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Browse Prior Art Database

Compatible Virtual Storage Device

IP.com Disclosure Number: IPCOM000082506D
Original Publication Date: 1974-Dec-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 15K

Publishing Venue

IBM

Related People

Stamper, DA: AUTHOR

Abstract

The images of data stored on direct access storage devices (DASD) can be mapped into the virtual pages of a virtual storage system. A DASD volume can be considered an address space. Data stored in the DASD volume space is addressed by a specific cylinder number, head number and record number. Each record begins with an identifier field which contains the sequential number of the record within the track (head) and the length of the data portion of the record. The data portion of the record follows the record identifier.

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Compatible Virtual Storage Device

The images of data stored on direct access storage devices (DASD) can be mapped into the virtual pages of a virtual storage system. A DASD volume can be considered an address space. Data stored in the DASD volume space is addressed by a specific cylinder number, head number and record number. Each record begins with an identifier field which contains the sequential number of the record within the track (head) and the length of the data portion of the record. The data portion of the record follows the record identifier.

Each track of the DASD is mapped into one or more pages of the virtual storage. For example, a DASD whose tracks have a capacity of 13,000 bytes would require four 4,000 byte pages for each track image. Within the 4,000 byte pages, the record identifiers and their data are recorded sequentially. The gaps appearing on the DASD tracks between the end of one record and the beginning of another record are not recorded.

A DASD volume which has 404 cylinders, 19 tracks per cylinder and 13,000 bytes per track would require 28,704 4,000-byte pages to contain all of its data. 404 cyl X 19 tracks X 4 = 28,704

By creating a translation table of cylinder and track addresses to page addresses (in groups of four pages), a program can address specific records as if they were on a DASD. The system then translates the cylinder and head address to a virtual memory address, and scans the group of our pages sequentially to find the specific record.

To maintain full compability to existing programs compiled to run on nonvirtual machines with real DASD, the system interprets real DASD channel programs to incorporate the full logic of the DASD to locate and read or write data into the virtual space.

A full volume of data on the above-described device would require a virtual address space well over 100 million bytes. Since the addressing capability of a virtual memory is much less than that (16 million with 24 bit addressing), a technique is required to "fold" the tracks into a smaller range of virtual space.

This range of virtual memory space is called a "window". The window would be large enough to contain one or more contiguous tracks of the DASD volume at one time. It could be made large enough to contain the entire data set, provided the data set was less than 16 million bytes. In OS/VS2, the window size was fixed at one track length. If the DASD volume being simulated has 13,000 byte tracks, a four page (16K) window is provided while a 7,000 byte track requires a two page window. While one track is addressable (in the window), the remainder of the DASD tracks (with content), exist in pages on auxiliary storage, unaddressable from virtual memory.

As a channel program is being interpreted, it may be necessary to move from one track to the next. At this time, the current track is "moved out" of the window. If the change bits are on for any of the four pages, they are updated on external storage....