By Susan Wooldridge
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The first and the last are the sources of greatest heat gain. If air-conditioning equipment breaks down, it is almost always necessary to shut down the entire computer at once to avoid heat damage. 2. Humidity. Recommended upper and lower limits are usually in the range of 40 to 60 per cent relative humidity. At the lower limit, the air may be dry enough to cause some problems with static electricity. For this reason, computer operators are sometimes requested not to wear nylon or other synthetic fabrics.
To summarise this discussion of tape files, and to illustrate typical file sizes and speeds, the details of an example file are given in Table 3. Table 3. Tape File Example The Tape Unit Transfer rate for reading or writing: 96-6 kch/s Packing density : 800 bpi Inter-block gap: 0-75 inches Tape length: 3,600 feet The File Number of records in the file: 292,000 Size of each record: 100 characters Blocking: 10 records per block, 1000 characters per block Number of characters in the file: 29-2 million File occupies approximately two reels of tape Access Times Time required to readfirstblock: 20-3 milliseconds Time required to read entire file (no allowance made for operator set-up and take-down time): 9-9 minutes 49 File Storage Devices Direct Access Devices Direct access devices are peripherals designed to overcome the problems of serial processing to which tape files are restricted.
Transfer rate is the speed at which data can be read and loaded into memory, or taken from memory and written. 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.