.ISO

Introduction

An .iso file is a copy of an optical disc, a CD, DVD or Blu-ray, stored sector by sector in one file. It is named after ISO 9660, the standard file system of CD-ROMs. ISO files are how operating systems and large software are usually distributed: they can be burned to a disc, written to a USB stick, mounted as a drive or started in a virtual machine.

Despite the name, the file system inside is not always ISO 9660. DVD and Blu-ray images usually use UDF, and many images carry both so that any system can read them.

Structure

The image is a run of 2048-byte sectors, the data area of each sector on the disc. The first 16 sectors, 32 KB, are a system area that ISO 9660 leaves free. Bootable images use it for a boot record, and hybrid images that also boot from USB put a hard disk partition table there.

From sector 16, at offset 0x8000, comes a set of volume descriptors, one per sector. Each starts with a type byte, the identifier CD001 and a version byte. The primary volume descriptor, type 1, gives the volume name, the size of the volume and the location of the root directory. A supplementary descriptor, type 2, adds Joliet long Unicode names. A boot record, type 0, points to the El Torito boot catalogue. A terminator, type 255, ends the set. Directories and files follow, each file stored as one run of whole sectors.

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