Most data CDs use the ISO-9660 format or one of its variants. ISO-9660 is based on the de facto standard High Sierra format that was developed by the CD-ROM industry as a cooperative effort because of the lack of formal standards that then existed for writing data to CDs. In the days before High Sierra came into use, it was quite common to find that you could not read the data on a particular CD-ROM because that CD was incompatible with your software.

The primary purpose of ISO-9660, which was adopted in 1984, was to standardize a common logical data format for data CDs and, at the same time, to facilitate data exchange among different computing platforms. As a least-common-denominator format, the original ISO-9660 format is feature-poor because it supports only features that are common across many platforms. For example, the MS-DOS 8.3 filenaming convention limited ISO-9660 to using 8.3 filenames.

At the time ISO-9660 was adopted, these limitations were not much of a problem. Most people ran either MS-DOS or a Mac using floppy disks or small hard disks, and the limitations of ISO-9660 were not onerous in those environments. But the world soon changed, and the strict limits enforced by ISO-9660 became a problem, particularly for those who wanted to use deeply nested directories and long filenames. Accordingly, the ISO-9660 specification was expanded to include three ISO-9660 Interchange Levels for naming files and directories on disc. From most to least restrictive, these ...

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