on the z/System) contain the Supervisor call number. SVC invokes a supervisory function—usually implemented as a "closed subroutine" of the system's SVC interrupt handler. Information passed to and from the SVC routines is passed in general purpose registers or in memory. Under OS/360 and successors, return from an SVC routine is, for type 2, 3 and 4 SVC routines, via an SVC 3 (EXIT) invocation, and for other SVC types by the privileged Load PSW (LPSW) instruction, and which is executed on behalf of the SVC routine by the control program's dispatcher or SVC interrupt handler. On non-IBM developed operating systems such as MUSIC/SP developed by McGill University in Montreal, Canada for IBM mainframes, and for non-IBM mainframes, VS/9, developed by Univac (from the TSOS operating system for the RCA Spectra 70 series computers) for the UNIVAC Series 90 mainframe line, and the B800 operating system (also developed from the TSOS operating system) for Fujitsu's mainframes, all use the LPSW instruction to exit from a Supervisor Call. The choice on whether to have a supervisor call return to the calling program directly through an LPSW instruction or through some other means such as a subroutine return instruction or a supervisor call itself, is a matter of design. There is no obvious "right" way to do this; there can be reasons for both methods. Using an LPSW instruction to exit an SVC routine allows for faster execution, but means actual testing of the routine has to be done on a dedicated machine running the code as part of an actual operating system supervisor. If the code was written as an ordinary subroutine it can be tested in the same manner as any ordinary program and potentially deployed without having to modify it. It also would allow metrics to be measured, as to how long a supervisor call routine took to complete its task, allowing for analysis of routines that are excessively long in execution time (or, ones that are very fast). In OS/360 and later incarnations of the OS, branch and link entry points are alternatives to SVC invocations for some supervisor mode routines. In MVS/SP V1R3 and later incarnations of the OS, Program Call (PC) entries have augmented SVCs for invocations of many supervisory functions by both authorized and unauthorized programs; and some functions may only be invoked by branch or PC entries, e.g. STARTIO. (This also has the advantage