VM (operating system)

Design

• CMS. Most virtual machines run CMS, a lightweight, single-user operating system. Its interactive environment is comparable to that of a single-user PC, including a file system, programming services, device access, and command-line processing.
• GCS, which provides a limited simulation of the MVS API. IBM originally provided GCS in order to run VTAM without a service OS/VS1 virtual machine and VTAM Communications Network Application. RSCS V2 also ran under GCS.
• A mainstream operating system. IBM's mainstream operating systems can be loaded and run without modification. The VM hypervisor treats guest operating systems as application programs with exceptional privileges – it prevents them from directly using privileged instructions, but simulates privileged instructions on their behalf. Most mainframe operating systems terminate a normal application which tries to usurp the operating system's privileges. The VM hypervisor can simulate several types of console terminals for the guest operating system, such as the hardcopy line-mode 3215, the graphical 3270 family, and the integrated console on newer System/390 and IBM Z machines. Other users can then access running virtual machines using the DIAL command at the logon screen, which will connect their terminal to the first available emulated 3270 device, or the first available 2703 device if the user is DIALing from a typewriter terminal.
• Another copy of VM. A second level instance of VM can be fully virtualized inside a virtual machine. This is how VM development and testing is done. This technique was used to develop S/370 software before S/370 hardware was available, and it has continued to play a role in new hardware development at IBM. The literature cites practical examples of virtualization five levels deep. Levels of VM below the top are also treated as applications but with exceptional privileges.
• A copy of the mainframe version of AIX or Linux. In the mainframe environment, these operating systems often run under VM, and are handled like other guest operating systems. There was also the short-lived IX/370, as well as S/370 and S/390 versions of AIX.
• A specialized VM subsystem. Several non-CMS systems run within VM-CP virtual machines, providing services to CMS users such as spooling, interprocess communications, specialized device support, and networking. They operate behind the scenes, extending the services available to CMS without adding to the VM-CP control program. By running in separate virtual machines, they receive the same security and reliability protections as other VM users. Examples include:
• * RSCS – communication and information transfer facilities between virtual machines and other systems
• * RACF — a security system
• * Shared File System, which organizes shared files in a directory tree
• * VTAM – a facility that provides support for a Systems Network Architecture network
• * PVM – a facility that provides remote access to other VM systems
• * TCPIP, SMTP, FTPSERVE, PORTMAP, VMNFS – a set of service machines that provide TCP/IP networking to VM/CMS
• * Db2 Server for VM – a SQL database system, the servers are often named similarly to "SQLMACH" and "SQLMSTR"
• * DIRMAINT – A simplified user directory management system.
• * MUMPS/VM — an implementation of the MUMPS database and programming language which could run as guest on VM/370. MUMPS/VM was introduced in 1987 and discontinued in 1991.
• A user-written or modified operating system, such as National CSS's CSS or Boston University's VPS/VM.

  Versions

Hypervisor interface

• Provides a fast path to VM-CP, to avoid the overhead of full simulation.
• Was first developed as a performance improvement for CP/CMS release 2.1, an important early milestone in CP's efficiency.
• Uses a non-virtualized, model-dependent machine instruction as a signal between CMS and CP: DIAG.