Supervisor Call instruction
In supervisor state, system programs are additionally able to use a small set of privileged instructions which are generally intended for supervisory functions.In OS/VS1, OS/VS2 (SVS), MVS/370 and subsequent versions of the OS, the MODESET function (SVC 107) obviated the need for many user-written SVCs as this system SVC accommodated both changes in mode (problem state to supervisor state) and key (8-15 [ user ] to 0-7 [ system ] ) in a single operation, and many user-written SVCs were originally intended for simple mode and key changes, anyway, and subsequently the only special requirement was that the jobstep be APF authorized[b][c] and that the MODESET-invoking program be resident in 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 OS/360 and later incarnations of the OS, branch and link entry points are alternatives to SVC invocations for some supervisor mode routines.The following information, part of a table for OS/360, augmented for MVS/370 and successor systems, gives an idea of the considerations involved in writing an SVC routine.The size restrictions on types 3 and 4 SVC routines are necessary because they are loaded into designated "transient areas" (PLPA in post-MVT) when invoked.Consequently, there were quite a number of unintentional system- and data-integrity exposures which were possible by employing certain sequences of SVCs and other instructions.Beginning with MVS/370, IBM considered it a product defect if a system design error would allow an application program to enter supervisor state without authorization.