Disk Images

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The best way for hobbyist os developers to test their systems is to use programs like QEMU, Bochs, VMware or Virtual PC.

All of these common tools rely on using disk images (either floppy or hard disk). A disk image is nothing more than a file whose content reflects the layout of a disk and that the tool will use as if it was a physical device. E.g. A floppy image typically is 1.44MB large and its 512 first bytes consist of the 'floppy's boot sector. The different sub-pages should tell you how to create such images from real disks and use them under the most common operating systems.


Tools

These programs can help you with the management of disk images

Linux Windows BSD Mac OS X File Systems Supported
BFI No Yes No No FAT
fat_imgen Yes Yes Yes Yes FAT12
File Disk No Yes No No Anything supported by Windows
FUSE 1 Yes Yes Yes Yes Literally anything, including your own FS
hdiutil No No No Yes HFS HFS+ HFS+J HFSX FAT UFS
ImDisk No Yes No No Anything supported by Windows
Loopback Device Yes No Yes Yes Anything supported by the kernel
MagicISO No Yes No No ISO 9660, Rock Ridge, HFS/HFS+, Joilet, UDF, XBOX DVD FS
mkisofs Yes Yes Yes Yes ISO 9660
MTools Yes Yes Yes Yes FAT
Ultimate No Yes No No MBR, GPT, eMBR, ISO 9660, FAT, ExFAT, NTFS, LEAN, FYSFS, SFS, Ext2
Virtual Floppy Disk No Yes No No Anything supported by Windows
Win Image No Yes No No FAT ISO 9660

Note 1: FUSE is not an application, but a shared library and a set of applications; one for each file system. You can also write your own if you'd like. They are easy to use as they mount a disk partition or image file on a directory, and you can manipulate the files in the image with standard tools afterwards.


If you want to write/burn an image to media you can use one of the following tools:

Linux Windows BSD Mac OS X Notes
dd Yes Yes (Using Cygwin) Yes Yes
Rawwrite No Yes No No Floppy images only
USBImager Yes Yes Yes Yes very small, dependency-free and has a simple GUI

Floppy disk images

You can use 'dd' to create a blank floppy image. 

dd if=/dev/zero of=floppy.flp bs=512 count=2880

CD images

As of version 0.95, GNU GRUB comes with support for no-emulation El-Torito CD boot. Creating a CD image is much easier than working with floppy images and trying to stuff GRUB in them (and you get 650 meg more space too). Putting GRUB on a CD is now a simple matter of making a skeleton directory tree for the CD filesystem layout, copying the "stage2_eltorito" file in there, and running mkisofs with a specialized command line. See the GRUB 0.95 info node Installation > Making a GRUB bootable CD-ROM for details. Much more humane than those floppy games.

There is also a tutorial about creating a no-emulation El-Torito CD with GRUB.

Handling of Partition Tables in images.

At the time of writing, only Apple's hdiutil and Plan 9's partfs can handle the mounting of partitions inside of an image cleanly. Linux users can use a hack to skip over the MBR sector. In general, you can solve the partitioning information problem by using a separate image for one partition, then insert it into a larger image with partitioning data at the right place. This works because offsets are generally relative to the start of the partition.

There's a tutorial on Bootable Disks.

Images with preinstalled GRUB

If you are looking for a ready-made floppy image with GRUB already installed, such has been set up by MartinBaute, and is available from the following mirrors:

The images have GRUB stage1 / stage2 and an empty config file set up, so all you have to do is adding your binaries and editing the config file (see the GRUB manual for details).

The CD/DVD image on rdos.net has an embedded bootable floppy image and the required files in the root directory.

The Ubuntu "grub-rescue-pc" package is GRUB2, and has floppy, CDROM, and USB images.

Pre-made Images to test your code with

The following is a list of pre-made images of various partitioning schemes and file systems.

The purpose of this list is to include images for testing your partitioning and file system code. In other words, can your code find and mount the given partitions/file systems on these images.

This is not a 'vanity' list, a list to simply add your image file. The ideal purpose of this list is for images with intentionally numerous partitions and/or partitioning schemes, and other types of images. However, images with single partitions are allowed. Please make sure--to the best of your knowledge--the partitioning scheme and file systems are correctly implemented before adding to this list. The intent is for others to test their code on your image, not to test your image with their code.

You are welcome to add your image URL, but please keep in mind the purpose of this list.

  • Please compress the image and point the URL to the compressed image. ZIP, GZ/BZ, etc.
  • Please do not include many other files. Keep it to the disk image itself. If you wish to include the source, place the source on the disk image itself.
  • Please use (somewhat) permanent links. Do not add your image file if the URL will be broken in the near future.
URL/Name Partition Scheme Sector Size File System(s) Compressed Uncompressed Notes
Ultimate Test eMBR 512 FAT, ExFAT, LEAN, FYSFS, SFS, Ext2 5.5 Meg 165 Meg Test image for Ultimate disk viewer
fysos GPT, floppy 512 FAT 8.2 Meg 10 Meg/1.44 Meg Single FAT 16 file system partition with a GPT partitioning Scheme
Ultimate Test (4k) GPT 4096 FAT, LEAN 8.6 Meg 86 Meg FAT 16 & Lean file systems partitioned with a GPT partitioning Scheme (Using 4k Sector sizes)

Boot EFI only. Legacy will not read 4k sector sizes.

Links

  • Related thread in the forum: Topic:10549 You gonna find a nifty trick Brendan uses to build his own iso images - with NASM. (download missing)
  • John Burger's Demonstration uses NASM source code to build his own ISO images too.
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