This a list of answers to Frequently Asked Questions about ELKS, also known as Linux-8086.

This FAQ is maintained by Alistair Riddoch, based on the origonal by Brian Candler; please send all updates to ajr@ecs.soton.ac.uk. The latest version of this document is available at http://www.ecs.soton.ac.uk/elks. Note that ELKS is in the very early stages of development and this information is likely to become out of date rapidly.

Additional information can be found at the ELKS home page at http://www.uk.linux.org/ELKS-Home.

There is a mailing list. To subscribe, send a message to majordomo@vger.rutgers.edu containing the words subscribe linux-8086 in the body. Archives of the mailing list can be found at http://epocha.pd.mcs.net/Linux8086/. At the moment this archive doesn't seem to be working. There was an unofficial linux-8086 mailing list archive at http://www.cyberpass.net/~dummy/robert/archives/linux-8086/ which seems to be down at the moment.

Note that ELKS is not Linux, and the mailing list is not a suitable place for posting questions about Linux (despite its name). These would be better sent to one of the Linux-specific newsgroups such as comp.os.linux.misc

Section 1 - General

Section 2 - Compiling and installing

Section 3 - Using ELKS

Section 4 - Unanswered questions

Section 1 - General

Q1.1. What is ELKS?

ELKS is the Embeddable Linux Kernel Subset, a project to build a small kernel subset Linux (which will provide more or less UNIX V7 functionality within the kernel) that can run on machines with limited processor and memory resources. The initial proposed targets are the Intel 8086 and eventually the 286's 16-bit protected mode. A kernel that can run on this kind of hardware is useful for embedded systems projects, for third world deployment where 80x86 x>0 machines are not easily available, and for use on various palmtops.

More information on the background, goals and current status of the project can be found at the ELKS home page.

Q1.2. How does ELKS compare with standard Linux?

ELKS is intended as a subset of true Linux, and ought to be small enough to be understood by one person, so it should be invaluable as a learning tool. It is in the very early stages of development, and big chunks are missing, such as swapping, shared libraries, and networking. Having said that, it is already able to boot, provide virtual consoles, mount a root minix filesystems floppy, provide basic serial and parallel I/O, and start various small programs.

Q1.3. Are there any ready-to-run distributions of ELKS?

Not yet. You need to use a standard Linux machine, download the sources, and cross-compile to generate the 8086 target code. Boot and root disk images are provided with the current version of ELKS, but all they do is act as a platform for testing the system, very little can be done with them. Michael Strates has volunteered to put together a distribution, which he is currently working on.

Q1.4. Can ELKS run on an 8088?

The 8088 is identical to the 8086, except it has an 8-bit external data bus instead of 16-bit (and thus is slower). So yes, ELKS will run on it.

Section 2 - Compiling and Installing

Q2.1. Where can I find the source?

The primary site ftp://linux.mit.edu/pub/ELKS is now back up. Other sites where ELKS and Dev86 can be found are:


If you know of any others or wish to set one up, please contact the maintainer.

Q2.2. How do I make an ELKS kernel?

Recent releases contain a conmpiled binary kernel which most people whould be able to use for initial testing. You should only need to compile your own if you wish to modify the kernel, or change the configuration options. The file boot in images.zip can be written to floppy with dd or RAWRITE.EXE to make a kernel boot disk.

If you want to compile your own kernel, download dev86/Dev86- and kernel/elks-0.0.67/elks-0.0.67.tar.gz (or the latest versions) from linux.mit.edu, or one of the other sites. Unpack Dev86 them into /usr/src (or any other convenient directory), and elks into /usr/src/linux-86
    tar -xvzf Dev86-0.0.13.tar.gz -C /usr/src
    tar -xvzf elks-0.0.67.tar.gz -C /usr/src
The development environment will be created in /usr/src/linux-86, and the kernel source in /usr/src/elks. Next you have to build the development tools, which include the bcc compiler:
    cd /usr/src/linux-86
    make install
Next, compile the kernel.
    cd /usr/src/elks
    # Build kernel
    make config
    make dep
Make config works in the same way as the main Linux kernel. The default configuration will work on almost all systems. Don't change any of the options unless you know what you are doing.

