International Business Machines Corporation

Charles Henry Gaudette

Raymond C Williams

The embodiment provides a bit position in a system control register (CR) which is set to enable, and is reset to disable, the HCA protect means. An AND gate has an input which receives the enablement or disablement signal from the control register HCA protect bit position. The AND gate also receives the store requests from the processor, and further receives a signal common to all addresses in the HCA area such as all zeros for address bits 8 through 22 to signal the first 512 bytes in main storage which in current IBM S/370 machines contains the HCA, which is also called the program save area (PSA). The AND gate is normally disabled by the CR bit and provides an inverted output to a store request gate which normally provides store requests to the processor store access controls. Writing into the HCA is only permitted under special programmed conditions when the control register bit is set to disable the HCA protect means. Whenever the HCA protection is enabled, the AND gate is enabled to provide an HCA store violation signal, which actuates a protection interrupt code generator to generate a storage protect interrupt code in the PSW (Program Status Word) that causes the processor to test each store request into the HCA. The CR protect bit is disabled before and enabled after each legitimate HCA change code in a system control program to form a window in the code.

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Yorktown Heights, NY, United States of America

Charles Henry Gaudette