A0, thus set offset 0x01F8 to 0xA0 and
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offset 0x01F9 to 0x05. This will allocate enough space for
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a 1.4Mb diskette.</P>
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<P>The major and minor device numbers must be set to the device
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you want to mount your root filesystem on. Some useful values
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to select from are:
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<BLOCKQUOTE><CODE>
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<PRE>
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device major minor
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/dev/fd0 2 0 1st floppy drive
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/dev/hda1 3 1 partition 1 on 1st IDE drive
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/dev/sda1 8 1 partition 1 on 1st scsi drive
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/dev/sda8 8 8 partition 8 on 1st scsi drive
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Once you have set these values then you can write the file to
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a diskette using either Norton Utilities Disk Editor, or a program called
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rawrite.exe. This program is included
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in several distributions, including the SLS and Slackware distributions.
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It is a DOS program which writes a file to the "raw" disk, starting
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at the boot sector, instead of writing it to the file system. If you use
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Norton Utilities, then you must write the file to a physical disk
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starting at the beginning of the disk.</P>
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<H2><A NAME="ss5.5">5.5 Q. How can I make extra copies of boot/root diskettes?</A></H2>
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<P>It is never desirable to have just one set of rescue disks - 2 or 3
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should be kept in case one is unreadable.</P>
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<P>The easiest way of making copies of any diskettes, including
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bootable and utility diskettes, is to use the dd command to copy the
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contents of the original diskette to a file on your hard drive, and
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then use the same command to copy the file back to a new diskette.
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Note that you do not need to, and should not, mount the diskettes,
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because dd uses the raw device interface.</P>
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<P>To copy the original, enter the command:
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<BLOCKQUOTE><CODE>
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<PRE>
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dd if=devicename of=filename
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where devicename the device name of the diskette
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drive
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and filename the name of the file where you
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want to copy to
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>For example, to copy from /dev/fd0 to a temporary file called
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/tmp/diskette.copy, I would enter the command:
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<BLOCKQUOTE><CODE>
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<PRE>
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dd if=/dev/fd0 of=/tmp/diskette.copy
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Omitting the "count" parameter, as we have done here, means that the
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whole diskette of 2880 (for a high-density) blocks will be copied.</P>
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<P>To copy the resulting file back to a new diskette, insert the new
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diskette and enter the reverse command:
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<BLOCKQUOTE><CODE>
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<PRE>
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dd if=filename of=devicename
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Note that the above discussion assumes that you have only one diskette
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drive. If you have two of the same type, then you can copy diskettes
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using a command like:
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<BLOCKQUOTE><CODE>
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<PRE>
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dd if=/dev/fd0 of=/dev/fd1
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H2><A NAME="ss5.6">5.6 Q. How can I boot without typing in "ahaxxxx=nn,nn,nn" every time?</A></H2>
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<P>Where a disk device cannot be autodetected it is necessary to supply
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the kernel with a command device parameter string, such as:
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<BLOCKQUOTE><CODE>
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<PRE>
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aha152x=0x340,11,3,1
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</PRE>
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</CODE></BLOCKQUOTE>
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This parameter string can be supplied in several ways using LILO:
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<UL>
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<LI>By entering it on the command line every time the system
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is booted via LILO. This is boring, though.</LI>
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<LI>By using the LILO "lock" keyword to make it store the
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command line as the default command line, so that LILO will use
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the same options every time it boots.</LI>
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<LI>By using the APPEND statement in the lilo config file. Note that
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the parameter string must be enclosed in quotes.</LI>
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</UL>
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</P>
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<P>For example, a sample command line using the above parameter string
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would be:
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<BLOCKQUOTE><CODE>
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<PRE>
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zImage aha152x=0x340,11,3,1 root=/dev/sda1 lock
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</PRE>
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</CODE></BLOCKQUOTE>
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This would pass the device parameter string through, and also ask the
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kernel to set the root device to /dev/sda1 and save the whole command
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line and reuse it for all future boots.</P>
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<P>A sample APPEND statement is:
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<BLOCKQUOTE><CODE>
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<PRE>
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APPEND = "aha152x=0x340,11,3,1"
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Note that the parameter string must NOT be enclosed in quotes on the
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command line, but it MUST be enclosed in quotes in the APPEND statement.</P>
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<P>Note also that for the parameter string to be acted on, the kernel
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must contain the driver for that disk type. If it does not, then there
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is nothing listening for the parameter string, and you will have to
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rebuild the kernel to include the required driver. For details on
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rebuilding the kernel, cd to /usr/src/linux and read the README, and
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read the Linux FAQ and Installation HOWTO. Alternatively you could
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obtain a generic kernel for the disk type and install that.</P>
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<P>Readers are strongly urged to read the LILO documentation before
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experimenting with LILO installation. Incautious use of the "BOOT"
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statement can damage partitions.</P>
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<H2><A NAME="ss5.7">5.7 Q. How can I create an oversize ramdisk filesystem?</A></H2>
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<P>An oversize ramdisk filesystem is a filesystem in a ramdisk larger
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than the size of the root disk it was loaded from. This can be
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extremely useful when using Ftape, which requires exclusive use
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of the floppy disk controller (see the Ftape HOWTO for details.)</P>
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<P>Two things are required: create an oversize file system on the
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root diskette, and then patch the kernel so that it will not
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try to load blocks off the end of the diskette.</P>
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<P>To create an oversize filesystem, two methods are possible:
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<UL>
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<LI>Use the "blocks" e2fsck parameter to specify the size of
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filesystem that you eventually want in the ramdisk. For example:
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<BLOCKQUOTE><CODE>
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<PRE>
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mke2fs /dev/fd0 3000
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</PRE>
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</CODE></BLOCKQUOTE>
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will create a filesystem on the diskette of 3000 1Kb blocks. The
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diskette only has 1440 blocks, but mke2fs does not care about this.
