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Browse Prior Art Database

Jump and Check Instruction

IP.com Disclosure Number: IPCOM000072879D
Original Publication Date: 1970-Oct-01
Included in the Prior Art Database: 2005-Feb-22
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Randell, B: AUTHOR

Abstract

The described instruction has particular utility in a demand paging system or other dynamic storage allocation system in which information is fetched to working storage as a result of an interrupt caused by an attempt by an instruction to access information not currently in working storage.

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This is the abbreviated version, containing approximately 90% of the total text.

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Jump and Check Instruction

The described instruction has particular utility in a demand paging system or other dynamic storage allocation system in which information is fetched to working storage as a result of an interrupt caused by an attempt by an instruction to access information not currently in working storage.

One of the tasks involved in designing an operating system is that of minimizing the chances, or better yet making it impossible, that the system gets itself into a situation where further progress is logically impossible. One of the best known problems in this area is that of the "mutual wait" when two tasks each get into the position of each waiting for the other.

With demand paging systems, the problem is that of guaranteeing that each task, even though subject to delays caused by page exceptions, does continue to make progress, and cannot be held up indefinitely.

To avoid the above problems, an additional instruction provides one extra flag, accessible to two new privileged instructions: Jump and Check Test Flag.

The Jump and Check instruction is used as the last instruction obeyed by the supervisor before a task is resumed after a page fault. Its actions are to set the flag to a zero and to cause the instruction sequencing mechanism to treat the immediately following instruction specially by setting the flag to one if the instruction fails to complete. (The flag is not affected by the completion or noncompletion of any succeeding instructions.)...