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The 2 GiB (or 4.2GB depending on implementation) file size limit

All numbers in ISO 9660 filesystems except the single byte value used for the GMT offset are unsigned numbers. Some operating systems may be incorrect and this will cause a 2 GB limit for single extent files.

Normally, a file on an ISO 9660 formatted disc cannot be larger than 232-1 in size, as the file's size is stored in a unsigned 32 bit value, for which 232-1 is the maximum.

It is, however, possible to circumvent this limitation by using the multi-extent (fragmentation) feature of ISO 9660 Level 3. With this, files larger than 4GB can be split up into multiple extents (sequential series of sectors), each not exceeding the 4GB limit. The free software mkisofs is able to create filesystems that use multi-extent files to support file sizes up to 8 TB.

Empirical tests with a 4.2 GB fragmented file on a DVD media have shown that Microsoft Windows XP supports this, while Mac OS X (as of 10.4. does not handle this case properly. In the case of Mac OS X, the driver appears to not support file fragmentation at all (i.e. it only supports ISO 9660 Level 2 but not Level 3). Linux supports multiple extents [3]; FreeBSD only shows and reads the last extent of a multi-extent file.

It appears that the spec does allow 4.2GB files that are not fragmented. From the ECMA-119 Spec (same as ISO9660) we find that the file size is: 9.1.4 Data Length (BP 11 to 18) This field shall specify as a 32-bit number the data length of the File Section. This field shall be recorded according to 7.3.3. ---And 7.3.3 Both-byte orders A numerical value represented by the hexadecimal representation (st uv wx yz) shall be recorded in an eight-byte field as (yz wx uv st st uv wx yz).

For an unsigned number the file size limit is actually (232)-1, or 4.2GB. Windows XP has been tested with single-extent files up to the 4.2GB size and they do work.

However, this 4.2G limit only works for non-multi-extent files. For multi-extent files (level 3 ISO) the maximum size for each file part excapt the last one must be limited to 4GB-one sector (0xFFFFF800 bytes). This is because each part of the file must completely fill each set of sectors to keep the file parts contiguous. A file size of 0xffffffff does not allow this since it will not fit in an integer number of sectors.

Note also that some other OS and file systems might place other limits on file sizes. For example, some versions of FAT and Macintosh HFS (not HFS+) are limited to 2G files.