Browse Prior Art Database

Dual Key Program Status Word

IP.com Disclosure Number: IPCOM000081495D
Original Publication Date: 1974-Jun-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 13K

Publishing Venue

IBM

Related People

Gum, PH: AUTHOR

Abstract

The IBM S/360/370 protection mechanism serves a number of purposes, including data security and privacy, prohibiting dynamic modification of (reentrant) system programs, and preventing problem programs from interfering with each other or damaging the control program. Restricting a program's right to store improves the availability of the system, since it means program errors cannot destroy data not ordinarily available to the program.

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Dual Key Program Status Word

The IBM S/360/370 protection mechanism serves a number of purposes, including data security and privacy, prohibiting dynamic modification of (reentrant) system programs, and preventing problem programs from interfering with each other or damaging the control program. Restricting a program's right to store improves the availability of the system, since it means program errors cannot destroy data not ordinarily available to the program.

Described is an enhancement of the protection mechanism primarily providing improved availability. The principle feature is the provision of two protection keys in the Program Status Word (PSW). The hardware would apply all checking rules to both keys exactly as it presently does for one key, with the additional rule that an exception is not recognized if checking succeeds with respect to either one of the keys for each individual storage reference. If both keys are the same, the results are the same as for the present single key PSW. If one of them is zero, checking with respect to the other is unnecessary. The use of a single key to control access to 2K increments of storage is unchanged.

The usefulness of the feature is best illustrated within the context of the design of the VS2 Release 2 operating system (virtual store). Recall that there are at least three distinct levels of trustworthiness in this system. The first level is comprised of supervisor functions executed with a protection key of zero, and privileged. The second level is comprised of the functions of several subsystems, often executing in the privileged state but generally with a nonzero protection key. The third level consists...