Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 (Tek) 9/28/84 based on 9/17/84; site orca.UUCP Path: utzoo!linus!decvax!tektronix!orca!andrew From: andrew@orca.UUCP (Andrew Klossner) Newsgroups: net.unix-wizards Subject: Re: VM/370 Security (and perform Message-ID: <1277@orca.UUCP> Date: Mon, 31-Dec-84 02:40:26 EST Article-I.D.: orca.1277 Posted: Mon Dec 31 02:40:26 1984 Date-Received: Tue, 1-Jan-85 05:59:10 EST References: <986@opus.UUCP> <13500029@ea.UUCP> <258@mhuxi.UUCP> Organization: Tektronix, Wilsonville OR Lines: 53 Not that many Unix inhabitants care, but here's two more cents worth about VM/370 before the end-of-Xmas news rush ... "(Virtual Machine). Under VM you can run any operating system you want, as long as you have the 370 code for it. CMS is one option. CP (Control Program) is also part of the package ... IBM developed VM, CP and enhanced CMS. VM is a system control program. It manages the resources of an IBM 370 type computer in such a way that multiple users have a functional simulation of a computing system (a virtual machine) at their disposal. Note that VM can run many different OSs concurrently. CP executes in a real machine controlling the resources of that machine. CP is used to create concurrent virtual machines. CMS is a single user OS that can execute in one or more virtual machines. CMS is considered to be a timesharing OS ... Hope that clarifies the discussion." Some corrections. VM/370 is the name of a system software package which consists primarily of two components: CP and CMS. CP is the kernel running on the real machine (unless it's running under itself ...). It is the part of VM/370 that presents several virtual machine environments. It is responsible for fielding traps and interrupts, doing resource allocation (like scheduling), and other "kernel" jobs. There is some user interface in CP, but not much ... it's mostly of the "simulate a front panel" variety, like the command to boot an operating system in your virtual machine. CMS is a single-user IBM 370 operating system. When it runs stand-alone on a 370, it supports a single user. Several instances of it can be running under CP, each in a distinct virtual machine. Since CMS doesn't have to support more than one user, it needn't be concerned with issues of resource fairness or protection. Thus, there is no distinct piece of the package called "VM"; the name refers to the entire system. Often users of such a system will use the terms "VM" and "CP" interchangeably. And the beauty of CMS is that it is *not* a timesharing OS. Incidentally, modern versions of CMS will not run stand-alone on a real computer. For reasons of efficiency, CP allows programs running in virtual machines to use an additional means of requesting services, implemented with the "diagnose" instruction (which on a real machine causes model-specific results for hardware maintenance, and is useless to production software). For example, one variant of "diagnose" means "read a record from the tape drive, block this virtual machine until complete." This is a good deal more efficient than going through the standard hardware sequence of events: start I/O; wait for an interrupt; handle the interrupt; test the I/O state; etc. CMS does all its standard I/O through diagnose, and so can no longer execute on a bare machine. -- Andrew Klossner (decvax!tektronix!orca!andrew) [UUCP] (orca!andrew.tektronix@csnet-relay) [ARPA]