(If you're used to building Linux kernels with gcc, you'll be amazed at how quick this is! :-) The result is a floppy disk image called "Image", which you can copy to a formatted floppy and boot from.

    dd if=Image of=/dev/fd0
    make disk
You should now be able to boot with this disk, but you won't get much further without a root floppy disk for it to mount. You can either use the image file root provided in images.zip, or make your own. You'll need to use bcc to compile an 'init' program and make a root minix filesystem. This is explained further down.

Q2.3. I get an error saying that /usr/include/linux/vm86.h does not exist

Edit /usr/include/sys/vm86.h, and change #include <linux/vm86.h> to #include <asm/vm86.h>

Q2.4. How do I make an 'init' for ELKS?

After the ELKS kernel has booted it will run /bin/init.

A real version of 'init' is being prepared for inclusion in the tiny-utils package, also on linux.mit.edu. A very basic init and login is included in the sysutils part of the elkscmd package which is distributed with each release. The kernel code has recently been changed so that it loads /bin/init instead of /bin/sh.

To install init and login compile them both with

    bcc -0 -O -s init.c -o init
    bcc -0 -O -s login.c -o login
and copy them into the /bin directory of your root disk. login requires an /etc/passwd and a /etc/group file, and a /bin/sh (or other depending on the entry in /etc/passwd.

-0 selects 8086 code generation,
-O enables optimisation,
-ansi enables ansi-style function prototypes (only), and
-s strips symbols out of the binary.

As of kernel 0.0.56 sash now works and is being used as /bin/sh by most people. It can be found in elkscmd.tar.gz. For best results copy /etc/passwd and /etc/group from a standard linux distribution onto your root disk so that ls -l works okay, and /usr/lib/liberror.txt so that sash error reporting works.

A version of ash is also included which does work under ELKS, but is much larger, and may not work at all on machines with <640K of memory.

Q2.5. How do I make a root filesystem disk?

For most people the root filesystem included in images.zip should be ideal, and can be used to create a root disk under Linux as follows:-
    dd if=root of=/dev/fd0 bs=8192
or under DOS, RAWRITE.EXE can be used.

You need to create a minix floppy containing the appropriate bits and pieces. This file should contain init, login, and sh in /bin/ and passwd and group in /etc/, as well as three ttys for init in /dev/.
Note: /dev/tty is different to normal Linux which uses (5,0)

    fdformat /dev/fd0H1440                     # if floppy not yet formatted
    mkdir /usr/src/root86                      # make a directory
    cd /usr/src/root86
    mkdir dev                                  # create directorys in it
    cd dev
    mknod tty c 4 0                            # creatye the required devices
    mknod tty2 c 4 1
    mknod tty3 c 4 2
    mknod fd0 b 3 128
    cd ..
    mkdir bin
    cd bin
    cp <path-to-elkscmd-init> init       # copy in essential binaries
    cp <path-to-elkscmd-login> login
    cp <path-to-elkscmd-sash> sh
    cd ..
    mkdir etc
    cd etc
    cp <path-to-skeleton-passwd> passwd  # add required files for login
    cp <path-to-skeleton-group> group
    cd ..
    fdformat /dev/fd0H1440                     # if floppy not yet formatted
    mkfs -t minix -n14 /dev/fd0 1440
    mount -t minix /dev/fd0 /a                 # need a mount point "/a"
    cp -pR /usr/src/root86/* /a
    umount /a
Some distributions use /dev/fd0u1440 instead of /dev/fd0H1440.

You should removed passwords from the passwd file as they are not yet suported, and make sure the shell entry for each user is /bin/sh for future compatability.

