2.2 Tapes

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Tape was an alternate long- term storage medium whereby the data created by a program could be stored for reuse by the same program or by other programs requiring the same data.

This was the golden age of sequential processing. For example, consider a program that ran daily to maintain customer information on a master customer list. The customer master file was kept on tape, and when the update program called for it, the tape was mounted on a tape drive ”a read/write device the size of a refrigerator. The day s changes (adds, changes, and deletes) might be on cards (the transaction deck) or tape (the transaction tape). When the program ran, new customers would be added, old customers deleted, and existing customer information changed as needed.

Each record on the master file would be read from the tape, put into internal memory, modified as needed and then rewritten to a new tape mounted on another tape drive (the updated master, which would be used the next day). (See Figure 2-2.)

Figure 2-2. Tape Usage

Every record on the master was read and rewritten, whether it was to be changed or not.

Tapes started out 1/2-inch wide and approximately 600 feet long, with a density of 800 bytes per inch (bpi). This was great at the time, certainly a great improvement over carrying around trays of punch cards. However, as processing requirements grew there was an increased need for more storage capacity. Therefore the tapes were increased in length from 600 to 1200 feet. About the same time, the density doubled from 800 bpi to 1600 bpi.

Then, to answer the demand for increased storage, the tape length was increased again to 2400 feet and the density was increased to 6250 bpi. Notice that the demand for increased storage is becoming a persistent theme in data processing.

The tape reel size didn t change from the 1200 foot to the 2400 foot reel. The length of tape was simply doubled, as shown in Figure 2-3.

Figure 2-3. Tape Capacity Growth

Tape, of course, is a sequential storage medium. Sequential meaning one right after the other. This makes tape a fine candidate for long-term storage, but certainly limited in terms of the speed of data retrieval.

When a specific record must be read, and that record is near the end of the reel, then the tape must move through all preceding data to get to the required data (Figure 2-4). Tapes are still with us, and so is this limitation.

Figure 2-4. Sequential Data

Even though tapes were considered fast at the time, it still took too much time to access and read tape data. This is where disks came in.

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