Disk image
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Template:Optical disc authoring
A disk image is a computer file containing the complete contents and structure of a data storage device. The term has been generalized to cover any such file, whether taken from an actual physical storage device or not.
One of the most popular and commonly encountered types of images is a CD/DVD image. In simple terms, a CD/DVD-image is an exact replica of a CD/DVD, whereby all the data is stored in one file for the convenance of its users and as a courtesy to those who prefer CD/DVD-images over compressed archives (.ZIP, .RAR).
Creating a CD/DVD-image is actually very easy, 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, and Disk Utility and Roxio Toast, for Macintosh. For CDs that contain CD audio (Red Book standard), referred to as multi-track CDs (Data or Audio on the first track, Data or Audio on any subsequent tracks), a CD-image is required, as analog CD audio cannot be copied directly via the Windows interface.
An ordinary backup only backs up the files it can access; boot information, and files locked by an operating system or being changed at the time, may not be saved. A disk image contains all these, and faithfully replicates the data, so it is commonly used for backing up disks with operating systems, or bootable CD/DVDs.
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CD sector size FAQ
- A standard 74 min CD is made by 333,000 sectors.
- Each sector is 2352 bytes, and contains 2048 bytes of PC (MODE1) Data, 2336 bytes of PSX/VCD (MODE2) Data or 2352 bytes of AUDIO.
- The difference between sector size and data content are the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio.
- If you extract data in RAW format (standard for creating images) you always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on data type (basically, you extract the whole sector). This fact has two main consequences:
- You can record data at very high speed (40x) without losing information, but if you try to do the same with PSX or Audio you get unreadable CD (for PSX) or audio CD with lots of clicks because there are no error correction codes (and error are more likely to occur if you record at high speed.)
- On a 74 min CD you can fit very large RAW images,up to 333,000 x 2352 = 783,216,000 bytes (747 Mb). This should be the upper limit for a RAW image created from a 74 min CD. Remember that if you store standard data (backup files), you can burn only 333,000 x 2048 = 681,984,000 bytes (the well known 650 MB limit).
- Please note that an image size is ALWAYS a multiple of 2352 bytes (you extract SECTORS), if extracted in RAW mode.
Common formats
.CUE/.BIN
The .cue / .bin format developed by Jeff Arnold for CDRWIN can encode CD Image formats in either 2048 or 2324 bytes per sector. The BIN file is a binary copy of an entire CD/DVD disc. The BIN file contains all the data stored on the original disc, not only its files and folders but also its system-specific information such as, boot loaders, volume attributes and any other data. BIN files are usually bigger than .iso files because, in contrast to .iso files, they are not a bit for bit copy of the entire CD/DVD.
.bin files (or .RAW files) are images extracted in pure RAW format. That is 2352 bytes sectors, the full CD sector content: user data, sector header, error correction codes (ECC) and error detection codes (EDC). Once again, each sector is converted to digital data in the .BIN file, but more stuff is copied and the resulting file will be bigger. The .BIN file should be 251,000 x 2352 = 590,352,000 bytes big. This process will copy ANYTHING on the disc, so it is useful for exotic discs (multiple tracks, mixed track type Audio+Data or Data+Audio) and for non-PC CDs (PSX, VCD, MAC).
The .cue file is a cue sheet that describes the data stored in the .bin file. The .cue file is in fact 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
.ISO
Template:Main An ISO image (.iso) is an informal term for a disk image of an ISO 9660 file system. More loosely, it refers to any optical disk image, even a UDF image. Most CD/DVD authoring utilities can deal with ISO images: Producing them either by copying the data from existing media or generating new ones from existing files, or using them to create a copy on physical media.
Some of the common uses include the distribution of operating systems, such as Linux or BSD systems, and LiveCDs.
Most operating systems allow these images to be mounted as if they were physical discs, making them somewhat useful as a universal archive format. Console emulators, such as ePSXe, and many other emulators that read from CD/DVD, are able to run ISO/BIN (and other similar formats) instead of running directly from the CD drive, obtaining better performance.
.IMG
.img is the filename extension usually use for the disk image of floppy disks, and sometimes, hard disks. First populated by DOS-based software HD-Copy DiskDupe and now WinRaWrite, is a handy way to archive a floppy disk completely, including bootable ones. In fact there is no "format" in it, just a raw dump of the content of the disk.
Some newer software like WinImage supports zipped version of the format and the extension is .imz.
Apple Disk Image
Image:Apple Disk Image.png Template:Main 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 the 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. Template:-
Storage
An image is saved as a file, which for a full CD or system disk may be quite large (~10 MB to several GB). This file can be saved onto a hard drive, CD, DVD or other media, for later use.
Disk images can either be compressed using some type of compression algorithm like LZW, or uncompressed ("raw"). Images of CD-ROMs most often carry the file name extension .iso, referring to the ISO 9660 file system commonly used on such discs. The .iso format is the most common format for Linux distributions and other online images, however it is not a multi-track image format and cannot be used for CD's with CD Red Book audio. 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 CD's.
Usage
A common use of disk images is for remote distribution of software such as Linux distributions: installation floppy disks or CD-ROMs can be recorded as disk image files, transferred over the Internet, and the contents of the original disk(s) duplicated exactly by end users with their own floppy or CD-R drives, so users can burn the ISO images to convert them into LiveCDs to try an operating system without installing it in the hard disk .
Disk images are also routinely used in connection with software copyright infringement, creating an image of a software package and subsequently distributing it.
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).
See also
- ISO image
- DVD Decrypter
- DAEMON Tools
- RaWrite and RaWrite2
- QEMU
- LiveCD.
- Bootloader
- ROM image
- DiskImageMounter
- Loop device
- Disk cloning