(the directory is a listing of every account on the system, including virtual hardware configuration, user passwords, and minidisks). 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. Hypervisor interface IBM coined the term hypervisor for the 360/65 and later used it for the DIAG handler of CP-67. The Diagnose instruction ('83'x—no mnemonic) is a privileged instruction originally intended by IBM to perform "built-in diagnostic functions, or other model-dependent functions." IBM repurposed DIAG for "communication between a virtual machine and CP." The instruction contains two four-bit register numbers, called Rx and Ry, which can "contain operand storage addresses or return codes passed to the DIAGNOSE interface," and a two-byte code "that CP uses to determine what DIAGNOSE function to perform." A few of the available diagnose functions are listed below. CMS use of DIAGNOSE At one time, CMS was capable of running on a bare machine, as a true operating system (though such a configuration would be unusual). It now runs only as a guest OS under VM. This is because CMS relies on a hypervisor interface to VM-CP, to perform file system operations and request other VM services. This paravirtualization 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 (diagnose). Minidisks CMS and other operating systems often have DASD requirements much smaller than the sizes of actual volumes. For this reason CP allows an installation to define virtual disks of any size up to the capacity of the device. For CKD volumes, a minidisk must be defined in full cylinders. A minidisk has the same attributes as the underlying real disk, except that it is usually smaller and the beginning of each minidisk is mapped to cylinder or block 0. The minidisk may be accessed using the same channel programs as the real disk. A minidisk that has been initialized with a CMS file system is referred to as a CMS minidisk, although CMS is not the only system that can use them. It is common practice to define