Higher Level Subroutines Invoked on Behalf of Horizontal Microcode
Original Publication Date: 1987-Dec-01
Included in the Prior Art Database: 2005-Feb-02
By use of a programmable register bit, Internal Microprogram Interface (IMPI) subroutines are invoked on behalf of Horizonal Microcode (HMC) in a manner transparent to Vertical Microcode (VMC). In order to be transparent to VMC, no exceptions or task switching can be allowed during the execution of an IMPI subroutine for HMC. Exceptions which occur during an IMPI instruction indicate conditions which, in general, cannot be ignored. These include such things as page faults, invalid decimal data, and effective address overflow. Some of these can be avoided by doing checks in HMC prior to invoking an IMPI subroutine. Some of them, such as page faults, can be handled when they occur either by passing the exception back to VMC or by invoking a machine check.