METHOD FOR SWITCHING THE CONTROL OF CENTRAL PROCESSING UNITS IN A DATA PROCESSING SYSTEM, AND APPARATUS FOR INITIATING THE SWITCHING OF CPU CONTROL

A method is disclosed which improves, in a uniprocessor or multiprocessor system, the switching of CPU control between different system control programs SCP. An interface area PSA (42(0), 43(0)...42(n), 43(n)) for storing old and new program status words is provided in common main storage separately for each SCP and CPU. Switching from one SCP to another for any CPU is effected by changing the contents of a prefix register (21) in the respective CPU. This switching is not controllable by user instructions executed on the CPU. The method enables a preferred guest SCP (such as MVS/370) to operate in a system having a particular host SCP (such as VM/370) with an efficiency that is almost the same as if the preferred guest were operating as a standalone SCP on its own dedicated system. Also disclosed is apparatus for initiating the switching of CPU control between different SCPs.