XDVDFS Format

XDVDFS Format

Xbox Disc Video File System (XDVDFS) is the file system used on original Xbox game discs. This page documents the format that SimpleXisoDrive exposes, including the on-disk structures and the detection logic. Parsing is performed by the XISOSharp library.

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	Byte size of the root directory table
0x01C	8	Volume creation time	Windows FILETIME (int64)
0x024 - 0x7EB	-	Reserved	Not used
0x7EC	20	Second magic ID	MICROSOFT*XBOX*MEDIA

A descriptor is considered valid only when both magic IDs match the expected signature. The stored FILETIME becomes the volume creation time; the raw value 0 maps to 1601-01-01 UTC.

Discovery locations

XISOSharp probes the following locations, in this order:

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	XGD2 hybrid	32	0x89D80000	Hybrid dual-layer layout
5	XGD1 dual-layer	32	2048 x 32 x 6192 (405,798,912)	Game partition offset for dual-layer discs
6	Rebuilt XISO	0	0	Descriptor written at file offset 0; common for rebuilt images

The first location that reads and validates wins. The chosen byte offset is reported as VolumeInfo.DiscLseek and every subsequent read adds it. VolumeInfo.DescriptorSector records whether the descriptor was found at sector 32 or sector 0.

Failure diagnostics

If no location validates, XISOSharp reports an invalid volume (or throws XisoFormatException), and XisoVfsVolume surfaces it as an InvalidImageException stating that the file is not a valid Xbox ISO/XISO image, with the underlying reason as the inner exception.

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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; 0 means none
0x02	2	Right subtree	Index of the right child; 0 means 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	Raw XDVDFS attribute flags (see below)
0x0D	1	Name length	Number of bytes in the name
0x0E	N	Name	ASCII/Latin-1 bytes
...	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

Name handling

• Names are decoded from the length-prefixed bytes as Latin-1 (ASCII-compatible).
• Names are matched case-insensitively when resolving paths.
• Names containing path separators are rejected by the XISOSharp directory walk.

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. XISOSharp walks a directory iteratively:

1. read the entry at the current table offset;
2. skip . and .. entries while still following their children;
3. follow the left and right child indexes until an empty sentinel is reached.

The walk is hardened:

• every visited offset is tracked, so a circular or DAG-shaped table fails with a named error;
• each table is capped at a fixed number of entries;
• a first entry of all 0xFF (left child 0xFFFF) is treated as an empty directory;
• separator-bearing file names abort the walk.

Special cases

Case	Behavior
Left/RightSubTree == 0	No child; traversal stops down that branch.
First entry with left 0xFFFF	Empty directory table (all-0xFF padding).
Empty name	The root entry is synthesized; empty-named children are skipped.
FileSize larger than the image	Reads are clamped to the stream length.

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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 = DiscLseek + (StartSector * 2048) + O

XisoVfsVolume 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/XGD2-hybrid game partition offsets;
• XGD3 and GLOBAL partition layouts;
• CISO-compressed images (.cso), including split .1.cso part sets, decompressed on the fly;
• Xbox ISO images stored as CHD (.chd), decompressed hunk-by-hunk on demand (CHD V1–V5, all codecs).

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;
• CD, GD-ROM and other non-Xbox CHD containers (their decompressed data is not XDVDFS);
• differential child CHDs that require a parent (merge with the parent first);
• audio/video partition content (only the game partition file system is exposed).

If an image fails to mount, the error states that the file is not a valid Xbox ISO/XISO image and carries XISOSharp's diagnostic as the inner exception. See Troubleshooting.