How CMS Performs OS/MVS Simulation
When CMS OS Simulation is used, VM does not invoke MVS to handle the function. Instead, it calls a series of programs within the CMS operating system to simulate the function of the OS/MVS system. This simulation is not exact because OS Simulation calls CMS native functions to provide the actual services.
CMS tries to functionally simulate OS/MVS macro and SVC function interfaces in a way which provides results to OS programs executing under CMS equivalent to the results obtained running on an MVS system. MVS macros and SVC functions expand to call CMS OS Simulation routines which do setup, provide control block and device interface information, and/or invoke CMS native support routines to perform the required functions. At the same time, OS Simulation maintains pseudo-control blocks for the OS/MVS products so that they can query and manipulate data as if they were running in an OS/MVS type environment.
Programs using simulated OS/MVS macros run in an environment defined by CMS commands. OS Simulation runs on top of the CMS virtual machine configuration. The accessed disks and directories and the files residing on them comprise the files which OS Simulation will use. The FILEDEF and GLOBAL commands define the files that the particular program will use and associate them with particular DCBs the same way the data definition (DD) statements in job control language (JCL) define the datasets that the program would use if it ran in the MVS batch environment.