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Priority Circuit for Servicing Requests Queued by Sector

IP.com Disclosure Number: IPCOM000091174D
Original Publication Date: 1969-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Related People

McGraw, EM: AUTHOR [+2]

Abstract

The information transfer rate of direct access storage devices is increased by sector queuing of requests for data transfer. Sector queuing involves the partitioning of the tracks of the device into a number of fixed length blocks, termed sectors. A queue of requests is established for each sector, arranged in the same sequential order as the corresponding location in the device. These requests each have that particular sector as the starting point of the data transmission associated with the request. A particular request can involve a transmission spanning more than one sector. As each sector of the device approaches the read write head, the sector queue for that particular sector is interrogated and a request is extracted from the queue and is executed.

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Priority Circuit for Servicing Requests Queued by Sector

The information transfer rate of direct access storage devices is increased by sector queuing of requests for data transfer.

Sector queuing involves the partitioning of the tracks of the device into a number of fixed length blocks, termed sectors. A queue of requests is established for each sector, arranged in the same sequential order as the corresponding location in the device. These requests each have that particular sector as the starting point of the data transmission associated with the request. A particular request can involve a transmission spanning more than one sector. As each sector of the device approaches the read write head, the sector queue for that particular sector is interrogated and a request is extracted from the queue and is executed.

Priorities are assigned to certain requests so that these requests can take precedence over requests of nonpriority: Priority requests are placed upon a sector queue. The queue is rearranged so that the priority request is initiated in preference to any other request in that queue awaiting initiation. Thus, queue rearrangement in accordance with priority insures that priority requests are initiated before nonpriority requests. This does not, however, solve the problem of a nonpriority request having been initiated during an earlier sector period, which request can require data transmission over more than one sector and, therefore, overlap sectors in which a priority request is queued.

The circuit specifies those sector points where initiation of a priority request is to begin and detects, before the initiation of any requests, if the execution of a cyclically previous nonpriority request will, if executed, overlap a priority request.

Sector reservation register 10 comprises N triggers, one for each sector N of the device. The outputs of the triggers, sector 0... sector N-1, are connected to combinational shifter circuit 12. The lat-cyclically shifts the vector of bits from register 10 an amount specified by the sector position from device 14 which is an indication of the actual position of the device. Shifter 12 provides a cyclic counterrotational shift of M positions where M is the present sector number of the storage device. The outputs 0...N-1, of shifter 12 are connected to logic circuits which detect for the possible overlap of the execution of nonpriority requests with priority requests.

When a priority request is received during the enqueuing process and a corresponding sector is the first sector involved in that priority transmission, the trigger corresponding to that sector is turned on by raising the appropriate reserve sector line 16. The actual storage request is placed at the top of the queue for that sector to be processed before any previously enqueued nonpriority request for the same sector. With the request positioned in the queue for immediate service and with the appropriate sector reservation register tr...