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Accessing Data From Disk Files

IP.com Disclosure Number: IPCOM000048691D
Original Publication Date: 1982-Mar-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 37K

Publishing Venue

IBM

Related People

Reed, DG: AUTHOR [+2]

Abstract

Disk files identify data by so-called record identification signals. One of the channel commands between a host and a disk file is to search for such identifications. The comparison of the requested identification argument with the DASD (direct-access storage device) identification is performed in channel processing circuits. This arrangement is necessary because of the relatively short gaps between successive records on a DASD file. Some DASD control units are also connected to mass storage tape libraries, other tape off-loading devices, buffered or cached, interposed between selected hosts and the disk file or to another DASD. Such internal connections to the peripheral system generally do not have ready access to the channel.

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Accessing Data From Disk Files

Disk files identify data by so-called record identification signals. One of the channel commands between a host and a disk file is to search for such identifications. The comparison of the requested identification argument with the DASD (direct-access storage device) identification is performed in channel processing circuits. This arrangement is necessary because of the relatively short gaps between successive records on a DASD file. Some DASD control units are also connected to mass storage tape libraries, other tape off-loading devices, buffered or cached, interposed between selected hosts and the disk file or to another DASD. Such internal connections to the peripheral system generally do not have ready access to the channel. Accordingly, the control unit computer must do a compare of the internal identification with the DASD identification. To achieve this within gap processing time, a sequence of DASD operations is listed in the control unit. During the execution of one operation, that operation is fetched and analyzed for search identification. If it is a search identification internal SEARCH, that identification is loaded in the control memory for use by the control as the next received record from DASD. This arrangement eases the time constraints on the control unit computer speed.

A DASD has a plurality of concentric tracks sensed by a radially movable head. Record areas are defined by a plurality of relatively short gaps.

To identify record areas on the DASD for data transfer, a host supplies an external identification (EID) to the control unit. This EID is stored in a register which supplies its signal to a comparison. The head senses these signals from the record area and supplies the record identification through switch DC-EC to the compare circuit. When the compare circuit sees an equality, an identification equal (EIDE) signal is supplied to the control memory for use by the control unit computer processing signals between DASD and the host. Since EID and compare are electronic circuits, operations occur at electronic circuit speeds, allowing a relatively short gap on the DAS...