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System Allowing Direct Access to Sequential Files

IP.com Disclosure Number: IPCOM000086899D
Original Publication Date: 1976-Nov-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Chorosz, H: AUTHOR

Abstract

This is a system for quickly retrieving a record, identified by a key, within a previously selected cylinder. The cylinder structure is modified in such a way that the index pulses are shifted from one track to the next. Thus, when the control unit executes a command of a file-scan type, it is possible to pass from one track to another without having to wait for the end of a rotation.

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System Allowing Direct Access to Sequential Files

This is a system for quickly retrieving a record, identified by a key, within a previously selected cylinder. The cylinder structure is modified in such a way that the index pulses are shifted from one track to the next. Thus, when the control unit executes a command of a file-scan type, it is possible to pass from one track to another without having to wait for the end of a rotation.

It is often desirable to have access to a particular record in a sequential file of a disk, in order to bring modifications or corrections thereto. In such cases, the sequential access is too long. This subject system allows the accessing of a record, identified by a key, in a file containing records arranged in accordance with increasing key values, without scanning the whole file.

In the first step, the cylinder containing the desired record is selected. For this purpose, at file opening, a table is established (index cylinder) with the keys appearing in the first record of every cylinder. Then, it is only necessary to search in this table for the cylinder containing the desired record.

When the cylinder containing the desired record is located, the search for the track containing the record is performed at cylinder level. To allow this search, the following modifications are introduced.

(1) The cylinder structure is changed in accordance with the figure, i.e., the index pulses are shifted from one track to the following one. Preferably, the shifts are equal to a sector, so that successive sector pulses in the servo track correspond to successive index pulses in the data tracks (this implies that, at least, as far as the first record of each track is concerned, the search key is always in the first sector of the record), and the position of the index pulses are forward-shifted from the bottom to the top of the cylinder.

(2) The control unit is enabled to execute a channel command word of file- scan type, with a variant which allows starting from the lowermost track to the uppermost track of the cylinder.

(3) The sector identification circuits are modified to number the sectors in accordance with the shift of the beginning track.

The search proceeds as follows: The nu...