DEF File

What are DEF files and how to open them

Are you having problems opening a DEF file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your DEF files.

What is a DEF file?

DEF files have multiple uses, and 22DISK Format Definition is one of them. Read more about the other uses further down the page.

22DISK Format Definition File

The 22DISK format definition file is a plain‐text disk-layout descriptor used by Sydex's 22DISK utility to tell the program how to read, write, format or interpret CP/M formatted floppy disks. Each definition (a „format") describes the geometry, encoding, sector layout and other low‐level details of a specific CP/M disk type so that 22DISK can work correctly with non-standard or machine-specific disks. This makes it useful when copying files to or from CP/M disks on an IBM PC or compatible system using 22DISK under DOS.

Definitions are created by Sydex and supplied in files such as CPMDISKS.DEF or in user-created .DEF files. Users may edit or build custom definitions using a text editor. Once a custom definition is prepared, 22DISK uses a utility called GENINDEX to compile it into the master definition file recognized by the suite.

Inside each definition, keywords specify parameters: density (FM or MFM, high or low data rate), number of cylinders (tracks per side), number of sides, sectors per track and sector size, sector ordering for each side (SIDE1, SIDE2), ordering of side sequences (ORDER), and allocation constants like BSH, BLM, EXM, DSM, DRM, AL0, AL1, and an offset (OFS).

One technical way to recognize such a file: it begins with the ASCII signature „BEGIN" at the very start, and contains fields such as the strings „CYLINDERS", „DENSITY", „SECTORS", „SIDE1", „SIDE2", „SIDES", and „ORDER" somewhere in the body.

How to open DEF files

Important: Different programs may use files with the DEF file extension for different purposes, so unless you are sure which format your DEF file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested ten different DEF openers which you will find listed below.

Last updated: October 22, 2026

All known file formats using extension .DEF

While 22DISK Format Definition File is a popular type of DEF-file, we know of 9 different uses of the .DEF file extension. Different software may use files with the same extension for different types of data.

Chinese UCDOS Print Definition File

We know that one DEF format is Chinese UCDOS Print Definition File. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

MAGIX Music Studio Definitions

We know that one DEF format is MAGIX Music Studio Definitions. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

Micropad Keys Definition

We know that one DEF format is Micropad Keys Definition. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

Module-Definition File

Files that contain the .def file extension are most commonly associated with Microsoft Visual C++ definition files. These files list the exports and attributes of a specific program that is linked by an application linker.

The DEF file format is commonly used when creating a dynamic link library during the development of a software application.

DEF opener for Windows

We have identified one DEF opener that is compatible with this specific type of DEF file.

Personal Font Maker Definition File

We know that one DEF format is Personal Font Maker Definition File. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

Philips Ease Database Definition File

We know that one DEF format is Philips Ease Database Definition File. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

Sprint Converter Definition

We know that one DEF format is Sprint Converter Definition. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

ThunderByte Antivirus Definitions

We know that one DEF format is ThunderByte Antivirus Definitions. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

Various apps that use files with this extension

These apps are known to open certain types of DEF files. Remember, different programs may use DEF files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about DEF file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.