Microcode Overlay Protection
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
A mechanism protects Overlaid Horizontal Microcode against unauthorized modification. A portion of the Horizontal Microcode (HMC) on the processor resides in a hardware array, called Control Store. This array is loaded at IPL time with control words. These words, when executed by the hardware, support the processor by interpreting instructions. The remainder of the HMC resides in main storage in an area called the HMC Overlay Area. When these control words are to be executed, they are first brought into a reserved section of the control store. Every doubleword in main storage has associated with it a "tag" which may either be in an "on" or "off" state. This tag is currently used in the protection of S/38 Machine Interface (MI) objects, called Space Pointers. These objects use tags on a quadword (16-byte) basis.