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Utilizing Multiple Exposure

IP.com Disclosure Number: IPCOM000073454D
Original Publication Date: 1970-Dec-01
Included in the Prior Art Database: 2005-Feb-22
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Kusmiss, JM: AUTHOR

Abstract

A fixed head storage facility consists of a storage control unit and a multitrack storage module having a single recording element per track. This type of facility is used in operating systems in which the file(s) are used as an extension of main store, i.e., images of main store blocks are kept in the file and are read in and written out under control of the operating system; or in systems using dynamic relocation where the file contains images of virtual storage pages and segments. To control and schedule the I/O requests to the file in such environments, the file is uniformly formatted with each track being identical.

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Utilizing Multiple Exposure

A fixed head storage facility consists of a storage control unit and a multitrack storage module having a single recording element per track. This type of facility is used in operating systems in which the file(s) are used as an extension of main store, i.e., images of main store blocks are kept in the file and are read in and written out under control of the operating system; or in systems using dynamic relocation where the file contains images of virtual storage pages and segments. To control and schedule the I/O requests to the file in such environments, the file is uniformly formatted with each track being identical. In the case of virtual systems, the format as shown includes an RO Record, which is a track descriptor record, followed by page size blocks, alternated with dummy records so that records 1, 3 and 5 are page size blocks and records 2 and 4 are dummy records, and a gap record at the end of the track. This formatting gives an equal angular separation between records 1, 3 and 5. Each block is denoted by a slot and the same slot number denotes blocks in the same relative position on different tracks. The dummy records are to allow head switching between tracks and time for execution of other I/O instructions for subsequent blocks.

Input/output operations are initiated by a start I/O instruction in the CPU program and subsequently controlled by commands fetched from main store by a channel. The address portion of the start I/O instruction identifies the channel and I/O device to be used in the operation. The start I/O instruction causes the channel to fetch the channel address word from main store, the address portion designating the location in main store from which the channel subsequently fetches the first channel command word (CCW). CCW chaining is a process by which a new CCW is obtained upon completion of the operation for the current CCW without the issuance of a new start I/O instruction by the CPU program. An example of such a chain would be seek, set sector, search ID, tic and Read/Write. The seek command contains an address portion which designates a main store location at which is stored an address for uniquely selecting one of the plurality of heads associated with the selected device; the set sector command contains an address portion which designates a main store location at which is stored the identification of a relative angular track position known as a sector address. As soon as this track position is received, the storage facility signals the channel of that fact and the channel can disconnect from the control unit during the time that the storage facility is seeking the designated angular position. The channel is free during most of this rotational time allowing other operations to be performed. When the desired angular position is reached, the control unit raises a request in and the channel can then re...