XDVDFS Format

XDVDFS Format

Xbox Disc Video File System (XDVDFS) is the file system used on original Xbox game discs. This page documents the subset of the format that SimpleXisoDrive parses, including the on-disk structures and the detection logic.

All multi-byte integers are little-endian.

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Overview

Property	Value
Sector size	2048 bytes
Volume descriptor size	1 sector (2048 bytes)
Magic identifier	MICROSOFT*XBOX*MEDIA (20 bytes)
Directory structure	Binary search tree of directory entries
File names	ASCII, up to 255 bytes, optionally null-terminated
Supported access	Read-only

The image is divided into 2048-byte sectors. File data, directories, and the volume descriptor all live at sector-aligned positions. Directory entries store offsets as indexes multiplied by 4, a quirk inherited from the original format.

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Volume descriptor

The volume descriptor is a single 2048-byte sector with the following layout:

Offset	Size	Field	Description
0x000	20	Magic ID	MICROSOFT*XBOX*MEDIA
0x014	4	Root directory table sector	Sector where the root directory table begins
0x018	4	Root directory table size	Read and skipped by SimpleXisoDrive
0x01C	8	Volume creation time	Windows FILETIME (int64)
0x024 - 0x7EB	-	Reserved	Not used by SimpleXisoDrive
0x7EC	20	Second magic ID	MICROSOFT*XBOX*MEDIA

A descriptor is considered valid only when both magic IDs match the expected signature. If the stored FILETIME cannot be converted, CreationTime falls back to DateTime.MinValue without failing the mount.

Discovery locations

SimpleXisoDrive probes five locations, in this order, using the same sector-32 descriptor layout:

Order	Variant	Sector	Byte offset	Notes
1	Standard Xbox ISO	32	0	The common layout, descriptor at byte 65,536
2	GLOBAL partition	32	0x0FD90000 (265,879,552)	Matches extract-xiso's GLOBAL_LSEEK_OFFSET
3	XGD3	32	0x02080000 (34,078,720)	Matches extract-xiso's XGD3_LSEEK_OFFSET
4	XGD1 dual-layer / hybrid	32	2048 x 32 x 6192 (405,798,912)	Game partition offset for dual-layer discs
5	Rebuilt XISO	0	0	Descriptor written as sector 0; common for rebuilt images

The first location that reads and validates wins. On success the chosen byte offset is stored in IsoSt.VolumeOffset, and every subsequent read is shifted by it. VolumeDescriptor.Sector records whether the descriptor was found at sector 32 or sector 0; IsRebuiltXisoFormat() returns true only for sector 0.

Failure diagnostics

If no location validates, the application throws InvalidImageException with a message that includes:

• the image size in bytes and MB;
• one line per probed location describing why it failed (file too small, magic mismatch, I/O error);
• possible causes (not an Xbox ISO, corrupted or incomplete image, unsupported variant);
• the expected magic string.

If an early probe threw an exception and the final sector-0 probe also throws, the exceptions are combined into an AggregateException.

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Directory entries

Each directory table is an array of entries stored at the start of the directory's sector. The root entry is synthesized from RootDirTableSector; all other entries are read from disk.

Entry layout

Offset	Size	Field	Description
0x00	2	Left subtree	Index of the left child, or 0xFFFF if none
0x02	2	Right subtree	Index of the right child, or 0xFFFF if none
0x04	4	Start sector	First sector of the file data, or of the child directory table
0x08	4	File size	Size in bytes; 0 for directories
0x0C	1	Attributes	XisoFsFileAttributes flags (see below)
0x0D	1	Name length	Number of bytes in the name
0x0E	N	Name	ASCII bytes, optionally null-terminated
...	0-3	Padding	Zero bytes aligning the entry to a 4-byte boundary

Entry size calculation

EntrySize = 14 + NameLength
EntrySize += (4 - (EntrySize % 4)) % 4    // padding to 4-byte alignment

SimpleXisoDrive validates that the whole name and padding can be read; otherwise the read fails with EndOfStreamException.

Name handling

• Names are decoded as ASCII.
• If a null terminator (\0) appears, everything after it is discarded.
• The result is trimmed of surrounding whitespace.
• Names are matched case-insensitively when resolving paths.

Attributes

Flag	Value	Meaning
None	0x00	No attributes set
ReadOnly	0x01	Read-only file
Hidden	0x02	Hidden entry
System	0x04	System entry
Directory	0x10	Entry is a directory
Archive	0x20	Archive/backup flag
Normal	0x80	Normal file

The values map to Windows FileAttributes as follows:

• ReadOnly is always applied (the volume is read-only);
• Directory, Hidden, System, and Archive are applied when set;
• Normal is applied when none of Directory, Hidden, System, or Archive are present.

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Tree traversal

Child pointers are stored as indexes; the byte offset of a child entry within its directory table is:

childOffset = Index * 4

The first entry of a directory table is always at offset 0 of its start sector. Walking a directory therefore means:

1. read the entry at (StartSector of directory, 0);
2. process it;
3. follow the left and right child indexes until 0xFFFF sentinels are reached.

SimpleXisoDrive uses an iterative, stack-based traversal (not recursion) for both listing and lookup:

• Right children are pushed before left children so that left-hand entries are processed first.
• A visited set keyed by (EntrySector, EntryOffset) prevents infinite loops on circular trees.
• A self-reference check rejects child pointers that point back to the entry itself.
• A traversal that exceeds 100,000 nodes is aborted and logged as a possible corruption.

Special cases

Case	Behavior
Left/RightSubTree == 0xFFFF	No child; traversal stops down that branch.
Child offset equals the entry's own offset	Treated as invalid self-reference; ignored.
FileSize == uint.MaxValue (0xFFFFFFFF)	Logged as suspicious (a warning), entry is still used.
Empty name	Root entry uses an empty name; empty-named children are skipped when listing.
Directory read where StartSector is beyond EOF	Read fails, the entry is treated as missing.
GetFirstChild on a non-directory	Throws InvalidOperationException.

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File data access

File contents live at StartSector, and a read at logical offset O maps to the absolute byte position:

absolute = VolumeOffset + (StartSector * 2048) + O

The application clamps reads to the file size reported by the directory entry and stops at end of stream. Multiple files may share sectors when their sizes do not fill a sector; this is normal for XDVDFS and requires no special handling for read-only access.

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Supported variants and limitations

Supported:

• standard original Xbox game ISOs with the descriptor at sector 32;
• rebuilt XISO images with the descriptor at sector 0;
• dual-layer and hybrid dumps using the XGD1 game partition offset;
• XGD3 and GLOBAL partition layouts.

Not supported:

• writing, renaming, deleting, or metadata changes of any kind;
• non-XDVDFS disc layouts and non-Xbox ISO formats;
• encrypted or Redump-style images that have not been converted to XISO;
• audio/video partition content (only the game partition file system is exposed);
• split or compressed image containers (the .iso must be a single file).

If an image fails to mount, the error message lists every probed descriptor location, which is the fastest way to tell "wrong format" apart from "corrupted image". See Troubleshooting.