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Store Pad (Clear Line) Service Mechanism

IP.com Disclosure Number: IPCOM000045671D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 44K

Publishing Venue

IBM

Related People

Krygowski, MA: AUTHOR

Abstract

Certain processor operations require that main storage (MS) be initialized totally or in dIscrete parts. For example a program moving a short character string into a large MS area has need to pad the remainder of the MS area with a padding character. When each central processor (CP) in a multiprocessor (MP) configuration has a store in cache, it need not hold the padded part of the MS area in its local cache, since to do so would be time consuming. Since old copies of that MS area may reside In cache locations of other CPUs, directory searches must be made to insure that any old copies in various cache locations are invalidated. The pad character must be stored in such a way that there is no chance of old modified data being able to overwrite the new padded MS area.

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Store Pad (Clear Line) Service Mechanism

Certain processor operations require that main storage (MS) be initialized totally or in dIscrete parts. For example a program moving a short character string into a large MS area has need to pad the remainder of the MS area with a padding character. When each central processor (CP) in a multiprocessor (MP) configuration has a store in cache, it need not hold the padded part of the MS area in its local cache, since to do so would be time consuming. Since old copies of that MS area may reside In cache locations of other CPUs, directory searches must be made to insure that any old copies in various cache locations are invalidated. The pad character must be stored in such a way that there is no chance of old modified data being able to overwrite the new padded MS area. This article addresses the problem and can enable improved execution.

A store pad command is issued by the requesting processor to a storage controller (SC) in the MP to signal the intent to do a pad character store operation in MS. The command Is sent for each storage unit of data (e.g., line of 128 bytes) represented by each cache directory entry. Also, the store pad requesting processor searches its local directory and automatically invalidates any hit location. The store pad command execution is done by the SC In two phases: 1. the cross-interrogation (XI) phase, and 2. the data store phase.

In the XI phase, copy directories for all CPs residing in the SC(s) in the MP are searched. If the search result finds a copy directory entry having an equal address (indicating a XI hit) , the copy directory entry is invalidated. For each copy directory where there is a XI hit (other than the requestor's), the copy directory control communicates with the associated processor directory to invalidate the cache entry. The requestor is signalled to start the second phase concurrently if the XI hit status does not reflect an exclusivity condition.

If there is no XI hit, the operation terminates the XI phase and signals the requestor to start the second phase.

If a XI hit indicates an exclusivity condition, a wait occurs before starting the second phase in order to guarantee that there is no window condition in which the obsoleted changed version in a remote cache can overwrite the store pad data.

The second phase begins with the SC notifyi...