The key to VM/ESA is the Control Program (CP). CP is primarily a real machine resource manager. It does not support any job scheduling for virtual machines and offers limited data management support. The only task management functions CP supports are those that are required to allocate real processors to concurrently operating virtual machines.
CP performs the following major functions:
- Allocates real machine resources to concurrently operating virtual machines
- Simulates the System/370, ESA/370, and ESA/390 architectures for virtual machines
- Handles certain real machine hardware interrupts and all intercepts
- Schedules and initiates certain real I/O operations
- Manages real storage, Expanded Storage, and auxiliary storage to support virtual storage for each virtual machine
- Handles real machine hardware errors
- Defines or redefines some system characteristics dynamically.
VM/ESA is a virtual storage operating system. The CP component of VM/ESA (the CP nucleus) consists of a nonpageable portion, which is resident in real storage during real machine operation, and a pageable portion, which is made up of the least frequently used routines that are paged in as required.
The CP nucleus is contained on a CP system residence direct access volume, which must be online during the entire time VM/ESA is operating. An initial program load (IPL) procedure must be performed to load the resident portion of the CP nucleus into real storage of the real machine before any user establishes a connection with CP (that is, logs on the virtual machine to be used). Before an IPL, perform a system generation to define the system and its operating characteristics. If you use a generic CP nucleus, you can use a system configuration file to define many of the system's characteristics, and let CP issue sense ID requests to determine the rest dynamically. The real machine operator controls real machine operations using a set of CP commands.
Usually, the Control Programs and problem programs that run in virtual machines under CP control operate in page mode in the real machine. CP implements demand paging, which allocates real storage to the virtual machines' main storage. Thus, auxiliary storage must be made available to CP to contain the pages of these programs.
Because the virtual storage addresses of the virtual machines that operate in page mode are not the same as the real storage addresses of the page frames to which their virtual storage pages are allocated, they are called virtual=virtual (V=V) machines.
CP provides a local spooling facility (transcription of unit record data to and from direct access devices). Remote spooling facilities are provided by a licensed program that is ordered separately, Remote Spooling Communications Subsystem (RSCS), Program Number 5664-364. These spooling facilities require that direct access space be made available to CP for spooled data.
----from "VM/ESA Introduction and Feature Summary"