Call instruction, return instruction and trap procedure for ring crossing architecture

Claim 1

In a data processing system having a ring architecture comprised of a plurality of protection rings, each of said protection rings determining a level of privilege for execution of procedures (including programs) related to each of said protection rings, each procedure having a ring number associated therewith identifying a level of protection, wherein procedures can execute at a level of privilege of said procedure ring number or a lower privilege level, each procedure having attributes associated therewith defining execution restrictions thereof, a method for causing a first procedure executing at a first level of privilege to utilize a second procedure which is executed at a higher level of privilege, said method comprising steps of: comparing attributes of said first and said second procedure to determine when a predetermined relationship exists between said first and said second procedure attributes by means of a test procedure executed by said data processing system under hardware control without intervention of a software procedure; providing a stack means associated with each level of privilege for storing information related to procedures being executed at an equivalent level of privilege, wherein a stack means associated with a highest level of privilege is implemented by dedicated memory locations in a main memory unit of said data processing unit; when said predetermined relationship exits between said first and said second procedure attributes, entering control information in a second stack means associated with said second procedure under data processing system hardware control; when, required by said second procedure, entering data information in said second stack means under data processing system software control; entering address information identifying a location of control information in a first stack means associated with said first procedure; executing said second procedure, said second procedure using control information from said second stack means; and, after completion of said second procedure, returning to execution of said first procedure by means of a procedure activated in response to said second procedure completion and executed under data processing system hardware control.