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430 FILE SYSTEMS CHAP. 6 6.4 EXAMPLE FILE SYSTEMS In the following sections we will discuss several example file systems, rang- ing from quite simple to highly sophisticated. Since modern UNIX file systems and Windows 2000’s native file system are covered in the chapter on UNIX (Chap. 10) and the chapter on Windows 2000 (Chap. 11) we will not cover those systems here. We will, however, examine their predecessors below. 6.4.1 CD-ROM File Systems As our first example of a file system, let us consider the file systems used on CD-ROMs. These systems are particularly simple because they were designed for write-once media. Among other things, for example, they have no provision for keeping track of free blocks because on a CD-ROM files cannot be freed or added after the disk has been manufactured. Below we will take a look at the main CD- ROM file system type and two extensions to it. The ISO 9660 File System The most common standard for CD-ROM file systems was adopted as an International Standard in 1988 under the name ISO 9660 . Virtually every CD- ROM currently on the market is compatible with this standard, sometimes with the extensions to be discussed below. One of the goals of this standard was to make every CD-ROM readable on every computer, independent of the byte order- ing used and independent of the operating system used. As a consequence, some limitations were placed on the file system to make it possible for the weakest operating systems then in use (such as MS-DOS) to read it. CD-ROMs do not have concentric cylinders the way magnetic disks do. Instead there is a single continuous spiral containing the bits in a linear sequence (although seeks across the spiral are possible). The bits along the spiral are divided into logical blocks (also called logical sectors) of 2352 bytes. Some of these are for preambles, error correction, and other overhead. The payload por- tion of each logical block is 2048 bytes. When used for music, CDs have leadins, leadouts, and intertrack gaps, but these are not used for data CD-ROMs. Often the position of a block along the spiral is quoted in minutes and seconds. It can be converted to a linear block number using the conversion factor of 1 sec = 75 blocks. ISO 9660 supports CD-ROM sets with as many as 2 16 1 CDs in the set. The individual CD-ROMs may also be partitioned into logical volumes (partitions). However, below we will concentrate on ISO 9660 for a single unpartitioned CD- ROM. Every CD-ROM begins with 16 blocks whose function is not defined by the ISO 9660 standard. A CD-ROM manufacturer could use this area for providing a
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SEC. 6.4 EXAMPLE FILE SYSTEMS 431 bootstrap program to allow the computer to be booted from the CD-ROM, or for some other purpose. Next comes one block containing the primary volume des- criptor , which contains some general information about the CD-ROM. Among this information are the system identifier (32 bytes), volume identifier (32 bytes), publisher identifier (128 bytes), and data preparer identifier (128 bytes). The manufacturer can fill in these fields in any desired way, except that only upper
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