What is the ASI file format?
.asi is a game plugin file used to inject custom code into Windows-based PC games. Despite its extension, an .asi file is technically a standard .DLL - a compiled native binary in Windows Portable Executable (PE) format - with the extension renamed from .dll to .asi as a modding convention.
.asi files do nothing on their own. They require a dedicated ASI Loader placed in the game's directory, typically disguised as a system library such as dinput8.dll, version.dll, or vorbisFile.dll. At game startup the loader calls LoadLibrary() on every .asi it finds in the folder, injecting each plugin's code directly into the running game process.
The convention originated in the mid-2000s modding scene around Rockstar's Grand Theft Auto series (GTA III, Vice City, San Andreas) and has since spread to many other PC titles. Widely used loaders include Ultimate ASI Loader and Silent's ASI Loader.
Because an .asi plugin runs arbitrary native code inside the game process with full access to memory and system APIs, only files from trusted sources should be installed. The file begins with the standard MZ magic bytes (4D 5A) at offset 0 and the PE\0\0 signature further in - identical to any .dll or .EXE. It can be inspected with PE tools such as CFF Explorer, but it requires the target game and its ASI Loader to actually execute.
Security & safety
RISK: HIGHAn .asi is native executable code (a DLL) that runs inside the game with your user privileges, so it has the same risk profile as installing any unknown DLL: a malicious .asi can do anything your account can. Game 'trainers', 'cheats' and pirated-mod .asi files are a well-known malware/PUP vector. Only install .asi plugins from reputable sources (established modding communities, GitHub releases, the mod author's official page), prefer open-source loaders (Ultimate ASI Loader), scan unknown files, and inspect an .asi's imports in a PE viewer if unsure. Never run game mods on an account with sensitive data, and avoid 'free .asi' download portals bundled with adware.
Format details
in a nutshellFULL NAMEASI Game Plugin (renamed Windows DLL)aka ASI script, ASI mod
DEVELOPERCommunity modding convention (loaded via ASI Loader); built on Microsoft's PE/DLL formatsince Early-mid 2000s modding scene (GTA III/Vice City/San Andreas era ASI loaders)
CATEGORYGame Files
MIME TYPEapplication/x-msdownload
TYPEWindows dynamic-link library (PE) renamed .asi, loaded into a game process
This extension is also used by…
- Assembly include / source file (Alpha Software, MASM-style) - Some assembler/IDE toolchains use .asi for an assembly 'include' or header source file (the meaning recorded in this site's db, dev 'Alpha Software'); it is plain text, not an executable.
- Velvet Studio / tracker sample/instrument - Older music tracker tools used .asi for an instrument or sample file; opened in that tracker only.
MAGIC BYTES · FILE SIGNATURE
OFFSET
0001
HEX
4D5A
ASCII
MZ
An .asi is a standard Windows PE dynamic-link library with the extension changed to .asi, so it begins with the "MZ" (4D 5A) DOS header and contains the "PE\0\0" (50 45 00 00) signature further in, exactly like a .dll/.exe. The .asi extension is purely a marker telling an ASI Loader to inject this library into the game. (The unrelated Alpha Software 'assembly include' meaning in this site's db is a plain-text source file with no magic - see also_used_by.)
Programs that open ASI files
Windows3 apps
Ultimate ASI Loader
Open-source Place the loader (a renamed proxy DLL like dinput8.dll/version.dll) in the game folder, then drop the .asi beside it; it loads automatically when the game starts. The maintained, general-purpose loader.
Silent's ASI Loader
Free A lightweight ASI Loader popular for GTA San Andreas (also bundled inside CLEO). Install it in the game directory so .asi plugins load at launch.
A PE viewer (CFF Explorer / PE-bichu) - inspection only
Free To inspect (not run) what's inside an .asi, open it in a PE explorer like CFF Explorer - it's a DLL, so you can see its imports/sections. Useful for vetting an unknown plugin. Technical details
deep spec| File structure | Windows Portable Executable (PE) - a standard DLL with the extension renamed to .asi |
| Magic bytes | 4D 5A ("MZ") at offset 0; PE signature 50 45 00 00 at the offset stored in the MZ header |
| PE variant | PE32 (32-bit / x86) or PE32+ (64-bit / x64) depending on the target game architecture |
| Encoding | Binary - compiled native machine code (typically C or C++) |
| Byte order | Little-endian (Intel x86/x64 convention, inherited from PE format) |
| Typical file size | Tens of KB to a few MB (a compiled native plugin) |
| Compression | None in the PE wrapper by default; individual sections may be packed by the author |
| Code signing | Optional Authenticode signature; most community-made .asi plugins are unsigned |
| MIME type | application/x-msdownload (same as .dll and .exe) |
| Loading mechanism | An ASI Loader (shipped as dinput8.dll, version.dll, vorbisFile.dll, etc.) calls LoadLibrary() on each .asi in the game folder at startup |
| Execution context | Runs as a DLL inside the host game process with full access to game memory, threads, and Windows APIs |
| Entry point | Standard DLL entry point (DllMain); no special ASI-specific export is required |
| Load order | ASI Loaders typically inject .asi files in alphabetical order from the game directory |
| Associated OS | Windows (the PE format is Windows-specific) |
| Security risk level | High - runs arbitrary native code with full process rights; install only from trusted sources |
| Released | Early-mid 2000s modding scene (GTA III/Vice City/San Andreas era ASI loaders) |
| Latest version | Tied to the Windows PE/DLL format; Ultimate ASI Loader actively maintained (2020s) |
| Specification | github.com |
ASI conversions
Community Q&A
asked by users
R
Z
Frequently asked questions
What is a .asi file?
A game plugin - technically a Windows DLL renamed to .asi - that an 'ASI Loader' injects into a PC game to add mods or features. It's most associated with Grand Theft Auto mods but used by many games. (The site's 'assembly header' meaning is a separate plain-text source file.)
How do I install / use a .asi file?
You don't open it - you install it. Put the .asi in the game's folder together with an ASI Loader (e.g. Ultimate ASI Loader), and the loader runs the plugin when the game starts. Without a loader, nothing happens.
Why isn't my .asi mod working?
Usually there's no ASI Loader installed, or the wrong one. Install Ultimate ASI Loader or Silent's ASI Loader in the game directory, make sure the .asi is in the right folder, and check the .asi matches the game's architecture (x86 vs x64).
Is a .asi the same as a .dll?
Yes - an .asi is a normal Windows DLL with a different extension. Renaming it to .dll produces a valid DLL. The .asi name just tells the ASI Loader to load it into the game.
Are .asi files safe?
They run native code with full user privileges, so only as safe as their source. Reputable mods are fine, but .asi 'trainers/cheats' from unknown sites are a common malware vector. Get plugins from trusted modding communities and prefer open-source loaders.
Where do I put a .asi file for GTA?
In the game's main folder (where the .exe is), alongside an installed ASI Loader; some setups use a 'scripts' or 'plugins' subfolder. The loader picks up every .asi at launch in alphabetical order.
References
1Microsoft - PE Format (Portable Executable)learn.microsoft.com
2Ultimate ASI Loader (ThirteenAG) - GitHubgithub.com
