What GCS Is

The Group Control System (GCS) is:
  • A component of z/VM. It consists of a named, shared segment in storage that you can IPL and run in a virtual machine.
  • A virtual machine supervisor. It bands many virtual machines together in a group and supervises their operations. See Figure 2.
  • An interface between applications. Some of the applications are:
    • Virtual Telecommunications Access Method (VTAM®®)
    • Remote Spooling Communications Subsystem (RSCS)
    • NetView®®
    • z/VM®'s Control Program (CP), Figure 1.
GCS provides multitasking services that allow numerous tasks to remain active in the virtual machine at one time.
Figure 1. Group Control System, an Interface between Applications and the Control Program
Group Control System, an Interface between Applications and the Control Program

The specific function of GCS for z/VM is to support a native VM/SNA network — a network that functions as part of your z/VM system without help from a second operating system. This System Network Architecture (SNA) network relies on ACF/VTAM, VTAM SNA Console Support (VSCS), and other network applications to manage its collection of links between terminals, controllers, and processors. In turn, ACF/VTAM, VSCS, and the others rely on GCS to provide services for them. This arrangement eliminates your need for VM/VCNA (VTAM Communications Network Application) and a second operating system like VS1 or VSE.

GCS runs in an XA machine and is installed with z/VM. This allows all virtual machines in a GCS group to run in XA mode or XC mode.
  • XA mode entails running with the full capabilities of the Extended System Architecture. Either 24-bit or 31-bit addressing can be used (thus allowing addresses below and above 16MB), as well as the more efficient XA I/O using the Channel Subsystem.
  • ESA/XC architecture is a virtual machine architecture in which DAT-off programs can create and access additional address spaces called data spaces. These additional address spaces can also be shared with programs running in other virtual machines. GCS applications must not be in Access Register (AR) mode when using GCS supervisor services, whether using a branch or an SVC interface, or returning from a user exit. GCS will abend any application that attempts to use GCS supervisor services while in AR mode.
  • For migration purposes GCS supports mixed mode groups which include XC and XA mode.
Figure 2. A Virtual Machine Group and Supported Applications
A Virtual Machine Group and Supported Applications