Alternatively, if you have a 2.0.x kernel and mount-2.5k, you can use the loopback filesystem to create a disk image on your hard disk, and then dump it to floppy

    dd if=/dev/zero of=root.image bs=18k count=80
    losetup /dev/loop0 root.image
    mkfs -t minix /dev/loop0 1440
    mount -t minix /dev/loop0 /a
    umount /a
    dd if=root.image of=/dev/fd0
As of ELKS 0.0.53 floppy sizes other than 1.44M now work. Just follow the above intructions substituting the appropriate sizes in where required. However, because of the way 360K disks are written by 1.2Meg drives, the instructions below must be followed if you are trying to boot a machine with a 360K drive.

Q2.6. What if I have an XT with a 360K disk drive?

Preparing boot and root images to run on a different machine requires a little care, especially if the target machine has a 360K drive. 360K drives (5.25" DD) have 40-track heads, and 1.2MB (5.25" HD) have 80-track heads. A 1.2MB drive can read 360K disks but not reliably write them, since it will write magnetic tracks which are half the width expected by a 360K drive.

One solution is to fit a 360K drive as the second drive in your Linux PC - you can then format 360K disks with fdformat /dev/fd1d360 and create filesystems as usual.

Otherwise, you will need to create a floppy disk image file, transfer it to the PC which has the 360K drive (e.g. using a serial cable), then use "rawrite" under DOS to recreate the disk from the image file.

Boot disks

The 'Image' file can be written to any type of floppy disk and will boot successfully, so all you need to do is transfer the file and use 'rawrite' to make the target disk.

Root disks are now autodetected so you no longer have to re-compile for disk sizes other than 1.44Mb.

Root disks

You can prepare a 360K root image on a larger drive - when you enter the mkfs command substitute '360' for the size of the filesystem. Once you have copied the files to this disk, unmount it, then you can create a disk image file from it:
        dd if=/dev/fd0 of=root.image bs=1k count=360
This image can then be transferred to the target and written out using 'rawrite' as before. If you use the loopback filesystem this step is not necessary.

RAM limitations

0.0.44 expected there to be 640K RAM available. However if you do not have 640K the problems with DEF_INITSEG and DEF_SETUPSEG have now been fixed in later versions so you will not have to change anything.

Q2.7. How do a change root device of boot images?

Version 0.0.62 boot disk was compiled to use the second floppy drive as a root disk because the wait_for_keypress() function was broken. You can get round this if you only have one floppy drive by using a hex editor to change the root device no. in the image.

If you get an error about root disk mount failing on device 03c0, load the boot image into a hex editor and look for c003h somewhere near the start of the image and change it to 8003h. (c003h is 03c0 in Intel's backwards byte ordering). This should work for any version, but it is not clear whether the address of the byte to edit varies between version. jose@cnct.com reports that in 0.0.62 boot image, the two bytes are at 0adf - oade.

Q2.8. How do I install to ELKS to harddisk?

As of version 0.0.65 it is possible to install ELKS root disk on a harddisk and then create a boot floppy that uses this. There still some bugs but it is more usable than a root floppy.

First create a partition with fdisk. Any version of fdisk will be able to do this, but the current root disk only supports hd?1-4, which are primary partitions, and if you already have a DOS primary partition you will not be able to make a second primary partition with DOS fdisk. The version of fdisk included on the ELKS 0.0.67 root disk is still young code, but seems to work fine on my system.

Having created the partition, boot ELKS from boot and root floppies, and create a filesystem using mkfs. It is safe to use ELKS fdisk to check you know the correct device name for the partition you want to create the filesystem on. To create the filesystem type

    mkfs /dev/hd?? <size>
where hd?? is the correct partition, and size is the size of the partition in Kbyte blocks. <size> cannot be more than 32M. E.g. To make a filesystem on the first partition on the slave disk, which is a 20M partition type:
    mkfs /dev/hdb1 20000

Now mount the partition on the /mnt mount point by typing

    mount /dev/hd?? /mnt
where hd?? is the correct partition. The root directory of the new partition may look a bit corrupted and it may not be possibly to use ls from /mnt. I am not sure why mkfs is generating slightly corrupt filesystems, but I am working on it.

