disk image

One of the most popular and commonly encountered types of disk images is a CD/DVD image. In simple terms, a CD/DVD-image is an exact digital replica of a CD/DVD, whereby all of the data is stored in one file to completely preserve the data structure and integrity of the CD/DVD. CD/DVD images are essential for retaining copy-protection data and multi-track data/audio.

Images of CD-ROMs are usually .ISO files, referring to the ISO 9660 file system commonly used on such disks. The .ISO format is the most common format for software disk images, but does not support multi-track data or audio CDs, a disadvantage.

Macintosh computers are able to create and mount disk images in the .DMG format.

Other common disk image formats include: .BIN/.CUE, .IMG, .NRG^[1], .MDF/.MDS^[2], .DAA used by PowerISO^[3], and .CCD/.IMG/.SUB^[4].

Usage

Creating a CD/DVD-image is actually very easy and not as complicated as one might think, all that is required is an appropriate CD/DVD-imaging program and enough disk space to save the image onto. Some common disk-imaging programs are Alcohol 120%, CDRWin, CloneCD/DVD and Nero Burning ROM for Windows, Disk Utility and Roxio Toast for Macintosh, K3B and x-CD roast for Linux.

For audio CDs, known as multi-track CDs (data or audio on the first track, data or audio on any subsequent tracks), a CD image is required, as CD audio cannot be copied directly via the standard Windows, Macintosh or Linux interfaces. This is because CD-audio is physically stored in the dips and pits on the surface of the disc, much like the grooves on a vinyl record.

Data Storage

An ordinary backup program only backs up the files it can access; boot information and files locked by the operating system (such as those in use at the time of the backup) may not be saved. A full disk image contains all these, faithfully replicating all data. For this reason, it is commonly used for backing up disks with operating systems, or bootable CDs and DVDs.

As most operating systems allow disk images to be mounted as if they were physical discs, formats such as .ISO may be used as a universal archive format.

Software Distribution

A common use of disk images is for distribution of large programs or software systems over the internet. Installation disks and LiveCDs for operating systems such as Linux or BSD systems are typically available as .ISO images. Installation floppy disks or CD-ROMs are stored as disk image files, transferred over the internet, and burned or copied onto actual disks by end users.

Disk images, most often .ISO or .BIN/.CUE files, are also popular for peer-to-peer file sharing of software.

Virtual Disks

Another common use is to provide virtual disk drive space to be used by emulators (e.g. QEMU) and virtual machines. This can prevent the CD from getting damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight and bootable storage device which has a higher storage capacity than that of a CD (i.e. a USB keydrive).

Emulation also provides a performance boost, as hard disks transfer data at exponentially higher rates than optical disk drives. Console emulators, such as ePSXe, and many other emulators that read from CD/DVD, are able to read ISO/BIN (and other similar formats) instead of running directly from the CD drive, obtaining better performance.

As most operating systems allow these images to be mounted as if they were physical discs, formats such as .ISO may be used as a universal archive format.

Formats

An image is saved as a file, which for a full CD or system disk may be quite large (600 MB to several GB). This file can be saved onto a hard drive, CD, DVD or other media, for later use.

Images of CD-ROMs most often carry the filename extension .ISO, referring to the ISO 9660 file system commonly used on such disks. The .ISO format is the most common format for Linux distributions and other disk images distributed over the internet, but it is not a multi-track image format and thus cannot be used for Audio CDs. Other common CD image formats are .NRG (Nero Burning ROM's proprietary format), .BIN/.CUE, .MDF/.MDS (Alcohol 120%'s proprietary format) and .CCD/.IMG/.SUB (CloneCD's proprietary format). All of these support multi-track CDs.

Disk images can either be compressed using some type of compression algorithm like LZW, or uncompressed ("raw").

.ISO

Main article: ISO image

A .ISO file is a disk image of an ISO 9660 file system. The term ISO image is also used informally to refer to any optical disk image, even if the disk uses another file system, such as UDF. The format is used by most CD/DVD authoring utilities. .ISO files may be created from existing physical media or from files and folders on a computer. .ISO images can be mounted in "virtual drives" which emulate physical hardware, or burned to CDs or DVDs.

.ISO files are often used to distribute large programs or software systems over the internet. Operating systems such as Linux or BSD systems installation disks and LiveCDs are typically available as .ISO images. The format is also popular for peer-to-peer file sharing of pirated commercial software.

On Mac OS X, ISO images often have the .cdr extension.

.BIN/.CUE

The .BIN/.CUE image file format, developed by Jeff Arnold for the CDRWin program,^{citation needed]} consists of two files which contain binary data and metadata, respectively. The filenames typically match (e.g. image.bin and image.cue).

The .BIN file contains an exact copy of all data stored on an optical disk in raw, unprocessed form. For this reason, some programs use the .RAW suffix for these files. The file contains all data stored on the original disk - not only its files and folders, but also system-specific information such as boot loaders, volume attributes, error correction codes and other metadata on the disk itself. Of the 2352 bytes in each CD sector, only 2048 contain user data; the additional 304 bytes per sector primarily contain error correction information. Other formats, such as .ISO, do not store all this (mostly redundant) information, resulting in a smaller file. The .BIN format is useful for exotic disks, such as those containing multiple tracks or mixed track types (Audio+Data or Data+Audio), and for non-PC CDs (e.g. PSX, VCD, Mac).

The .CUE file is a "cue sheet" that describes the data stored in the .BIN file. The .CUE file is a plain text file. A typical .CUE file is as follows:

FILE "IMAGE.BIN" BINARY
  TRACK 01 MODE1/2352
  INDEX 01 00:00:00

The file would be saved as IMAGE.CUE to match the file name prefix of IMAGE.BIN.

.IMG

.IMG is the filename extension usually used for the disk image of floppy disks, and sometimes, hard disks. A .IMG file contains a raw dump of the content of a disk. First popularized by the DOS-based programs HD-Copy (for hard drives) and DiskDupe (for floppy disks) and now utilized by WinRaWrite, the .IMG format is a handy way to archive a floppy disk completely, including bootable ones.

The img extension was also used by the Macintosh application ShrinkWrap by Aladdin Systems (now Allume Systems), for Mac OS 9 and older.

Some newer software, like WinImage, supports a zipped version of the format, which uses the .IMZ extension.

Apple Disk Image

Main article: .dmg

For Macintosh computers, disk images have been available as a feature of the operating system since before System 7. Mounting a compatible disk image is achieved by double-clicking (opening) the image file. The archived volume is then available as a normal disk volume, indistinguishable at first glance from a physical volume. Modern Apple Disk Images for Mac OS X typically end in .DMG. Using NetBoot, client computers can start up over a network from a server-based disk image that contains system software. The default handler for disk images in Mac OS X is DiskImageMounter. Images may be created with Disk Utility in Mac OS X and with Disk Copy in older versions of the Mac OS.

See also

• Archive formats
• Bootloader
• Disk cloning
• ISO image
• Loop device
• ROM image
• Mini Image

Reference

• http://forums.afterdawn.com/thread_view.cfm/19244