Basic Principles of Operation Storage organization




Basic Principles of Operation Storage organization

A tape consists of vertical columns called frames, and horizontal rows called channels or tracks. Older tapes had 7 tracks and used 6-bit BCD code format for data recording. Hence, letter A is represented on this tape by its BCD code 110001. The first six tracks record the 6 bits of BCD code and the seventh track records parity bit.

A parity bit or check bit enables detection of errors that may occur due to loss of a bit from a string of 6 or 8 bits during data input or output operations. If the basic code for a character requires an odd number of 1 bits (such as characters 1, 2, or A in BCD), an additional 1 bit is added to the check bit location so that there will always be an even number of 1 bits. This is an example of even-parity. Similarly, in odd-parity, the check bit is 0 if the total number of 1 bits for representing a character is odd and it is 1, otherwise. The tape uses the parity bit for an even parity.

Most modern magnetic tapes have 9 tracks and use the 8-bit EBCDIC code format for data recording. The fourth track records parity bit, which produces an odd parity in this example. Notice that, it represents letter A by its 8-bit EBCDIC code 1100001.

A magnetic tape is a continuous medium, which records data serially. There is no addressing. In this situation, how different pieces of data (known as records) are identified on a tape? for this, it separates records by blank spaces called inter-record gaps (IRG). A tape drive creates IRGs automatically as it writes data on tape. Each IRG is about 0.5 inches. While reading data from a moving tape, a tape drive stops tape movement when it encounters an IRG. The tape remains stationary until the record processing is over, and then moves again to read the next record. This process repeats for all repeats for all records in a file.

Records are varying lengths. When a tape contains a large number of very short records with an IRG after each record, more than half of the tape is unused (occupied with IRGs). Moreover, tape I/O operation also becomes very slow due to frequent interruption in tape movement during I/O operations. To overcome these problems, it groups records in blocks, and separates blocks by blankspaces called inter-block gaps (IBG). The process of grouping two or more records together to form a block of data is known as blocking, and the number of records grouped together in each block is called blocking factor. Blocking factor generally depends on record length. To reduce wastage of tape storage space, the rule followed generally for deciding blocking factor is- a block should be at least ten times as long as the IBG.