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Hardware Skip-On-Write

IP.com Disclosure Number: IPCOM000042330D
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Peterson, RA: AUTHOR [+2]

Abstract

In a computer system it is often necessary to transfer data from main storage to auxiliary storage (Direct Access Storage Device (DASD)), and vice versa. Blocks of data are paged into main storage from auxiliary storage when needed by the operating system. When the main storage pages are no longer needed, they are overwritten or, if altered, they are paged back out to auxiliary storage. A data file consists of one or more pages of main storage or blocks of auxiliary storage. When the file is to be read or written to or from auxiliary storage, a multiple block command is used to read or write the entire file. Fig. 1a shows an 8-block read from auxiliary storage (DASD) to main storage. The data blocks are contiguous on the DASD but need not be contiguous in main storage.

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Hardware Skip-On-Write

In a computer system it is often necessary to transfer data from main storage to auxiliary storage (Direct Access Storage Device (DASD)), and vice versa. Blocks of data are paged into main storage from auxiliary storage when needed by the operating system. When the main storage pages are no longer needed, they are overwritten or, if altered, they are paged back out to auxiliary storage. A data file consists of one or more pages of main storage or blocks of auxiliary storage. When the file is to be read or written to or from auxiliary storage, a multiple block command is used to read or write the entire file. Fig. 1a shows an 8-block read from auxiliary storage (DASD) to main storage. The data blocks are contiguous on the DASD but need not be contiguous in main storage. DASD Blocks 1 through 8 are paged into main storage locations A through H, all in one command. The blocks of data remain in main storage according to a least- recently-used algorithm. Pages modified are pinned or held in memory until they are written back out to DASD. Unpinned pages may be overwritten. Some portions of the data file may not be used and may therefore be paged out to make room for other data. Fig. 1b shows main storage pages A, D, E, and G being written back out to auxiliary storage. Pages B, C, F, and H are no longer in main storage, having been overwritten due to their lack of usage. Note that D and E can be written out using one command. However, a gap exists between Pages A and D, E and G. Therefore, to write Pages A, D, E, and G to DASD requires three commands. Thus, the operating system needed one command to read the data file but needed three commands to write out updated pages of that file. For the writes, more than one command is needed because the data contained gaps and could not be written contiguously to the disk. In this instance the commands would be as follows: 1. Read DASD Blocks 1-8 into Memory Locations A-H. 2. Write Memory Location A to DASD Block 1. 3. Write Memory Locations D and E to DASD Blocks 4 and 5. 4. Write Memory Location...