DD File

What are DD files and how to open them

Are you having problems opening a DD 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 DD files.

What is a DD file?

DD files have multiple uses, and AccWare Data Dictionary is one of them. Read more about the other uses further down the page.

AccWare Data Dictionary

.DD files are data files used by AccWare. Data files are used by a specific application, these are stored in the computer where the program can be accessed.

AccWare is accounting software designed for businesses to process accurate financial information. The software was created by Berwyn Myers in 2008.

How to open DD files

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

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

Last updated: October 22, 2026

All known file formats using extension .DD

While AccWare Data Dictionary is a popular type of DD-file, we know of 4 different uses of the .DD file extension. Different software may use files with the same extension for different types of data.

Disk Doubler compressed data

Disk Doubler compressed data files store data compressed by the classic Mac OS program DiskDoubler. It was created by Salient, later acquired by Symantec. DiskDoubler reduces file storage needs by replacing ordinary files with compressed versions, decompressing on demand when those files are opened. The user sees icons tagged with "DD" and transparent handling: compressed files decompress smoothly before use and recompress afterwards.

DiskDoubler processors write these compressed data files. When a file is first compressed, DiskDoubler adds a header; when the operating system or an application requests that file, DiskDoubler or its expand-only utility (often called DDExpand) decompresses it to its original contents. DiskDoubler also supports combining multiple files into a single archive, though many compressed files are kept individually so the system can handle them file-by-file.

Files in this format begin with the four-byte signature 0xABCD0054 at offset zero. Later in the file is a checksum: at offset 82 (counting from zero) a 16-bit big-endian CRC16 is stored, matching a CRC computed over the first 82 bytes.

DiskDoubler Compressed Data

DiskDoubler Compressed Data files store data compressed by the DiskDoubler utility developed for classic Mac OS. They are created when individual files are compressed so that they can be used normally: when an application opens one, it is automatically decompressed, and when writable changes are saved or the file is closed, it may be recompressed. DiskDoubler could also produce archive-style files combining multiple files before compression.

DiskDoubler was written by Salient Software (later Symantec) and became popular in the 1990s. The program offered several compression methods (e.g. DD1, DD2, DD3, AD1, AD2) and supported both standalone compressed files and combined archives. Some versions also created self-extracting archives.

Files start with a magic number: the first four bytes read as 0xABCD0054 in big-endian order. Some variants include a CRC checksum in the header (e.g. at offset 82), while older files may have zero in place of the CRC.

MS-ISAM Data Dictionary

We know that one DD format is MS-ISAM Data Dictionary. 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 DD files. Remember, different programs may use DD 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 DD file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.