This began to fulfill some of the requirements for a file system, but remains extremely primitive. Sony and other record labels use the Sony definition of CD Text, which is also supported by many home CD recording tools. Each track contains subcode blocks of stereo bit audio samples, which are played at This information is stored in the R through W sub- channels associated with the audio samples.
For each subcode block of samples, there is a total of 96 bytes of graphics information. InPhilips defined an extension to this specification to store textual information on the disc. The principle differences between the two are the inclusion of time zone information and additional identification fields.
ISO is currently the most widely supported file system interchange standard that is supported by most computers and other systems with CD drives e. This is generally true even when a proprietary or real-time operating system is being used. All personal computers since support the ISO file system.
Some attempts have been made by Microsoft and others to support the use of alternate character sets, but this is not part of the standard and has differing levels of success when used in non-Microsoft environments. There are only three structures that define the ISO file system: the volume descriptor, the path table, and the directory entry.
The volume descriptor must be located at the 16th sector from the beginning of the track and points to all other structures. This means that for the first session on a disc starting at sector zero, the volume descriptor is located in sector The volume descriptor contains many important data items e.
If there are hex digits 01 43 44 30 30 31 01 in the contents of sector 16, there is an ISO file system on the disc. This can be positive or negative. Offset 23F in hex for bytes is the application identifier.
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Many CD writing applications insert information here to indicate the software that created the disc. The root directory consists of a list of directory entries concatenated together in one or more sectors. The beginning sector is at offset A0 in hex in the volume descriptor as a 4-byte integer in little-endian format. The length of the root directory is at offset A8 in hex as a 4- byte integer in little-endian format. By convention, the ISO file names are limited to 8 characters, with a 3-character extension separated by a period. Directory names are not allowed to have extensions.
Not all writing software respects these limits and can extend the file name to as many as characters.
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File names only use upper- case letters, s, and a small of special characters. Again, not all writing software respects this, so it is not unusual to find an ISO file system with lower-case letters in the file names. ISO files must be less than 4 GB in size; however, this is often restricted by writing software of less than 2 GB.
Because the ISO file system is not intended to be updated, the cre- ation time of the file on the disc is always equal to the last modified time and no last access time is recorded.
Until the advent of drag-and-drop recording, it was unusual to find an ISO file system where all of the files were not stored in a single con- tiguous range of sectors. While this is provided for in the ISO specifica- tion, it is rarely done. Currently, only drag-and-drop writing software creates fragmented files in ISO file systems.
This is ificant for forensic examiners because, even in cases where part of a disc has been destroyed, your ability to recover the contents of the remainder of the disc is excellent. Even without a directory, just examining the disc for file headers on sector boundaries is usually good enough to recover most common file types e. Joliet Joliet is an extension of ISO that was defined by Microsoft for the Windows 95 operating system and uses a parallel directory structure to enable both standard ASCII file names and longer Unicode file names.
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The definition of the Joliet extension specifically addresses using up to character Unicode file names and removing the restriction on a maximum directory depth of eight levels. Some writing software extends this further to allow the file name to be over characters in length, which appears to function correctly with current versions of Windows.
The volume descriptor for Joliet is required to be in a sector following an ISO volume descriptor in sector 16; usually in sector 17, 18, or This volume descriptor contains 8 bytes of the sector containing the hex values 02 43 44 30 30 31 The same fields that are defined for the ISO volume descriptor.