Floppy Disk Images

What Are Disk Images?

A disk image is simply a single file that contains an entire floppy disk's data. They are called "images" because typically the file contains a direct 'image' of each sector that existed on the original media. This means that even the boot sector is contained in the image file, and hence if you restore the image back onto an original floppy disk it would boot just as the original did! It also contains empty sectors if those sectors were empty on the original media.

Images of floppy disks have existed since the DOS days, primarily through backup software, often with compression applied to reduce the size of the image file. Floppy disks, as you know, used to be the primary storage mechanism for data in the 80s, and these days both the drives and magnetic media can be problematic to get working reliably.

These days we use floppy disk images mostly for two reasons:

1) For historical preservation.

2) For convenient propagation of a floppy disk's contents, e.g. to be able to download a floppy from the web for use on a modern system running a DOS emulator or on an old system with a modern 'floppy drive emulator', such as a Gotek drive.

 

Common Formats of Disk Image

This article will not go into technical detail on the constructs that make up a floppy disk image format. Instead, I will summarize the most common types you will see and how best to use them.

.IMG files

Files with a .IMG extension are usually raw disk images which contain a sector by sector copy of the original media. These are not just used for floppy disk images, but also hard disk, optical disks and even bitmap (graphics) images.

Because it's a raw copy of each sector, the actual format of the data held within the .IMG file is based on the file system of the disk from which the image was taken. In DOS terms, this will always be the FAT12 file system. Most raw floppy disk images start with the FAT boot sector.

If you see a file extension .IMZ, this is a GZIPped version of a floppy disk image, so you would usually need to 'gunzip' the file to extract its compressed contents first.

DskImage is a DOS utility used to create and restore disk images to and from a single image file in this format. It creates raw files, so no header or metadata is stored in the imaged file; just the contents of each sector on the floppy disk.

For Windows, WinImage can read and write .IMG files.

.IMA Files

These are dumps of raw floppy disk sectors, usually from 1.44 MB DOS-formatted floppy disks. Raw means there is no header at the start of the file, and no metadata (data that describes the data) stored either.

This format is supported by the Windows tool, WinImage, as well as other software utilities. These files usually have a .IMA file extension, but on occasion this is changed to .IMG.

TELEDISK (.TD0 files)

This is one of the oldest disk image formats as it was used on other non-DOS computers to create images from mainframes and DEC workstations. It became a standard for people to preserve system (boot) disks for old computers. Originally TeleDisk was a shareware program sold by Sydex. A later variant of the TeleDisk utility is called ImageDisk, which is free to use and is superior to the original.

The precise image format used by TeleDisk is undocumented, whereas the later ImageDisk (.IMD) format is open source. By converting a .TD0 to the .IMD format you can gain access to old media if your version of TeleDisk is incompatible with the same version that created the file (a common problem with the TeleDisk format).