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E2fs will care about it if you try to use more than 1440 blocks
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of data (allowing for blocks used as inodes and reserved etc),
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but up to this point it is quite safe. You will soon find out if
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you try to load too much on the diskette because an I/O error will result.</LI>
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<LI>Create a partition on your hard disk as large as the filesystem
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you want on the ramdisk. Then create a filesystem on it and load in
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the files you want. Then use dd to copy only the first 1440 blocks
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to diskette, and then check that there was nothing in the uncopied
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part of the filesystem. For example:
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<BLOCKQUOTE><CODE>
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<PRE>
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dd if=/dev/hdb of=/dev/fd0 bs=1024 count=1440
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dd if=/dev/hdb of=tailpart bs=1024 skip=1440
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cmp -l tailparm /dev/zero
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</PRE>
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</CODE></BLOCKQUOTE>
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</LI>
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</UL>
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Of the two, I prefer the first method - it appears easier and safer.</P>
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<P>The second thing required to get an oversized filesystem is to get the
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kernel to stop loading at the end of the physical diskette when it
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tries to load the root diskette into ramdisk. To do this, a simple
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patch can be applied to the ramdisk driver, which should be found in
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/usr/src/linux/drivers/block/ramdisk.c. The following patch has been
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contributed by Bruce Elliot. It is for kernel version 1.2.0, but it
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should be fairly easy to apply to later versions. Even if the patch
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will not apply, the code is not complex, so the patch could easily
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be modified until it worked.</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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=================================================================
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X--- ramdisk.c~ Mon Jan 23 13:04:09 1995
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X+++ ramdisk.c Mon May 29 00:54:52 1995
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X@@ -113,6 +113,7 @@
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X (struct ext2_super_block *)&sb;
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X int block, tries;
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X int i = 1;
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X+ int fblocks;
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X int nblocks;
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X char *cp;
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X
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X@@ -168,12 +169,16 @@
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X nblocks, rd_length >> BLOCK_SIZE_BITS);
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X return;
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X }
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X- printk("RAMDISK: Loading %d blocks into RAM disk", nblocks);
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X+ fblocks = blk_size[MAJOR(ROOT_DEV)][MINOR(ROOT_DEV)];
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X+ if (fblocks > nblocks)
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X+ fblocks = nblocks;
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X+ printk("RAMDISK: Loading %d blocks into %d block filesystem "
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X+ "in RAM disk", fblocks, nblocks);
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X
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X /* We found an image file system. Load it into core! */
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X cp = rd_start;
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X- while (nblocks) {
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X- if (nblocks > 2)
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X+ while (fblocks) {
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X+ if (fblocks > 2)
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X bh = breada(ROOT_DEV, block, BLOCK_SIZE, 0, PAGE_SIZE);
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X else
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X bh = bread(ROOT_DEV, block, BLOCK_SIZE);
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X@@ -184,7 +189,7 @@
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X }
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X (void) memcpy(cp, bh->b_data, BLOCK_SIZE);
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X brelse(bh);
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X- if (!(nblocks-- & 15)) printk(".");
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X+ if (!(fblocks-- & 15)) printk(".");
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X cp += BLOCK_SIZE;
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X block++;
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X i++;
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=================================================================
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</PRE>
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</CODE></BLOCKQUOTE>
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With this patch, the kernel stops loading at the end of the
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physical diskette, leaving a filesystem larger than the disk.</P>
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<P>Some warnings: I have been able to create stable ramdisk filesystems
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in this fashion of 3500 blocks, but if I try 3600 or more then the
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kernel collapses with an error something like "fixup table corrupt".
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I have not been able to track down why, but obviously something is
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overflowing. Up to 3500 blocks, though, I have had no problems.</P>
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<H2><A NAME="ss5.8">5.8 Q. At boot time, I get error <CODE>A: cannot execute B</CODE>. Why?</A></H2>
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<P>There are several cases of program names being hardcoded in various
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utilities. These cases do not occur everywhere, but they may explain
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why an executable apparently cannot be found on your system even though
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you can see that it is there. You can find out if a given program
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has the name of another hardcoded by using the "strings" command and
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piping the output through grep.</P>
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<P>Known examples of hardcoding are:
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<UL>
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<LI>Shutdown in some versions has <CODE>/etc/reboot</CODE> hardcoded, so
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<CODE>reboot</CODE> must be placed in the /etc directory.</LI>
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<LI>Init has caused problems for at least one person, with the kernel
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being unable to find init. </LI>
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</UL>
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To fix these problems, either move the programs to the correct directory,
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or change configuration files (e.g. inittab) to point to the correct
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directory. If in doubt, put programs in the same directories as they
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are on your hard disk, and use the same inittab and <CODE>/etc/rc.d</CODE>
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files as they appear on your hard disk.</P>
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