Browse Prior Art Database

Reference and Change Bit Recording

IP.com Disclosure Number: IPCOM000052249D
Original Publication Date: 1981-May-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 4 page(s) / 64K

Publishing Venue

IBM

Related People

Hoffman, RL: AUTHOR [+3]

Abstract

In a computer system with virtual storage, any reference to data or instructions which do not reside in main storage causes a fixed size block of data or instructions, i.e., a page, to be moved from secondary storage into main storage. In order to accommodate the new page in main storage, it may be necessary to move some other page from main storage to the secondary storage. To aid the page-replacement process, it is desirable to keep track of those pages which have been referenced. A single reference bit for each page is used for this purpose and is set whenever an address in the page is referenced. When a page has to be replaced, the page-replacement mechanism selects a page with its reference bit not set.

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Reference and Change Bit Recording

In a computer system with virtual storage, any reference to data or instructions which do not reside in main storage causes a fixed size block of data or instructions, i.e., a page, to be moved from secondary storage into main storage. In order to accommodate the new page in main storage, it may be necessary to move some other page from main storage to the secondary storage. To aid the page-replacement process, it is desirable to keep track of those pages which have been referenced. A single reference bit for each page is used for this purpose and is set whenever an address in the page is referenced. When a page has to be replaced, the page-replacement mechanism selects a page with its reference bit not set. The page replacement mechanism then resets all page reference bits and only tests the reference bits when another page needs to be replaced.

Because it is only necessary to move pages in main storage which have been changed to secondary storage when a page is to be replaced, pages in main storage which have been changed are noted through the facility of a change bit for each page which is set whenever any byte within the page is changed.

In order to avoid severe performance degradation that would occur if the reference and change bits were kept in main storage, most systems use a separate storage array for the reference and change bits to allow these bits to be set in parallel with a main storage operation. The separate storage array and all of the necessary controls, of course, result in an added cost. The present arrangement enables reference and change bit recording to take place without an added storage array and without significant performance degradation.

The virtual address translation apparatus described in IBM Technical Disclosure Bulletin 22, 743-749 (July 1979) utilizes a lookaside buffer (LB) for storing resolved or translated main storage addresses. A primary directory (PD) maintained in main storage includes one entry for each page frame in main storage. Some of the information contained in the primary directory is also contained in the lookaside buffer. A primary directory entry is illustrated in Fig. 1, and It contains the reference and change bits 41 and 42, respectively. Lookaside buffer entries contain the change bit but not the reference bit. The reference bits are maintained only in the primary directory and are updated only when a lookaside buffer entry is replaced or when executing a read reference and change reset reference instruction (RRCRR). The change bits are in the primary directory and lookaside buffer, but are maintained only in the lookaside buffer. The change bits in the primary directory entries are updated only when a lookaside buffer entry is replaced, if changed, or when executing a read reference and change reset reference instruction. This allows reference and change bit recordings to take place without additional storage arrays and without significant...