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By Susan Wooldridge

ISBN-10: 0434984558

ISBN-13: 9780434984558

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In addition, there will usually be records on the master file that need no processing at all. These are moved intact from the input area GRANDFATHER f master FATHER ( master out out SON Fig. 21. Tape update processing. in main memory to the output area. When a block of records has been assembled, it is written to a fresh reel of tape. The next week, this tape becomes the input master and a new output master is created, as shown in Fig. 21. There are two important characteristics of magnetic tape processing 48 Data Processing Made Simple that are illustrated in the example given above, and need to be empha­ sised.

46 Data Processing Made Simple The speed at which tape is written or read is called the transfer rate. Transfer rate depends on the speed at which the tape is travelling, and on the packing density. Typical transfer rates are in the range of 20 kch/s (20,000 characters per second) to 100 kch/s (100,000 characters per second). One of the biggest risks in any installation is the possibility of a human or program error that causes a tape to be written on rather than read from. The data previously on the tape would be destroyed.

This is in the order of 150,000 to 800,000 characters or bytes per second. Average access time is therefore the sum of average seek time, average latency, and transfer rate. The transfer rate is by far the smallest component, and is often disregarded in calculations. Facts and figures about three popular disk devices are given in Table 4. A similar type of device is the fixed disk. Its operational principles File Storage Devices 53 are the same as those of exchangeable disks, except that (as the name implies) the disk is permanently fixed in place and cannot be removed to be replaced with a new one.

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Data Processing. Made Simple by Susan Wooldridge

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