What is the SDI file format?
.sdi is an operating system image file used in Microsoft's Windows deployment infrastructure. An .sdi file is typically distributed alongside additional BLOB (Binary Large Object) entries - including BOOT and PART blobs - that together define a bootable RAM disk image. The BOOT blob initiates the load sequence, while the image payload itself is stored in the PART blob.
.sdi files are used to create Windows system RAM disks: the file is loaded into memory at boot time, presenting a virtual disk that WinPE (Windows Preinstallation Environment) can start from before any storage driver is active. This makes .sdi files essential for PXE (network) boot scenarios, where a deployment server streams the image to a client over the network. .sdi images are commonly used in enterprise environments with Windows Deployment Services (WDS).
The file begins with the 8-byte ASCII signature $SDI0001, followed by a 4 KB extensible header and a table of blob entries at offset 0x400. The best-known instance is boot.sdi, located at \Boot\boot.sdi on Windows installation media and in WDS shares, created and managed using tools in the Windows Assessment and Deployment Kit (ADK) such as DISM. For most deployment scenarios, .sdi works alongside .WIM images, which carry the OS payload.
Security & safety
RISK: LOWSDI files do not execute on their own; they require WDS/WinPE boot infrastructure. Low malware risk for end users.
Format details
in a nutshell- Fidonet Software Distribution Network Info - Legacy 1990s BBS format used to index files available for download on the Fidonet network; entirely unrelated to Microsoft SDI.
Programs that open SDI files
Windows3 apps
Windows ADK / DISM
Free Used internally during WinPE creation; boot.sdi is placed in the boot media automatically.
Windows Deployment Services (WDS)
Built-in Serves boot.sdi over PXE to booting clients; no manual opening required. Technical details
deep spec| File signature | `$SDI0001` (8 ASCII bytes at offset 0) |
| Header size | 4 KB extensible header |
| Byte order | Little-endian |
| Blob types | `BOOT`, `LOAD`, `PART`, `DISK`, `WIM` (indexed in table of contents at offset `0x400`) |
| Blob entry size | 0x40 bytes per entry |
| Compression | None (blob payloads uncompressed; embedded WIM may apply its own compression) |
| Typical size | 200 KB - 50 MB (standard `boot.sdi` ~3 MB) |
| Associated OS | Windows |
| Primary use case | WinPE RAM disk creation for PXE network boot and local OS deployment |
| Toolchain | Windows ADK / DISM |
| Standard media location | `\Boot\boot.sdi` on Windows installation media |
| Deployment partner | Paired with `.wim` images in WDS for full OS network delivery |
| Released | ~2001 (Windows XP era, used in WinPE/WDS) |
| Latest version | N/A (format stable, used in Windows 11 deployment) |
SDI conversions
Community Q&A
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