Similarly before the unused length towards the end of

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Unformatted text preview: o store several files. In this case we face the problem of uniquely identifying the files stored on a tape and separating one file from the other when they are sequentially stored one after another on the tape. To take care of this problem, the computer adds a file header label and a file trailer label respectively at the beginning and at the end of a particular tape file. The file header label identifies the beginning of a file and contains the file's attributes such as the name of the file (file identifier), the date it was created, etc. The file trailer label signifies the end of a file and contains the total number of records in the file. The data organization on a magnetic tape along with file header and file trailer labels is shown in Figure 8.5. A tape is wound on a spool and during tape I/O operation it moves from a supply spool to a take-up spool. To allow for threading on the take-up and supply spools, the first and the last several feet of tape are unused (not used for any type of information recording). After the unused length towards the beginning of tape, a reflective metal foil known as the beginning of tape (BOT) marker is used to indicate to the equipment the beginning of usable tape. Similarly, before the unused length towards the end of tape, another reflective metal foil known as the end of tape (EOT) marker is used to indicate to the equipment the end of usable tape. These two markers are placed on opposite edges of the tape for machine identification purposes. Between the BOT marker and the first file header label is a tape header label, which contains the tape's attributes such as the tape identifier, the number of files it contains, the date it was last used, and other control information that helps to prevent an important tape from accidentally being erased. The data organization on a magnetic tape along with BOT and EOT markers and tape header label is shown in Figure 8.6. Storage Capacity The storage capacity of a tape depends on the following: 1. Data recording density, and 2. Tape length It is a multiple of the above two parameters. That is, Sto...
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This document was uploaded on 04/07/2014.

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