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Read Modify Command

IP.com Disclosure Number: IPCOM000078733D
Original Publication Date: 1973-Feb-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Page, DR: AUTHOR

Abstract

The "Read Modify" command for a disk or drum controller, is used in a paging environment to improve the performance of a paging device. When a Read Modify Channel Command Word (CCW) is detected by the channel, it is treated by the device control unit precisely as though it were a Read Key and Data command. In the case of Read Modify, however, the first four bytes which the control unit transfers to the channel will not be sent to main memory. Instead, the first of these bytes is discarded and the remaining three are ORed with the 24-bit data address portion of the CCW in the channel. The modified address is then used to transmit the rest of the data to main memory. In normal operation the first four bytes, which are used for the modification, would form the Key Area and the rest of the record would consist of the Data Area.

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Read Modify Command

The "Read Modify" command for a disk or drum controller, is used in a paging environment to improve the performance of a paging device. When a Read Modify Channel Command Word (CCW) is detected by the channel, it is treated by the device control unit precisely as though it were a Read Key and Data command. In the case of Read Modify, however, the first four bytes which the control unit transfers to the channel will not be sent to main memory. Instead, the first of these bytes is discarded and the remaining three are ORed with the 24-bit data address portion of the CCW in the channel. The modified address is then used to transmit the rest of the data to main memory. In normal operation the first four bytes, which are used for the modification, would form the Key Area and the rest of the record would consist of the Data Area. The count field of the CCW should be 4 greater than the number of bytes to be transferred to main memory.

When a disk is being used as a paging device, for example, a page is 4096 bytes long and one track of the disk can hold approximately 4600 bytes. Without Read Modify, the track is formatted to contain a single record length of 4096 bytes and reading the record requires between one and two revolutions of the disk after the seek is complete, with an average of 1.5 revolutions.

Using the Read Modify command the track can be formatted to contain four records, each of which consists of a 4-byte key and a 1K data area. The pa...