What is the AD file format?
.ad files are binary plug-in modules for After Dark, the classic screen saver application originally developed by Berkeley Systems in 1989. After Dark popularized the screen saver concept on personal computers and is especially famous for the Flying Toasters animation module.
Each .ad module is a compiled, platform-specific binary loaded at runtime by the After Dark control panel or screen saver engine. On the original Mac OS (System 6 through 9), modules used the HFS resource fork to store both animation code and embedded PICT graphics resources. The Windows version (introduced 1991) packaged modules as DLL-style binaries renamed with the .ad extension and placed in the Afterdrk installation directory.
Modules range from about 10 KB for simple animations up to around 500 KB for complex scenes. Classic Mac and Windows 3.x modules used 8-bit (256-color) graphics; later Windows editions supported 24-bit color.
Berkeley Systems sold After Dark to StarDock around 1997, and StarDock discontinued the product shortly after. A Japanese revival, After Dark X for Mac OS X, was published by Hitachi/Infinisys in the early 2000s but is also defunct. Hundreds of third-party .ad modules were created by enthusiasts and are archived at the Internet Archive for historical use.
Today the format is obsolete - modern operating systems provide built-in screen savers (.scr on Windows; native effects on macOS). Running legacy .ad modules requires the original After Dark software inside a Windows 95/98 virtual machine or DOSBox environment.
Security & safety
RISK: LOW.ad files are binary modules from the 1990s. The format cannot run on modern OSes without After Dark installed. Historic modules from original media are safe; .ad files from unknown internet sources could theoretically contain malicious code targeting the After Dark runtime, though this risk is now purely theoretical given After Dark is not installed on modern systems.
Format details
in a nutshell- Active Desktop (Microsoft) advertisement descriptor - Microsoft used .ad files in early Windows Active Desktop configurations as advertisement/channel descriptor files - unrelated to After Dark screensavers.
Programs that open AD files
Windows1 app
After Dark (original, via DOSBox or Windows 98 VM)
Free Download the After Dark installer from Archive.org, run in a Windows 98/ME virtual machine (VirtualBox) or DOSBox-X. Place .ad modules in the Afterdrk folder. 
macOS1 app
After Dark X (defunct)
Paid After Dark X was a macOS version but is now defunct; website inactive. Use a classic Mac OS 9 emulator (Basilisk II, SheepShaver) to run the original .ad modules. Technical details
deep spec| Format type | Binary plug-in module (proprietary; platform-specific compiled code + embedded graphic resources) |
| Platforms | Mac OS System 6-9 (resource-fork code resources); Windows 3.x, 95, 98, ME (DLL-style binary renamed `.ad`) |
| Byte order | Big-endian (Mac original); little-endian (Windows port) |
| Container (Mac) | HFS resource fork - code resource + embedded `PICT` graphic resources |
| Container (Windows) | PE DLL-like binary placed in the `Afterdrk` installation subdirectory |
| Magic bytes | None publicly documented; format specification was never published |
| Encoding | Binary (compiled native CPU code plus embedded graphics) |
| Typical file size | 10 KB - 500 KB (simple animations to complex multi-element scenes) |
| Color depth | 8-bit (256-color) for classic Mac and Windows 3.x modules; 24-bit in later Windows versions |
| Runtime loading | Loaded dynamically by the After Dark screen saver engine at runtime via plug-in architecture |
| Associated OS | Mac OS (System 6/7/8/9), Windows 3.x, Windows 95/98/ME |
| Released | 1989 (After Dark 1.0 for Mac); Windows version 1991 |
| Latest version | After Dark 4.0 (Windows, ~1994); After Dark X (macOS, early 2000s, defunct) |
AD conversions
Community Q&A
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