ISO Disc Image
An ISO file is a disc image containing an exact sector-by-sector copy of an optical disc's filesystem. FileDex provides reference information only.
Disc image format. Mounting or conversion requires filesystem parsing not available in browser WASM.
Common questions
How do I open or browse the contents of an ISO file?
On Windows 10/11, double-click the ISO to mount it as a virtual drive in File Explorer. On Linux, use `mount -o loop` to mount it to a directory. On macOS, double-clicking an ISO opens it in DiskImageMounter. 7-Zip can also extract ISO contents without mounting on any platform.
What is the maximum file size an ISO can contain?
Standard ISO 9660 Level 1 limits individual files to 4 GB minus 2 bytes due to 32-bit extent lengths. UDF bridge format removes this restriction, supporting files up to 16 EB in theory. Most modern disc authoring tools create UDF/ISO 9660 hybrid images to avoid the 4 GB ceiling.
Can I create a bootable USB drive from an ISO file?
Yes, tools like Rufus (Windows), dd (Linux/macOS), or balenaEtcher (cross-platform) can write an ISO image to a USB drive in a bootable layout. The ISO must contain El Torito or UEFI boot structures for the result to be bootable. Simply copying the ISO file onto a USB drive will not work — the image must be written at the block level.
Why is my ISO image larger than the files it contains?
ISO 9660 uses 2048-byte sectors, so every file rounds up to the nearest sector boundary, wasting space on small files. The filesystem also stores path tables, volume descriptors, and directory records that add overhead. Additionally, bootable ISOs embed boot images and catalogs that increase total size beyond the sum of visible files.
What makes .ISO special
Optical disc images in ISO 9660 format preserve the complete filesystem of a CD, DVD, or Blu-ray disc as a single file, readable by virtually every operating system without special drivers.
Continue reading — full technical deep dive
Filesystem Layout
ISO 9660 reserves the first 32,768 bytes (16 sectors x 2048 bytes) as system area, followed by the Primary Volume Descriptor starting with CD001 at offset 32769 — this 32 KB gap is why El Torito bootable ISOs can embed a boot catalog without conflicting with the filesystem. The Primary Volume Descriptor (PVD) declares the volume name, creation date, root directory location, and path table positions. A Supplementary Volume Descriptor may follow, enabling Joliet extensions for Unicode filenames up to 64 characters. The Volume Descriptor Set Terminator marks the end of the descriptor chain, after which the root directory record and file data occupy the remaining sectors.
Extensions and Hybrid Formats
The base ISO 9660 standard restricts filenames to 8.3 format with uppercase ASCII — severe limits that spawned three major extensions. Joliet adds Unicode support through a supplementary volume descriptor that duplicates the directory tree with UCS-2 encoded names. Rock Ridge embeds POSIX metadata (permissions, symlinks, deep directory nesting) in the system use area of each directory record, making Linux and Unix systems treat the ISO like a native filesystem. UDF (Universal Disk Format) supports DVD and Blu-ray media with files larger than 4 GB. Many disc images combine ISO 9660 with UDF in a bridge format, placing both filesystem structures in the same image for maximum compatibility.
Boot Specification
El Torito defines how BIOS firmware locates bootable code within an ISO image. A Boot Record Volume Descriptor at sector 17 points to a boot catalog, which lists one or more boot entries specifying emulation mode (floppy, hard disk, or no emulation) and the location of the boot image. Modern UEFI-bootable ISOs place an EFI System Partition as an embedded FAT image referenced by a second boot catalog entry. Hybrid ISO images created with isohybrid can also boot from USB drives by embedding an MBR partition table in the system area, coexisting with the ISO 9660 structures.
Sector Addressing
All ISO 9660 structures use 2048-byte logical sectors, matching the physical sector size of CD-ROM media. Directory records, path tables, and extent locations reference logical block addresses rather than byte offsets. This sector-aligned design means mounting an ISO requires no block translation layer — the OS reads the image file directly as if it were a block device. Both-endian encoding appears throughout the format: every multi-byte integer is stored twice, once in little-endian and once in big-endian order, ensuring portability across processor architectures without byte-swapping.
Technical reference
- MIME Type
application/x-iso9660-image- Magic Bytes
43 44 30 30 31CD001 identifier at offset 0x8001 in the Primary Volume Descriptor.- Developer
- ISO / Ecma International
- Year Introduced
- 1988
- Open Standard
- Yes — View specification
CD001 identifier at offset 0x8001 in the Primary Volume Descriptor.
Binary Structure
Sector-based image at 2048 bytes per logical block. Sectors 0-15 (bytes 0-32767) form the system area, reserved for boot code or platform-specific data. Sector 16 onward contains Volume Descriptors, each exactly one sector, identified by a 5-byte signature (CD001). The Primary Volume Descriptor at sector 16 points to the root directory record and path table. File extents are stored as contiguous sector runs referenced by logical block address and data length.
Attack Vectors
- Crafted ISO images with malformed directory records can trigger buffer overflows in vulnerable mounting implementations
- Bootable ISOs may contain OS installers or live environments that execute arbitrary code when booted
- Autorun.inf files embedded in the ISO can trigger automatic execution on older Windows systems when the image is mounted
Mitigation: FileDex does not execute, install, mount, or parse these files. Reference page only.