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186 lines
4.7 KiB
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186 lines
4.7 KiB
Plaintext
Building a Busybox Boot Floppy
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==============================
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This document describes how to buid a boot floppy using the following
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components:
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- Linux Kernel (http://www.kernel.org)
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- uClibc: C library (http://cvs.uclinux.org/uClibc.html)
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- Busybox: Unix utilities (http://busybox.net/)
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- Syslinux: bootloader (http://syslinux.zytor.com)
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It is based heavily on a paper presented by Erik Andersen at the 2001 Embedded
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Systems Conference.
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Building The Software Components
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--------------------------------
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Detailed instructions on how to build Busybox, uClibc, or a working Linux
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kernel are beyond the scope of this document. The following guidelines will
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help though:
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- Stock Busybox from CVS or a tarball will work with no modifications to
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any files. Just extract and go.
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- Ditto uClibc.
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- Your Linux kernel must include support for initrd or else the floppy
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won't be able to mount it's root file system.
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If you require further information on building Busybox uClibc or Linux, please
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refer to the web pages and documentation for those individual programs.
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Making a Root File System
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-------------------------
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The following steps will create a root file system.
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- Create an empty file that you can format as a filesystem:
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dd if=/dev/zero of=rootfs bs=1k count=4000
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- Set up the rootfs file we just created to be used as a loop device (may not
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be necessary)
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losetup /dev/loop0 rootfs
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- Format the rootfs file with a filesystem:
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mkfs.ext2 -F -i 2000 rootfs
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- Mount the file on a mountpoint so we can place files in it:
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mkdir loop
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mount -o loop rootfs loop/
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(you will probably need to be root to do this)
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- Copy on the C library, the dynamic linking library, and other necessary
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libraries. For this example, we copy the following files from the uClibc
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tree:
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mkdir loop/lib
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(chdir to uClibc directory)
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cp -a libc.so* uClibc*.so \
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ld.so-1/d-link/ld-linux-uclibc.so* \
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ld.so-1/libdl/libdl.so* \
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crypt/libcrypt.so* \
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(path to)loop/lib
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- Install the Busybox binary and accompanying symlinks:
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(chdir to busybox directory)
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make PREFIX=(path to)loop/ install
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- Make device files in /dev:
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This can be done by running the 'mkdevs.sh' script. If you want the gory
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details, you can read the script.
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- Make necessary files in /etc:
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For this, just cp -a the etc/ directory onto rootfs. Again, if you want
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all the details, you can just look at the files in the dir.
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- Run ldconfig so busybox and other binaries can have access to the libraries
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that they need:
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(path to)uClibc/ld.so-1/util/ldconfig -r loop/
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- Unmount the rootfs from the mountpoint:
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umount loop
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- Compress it:
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gzip -9 rootfs
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Making a SYSLINUX boot floppy
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-----------------------------
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The following steps will create the boot floppy.
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Note: You will need to have the mtools package installed beforehand.
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- Insert a floppy in the drive and format it with an MSDOS filesystem:
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mformat a:
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(if the system doesn't know what device 'a:' is, look at /etc/mtools.conf)
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- Run syslinux on the floppy:
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syslinux -s /dev/fd0
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(the -s stands for "safe, slow, and stupid" and should work better with
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buggy BIOSes; it can be omitted)
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- Put on a syslinux.cfg file:
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mcopy syslinux.cfg a:
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(more on syslinux.cfg below)
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- Copy the root file system you made onto the MSDOS formatted floppy
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mcopy rootfs.gz a:
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- Build a linux kernel and copy it onto the disk with the filename 'linux'
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mcopy bzImage a:linux
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Sample syslinux.cfg
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~~~~~~~~~~~~~~~~~~~
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The following simple syslinux.cfg file should work. You can tweak it if you
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like.
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----begin-syslinux.cfg---------------
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DEFAULT linux
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APPEND initrd=rootfs.gz root=/dev/ram0
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TIMEOUT 10
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PROMPT 1
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----end-syslinux.cfg---------------
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Some changes you could make to syslinux.cfg:
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- This value is the number seconds it will wait before booting. You can set
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the timeout to 0 (or omit) to boot instantly, or you can set it as high as
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10 to wait awhile.
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- PROMPT can be set to 0 to disable the 'boot:' prompt.
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- you can add this line to display the contents of a file as a welcome
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message:
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DISPLAY display.txt
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Additional Resources
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--------------------
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Other useful information on making a Linux bootfloppy is available at the
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following URLs:
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http://www.linuxdoc.org/HOWTO/Bootdisk-HOWTO/index.html
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http://www.linux-embedded.com/howto/Embedded-Linux-Howto.html
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http://linux-embedded.org/howto/LFS-HOWTO.html
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http://linux-embedded.org/pmhowto.html
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http://recycle.lbl.gov/~ldoolitt/embedded/ (Larry Doolittle's stuff)
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Possible TODOs
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--------------
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The following features that we might want to add later:
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- support for additional filesystems besides ext2, i.e. minix
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- different libc, static vs dynamic loading
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- maybe using an alternate bootloader
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