a concatenation of libraries all of which were identified as authorized, and this secure approach was completely under the installation's control. This approach generally simplified user controls over authorization, although some simple changes to the application were thereby required. In general, user installations favored this approach, and the overall reliability of the system was significantly improved thereby. Although mainframe applications are typically synchronous processes, the operating system itself is naturally asynchronous, although the system also supports many processes which are naturally synchronous. When an application requests a system service which is naturally asynchronous, such as input/output processing, a mechanism for synchronizing the application and the operating system must be employed. This essential mechanism is through functions which are built into the operating system, or are specifically supported by it, including: WAIT (temporarily halt application processing until an external event has occurred); POST (indicate occurrence of an external event so application processing may continue); and SYNCH (change system processing mode—supervisor to user and system key to user key—while preserving system integrity, and synchronously perform a function on behalf of the application, after which supervisor processing may continue). The OS/360 SVCs table below indicates the conditions under which these synchronizing facilities may be employed. Implementation SVC is a two byte instruction with the hexadecimal operation code ; the second byte of the instruction, the SVC number, indicates the specific request. The SVC number can be any value from 0 to 255, with the particular SVC number being up to the implementer of the operating system, e.g. on IBM's MVS, SVC 3 is used to terminate a program, while on the UNIVAC VS/9 and Fujitsu BS2000 operating systems, SVC 9 was used for the same purpose. When a program issues an SVC, an interrupt occurs. The PSW, an 8-byte (on the System 360 and S/370) or 16 byte (on the z/System), privileged register containing, among other things, the current address of the instruction to be executed, the privilege bit (1 if privileged), and storage key, is saved at a real address. This is locations 32-39 on the 360 and 370; 320-335 on the z/System. The PSW is then loaded from a different real address ; it is 96-103 on the 360 and 370, 448-463 on the z/system. Execution resumes at the address that was loaded into the PSW. Bits 24-31 of the saved PSW (real address 35 on the 360 and 370, 323