Create the essential directories on the new filesystem as follows:

    mkdir /mnt/bin
    mkdir /mnt/dev
    mkdir /mnt/usr
    mkdir /mnt/usr/lib
    mkdir /mnt/mnt
    mkdir /mnt/etc
    mkdir /mnt/root
    mkdir /mnt/home
    mkdir /mnt/home/user?....

Now copy essential files. You will need many more than these for a decent system but here are the essentials and some of the more useful ones.

    cp /bin/init /mnt/bin/init
    cp /bin/login /mnt/bin/login
    cp /bin/sh /mnt/bin/sh
    cp /bin/ls /mnt/bin/ls
    cp /bin/pwd /mnt/bin/pwd
    cp /bin/mount /mnt/bin/mount
    cp /bin/umount /mnt/bin/umount
    cp /bin/cp /mnt/bin/cp
    cp /etc/passwd /mnt/etc/passwd
    cp /etc/group /mnt/etc/group
    cp /usr/lib/liberror.txt /mnt/usr/lib/liberror.txt

Finally create the essentail devices:

    mknod /mnt/dev/tty c 4 0
    mknod /mnt/dev/tty2 c 4 1
    mknod /mnt/dev/tty3 c 4 2
    mknod /mnt/dev/fd0 b 3 128
    mknod /mnt/dev/fd1 b 3 192
    mknod /mnt/dev/hda b 3 0
    mknod /mnt/dev/hda1 b 3 1
    mknod /mnt/dev/hda2 b 3 2
    mknod /mnt/dev/hda3 b 3 3
    mknod /mnt/dev/hda4 b 3 4
    mknod /mnt/dev/hdb b 3 64
    mknod /mnt/dev/hdb1 b 3 65
    mknod /mnt/dev/hdb2 b 3 66
    mknod /mnt/dev/hdb3 b 3 67
    mknod /mnt/dev/hdb4 b 3 68

Now unmount the new partition with

    umount /dev/hd??

You will now need to re-compile a new kernel with the correct root device selected. To do this edit include/linuxmt/conmfig.h in the ELKS kernel sources, and changed the ROOT_DEV from 0x380 to the device number of the partition. The last two digits must be the minor number of the partition in hex. The minor number can be found from the list above in the far right column. E.g. The minor number for hdb3 is 67, which is 43 in hex, so the ROOT_DEV value is 0x343.


    make disk
to build a new kernel floppy, and reboot the ELKS machine with this new boot disk. The new root should mount, but may give a warning, and the system come up as normal, but much faster than from floppy. As before the root directory may be corrupted, but it should be possible to change to other directories normally, and the system should work otherwise normally.

Section 3 - Using ELKS

Q3.1. Can I run bcc binaries under Linux?

Yes - by loading a kernel module you can run ELKS binaries directly. This makes development of programs to run under ELKS much less painful, since you can test them on your normal Linux system (although they might not work under ELKS, if your program uses a kernel facility which is not yet implemented)
    cd /usr/src/linux-86/elksemu
    make module
    insmod binfmt_elks.o
Note that you will need to rerun the 'insmod' each time you reboot. There are some sample bcc programs you can try in /usr/src/linux-86/tests (such as a version of 'wc')

If when making the module you get an error saying "modversions.h: no such file or directory" then you need to edit the Makefile to remove the reference to this file, i.e.:

MODCFLAGS=-D__KERNEL__ -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer \
          -fno-strength-reduce -pipe -m486 -DCPU=486 -DMODULE -DMODVERSIONS

Q3.2. Can I boot ELKS under DOSEMU?

Yes. ELKS can boot inside dosemu running under Linux 2.0.7, and probably other combinations as well. Make sure your floppy drive is configured in /etc/dosemu.conf, and use the -A flag to dos or xdos if necessary to force a floppy boot.
    floppy { device /dev/fd0 threeinch }  

Section 4 - Unanswered questions

Answers to these questions would be appreciated!

Alistair Riddoch / last updated 2nd November 1998