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Instruction for Restricting Memory in a Shared Memory Multicomputing System

IP.com Disclosure Number: IPCOM000094431D
Original Publication Date: 1965-Jan-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 4 page(s) / 115K

Publishing Venue

IBM

Related People

Robelen, RJ: AUTHOR [+2]

Abstract

When two or more independently operating computers share a common section or unit of storage, procedural safeguards are required. These insure that the computers do not gain simultaneous or conflicting access to the shared section so that one computer obliterates information needed by another computer. But even with what might be considered reasonable safeguards, mishaps can occur if plural requests for access to the same section of storage can be concurrently pending. For example, if information indicative of the status of utilization of the shared section cannot be retrieved and conditionally modified by each computer during an interval in which the same information is not acceptable to other computers, it would always be possible for two computers to retrieve and modify the same status information.

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Instruction for Restricting Memory in a Shared Memory Multicomputing System

When two or more independently operating computers share a common section or unit of storage, procedural safeguards are required. These insure that the computers do not gain simultaneous or conflicting access to the shared section so that one computer obliterates information needed by another computer. But even with what might be considered reasonable safeguards, mishaps can occur if plural requests for access to the same section of storage can be concurrently pending. For example, if information indicative of the status of utilization of the shared section cannot be retrieved and conditionally modified by each computer during an interval in which the same information is not acceptable to other computers, it would always be possible for two computers to retrieve and modify the same status information. They thus condition their subsequent references to the shared section on the same status information. Thus, it would be possible for two computers to make time-interleaved references to the shared section under conditions calling for exclusive use of that section by each computer.

Such conflict can be avoided by special Lock and Unlock program procedures. If a computer requires access to a particular section (SS) of shared storage, a Lock Instruction (LI) is placed at an appropriate point in that computer' s program. As part of the procedure for executing LI, the computer must test and conditionally modify a Lock Control Word (LCW) associated with SS. This is in a section of storage accessible to all of the computers. Although the stored LCW cannot be retrieved and conditionally modified in one uninterrupted pass at storage, and can, therefore, be simultaneously held and tested by more than one computer before it is modified, the procedure for executing LI is such that it is virtually impossible for two computers to make corresponding modifications in LCW so that both would then make non-exclusive references to SS.

Essentially, the LI procedure and LCW information are so organized that each computer can unconditionally enter a test indication. This occurs in a single pass at the stored LCW, whereby plural test indications may be simultaneously present in one LCW, but thereafter each computer can only conditionally enter a lock indication in LCW to signal exclusive reservation of SS to the other computers. The conditions for entering the lock indication are such that it cannot be entered if another lock indication or test indication is present in LCW. Thus, when plural test indications are simultaneously pending, the computer s involved are forced to execute a conflict resolving procedure. By this, all but one of the test indications are removed, before one of the computers can set its associated lock indication.

These procedures are exemplified in the drawing. The elemental procedural steps are listed in individual boxes to the right of vertical line 1. The f...