The IBM VM CS450/550 Section 2 Stephen Kam

IBM VM - Origins Originally an experimental OS called “CP-67” Designed to run on the IBM System/360 Model 67 mainframe Intended for environments with multiple users Later called the VM/370 – first commercial release IBM System/360 Model 67

Concept of Virtual machines Can refer to several distinct meanings The original meaning of a “virtual machine” referred to the creation of multiple execution environments on a single computer Each virtual machine emulates the “real” machine exactly Gave users the illusion of having their own private computer complete with I/O and memory

Benefits of VM Allowed users to run many different operating systems simultaneously Ability to test new versions of operating systems while still running older version (debugging purposes) Switch to a new operating system without any system down-time (mission critical systems) Provided each user with their own “virtual” computer at a time when PCs were nonexistent and mainframes were expensive

CP and CMS VM/370 has two main components: the Control Program (CP) and the Conversational Monitor System (CMS) CP manages the resources of the real machine to create virtual machines (i.e access to CPU, I/O, and memory) – the “brains” of the VM CMS is a single-user OS designed to run on top of the CP – provides each user with an interactive personal computing environment

A VM/370 Environment Control Program (CP) CMS MVSSVSCMS

Processor Modes Control Program has a problem (user) mode and a privileged (supervisor) mode CP handles all instructions executed by a virtual machine in problem mode – always results in a system call This protects the CP and all the other virtual machines running from any malicious code Isolated the virtual machines from one another by securing their internal data and state of execution

Multiprocessing VM supports symmetric multiprocessing (SMP) Multiple processors allow many virtual machines to be in execution concurrently (one on each processor) But only one processor at a time can execute supervisor mode code To ensure this, the system uses a spinlock – only one processor can be locked at a time, while the others loop (spin) through attempts to acquire the lock

Virtual Memory Each virtual machine has its own simulated virtual memory Virtual Memory is divided into segments 4KB in size, and pages that are 4KB in size The control program manages paging on a demand basis (demand paging) Only the most recently referred to pages of virtual memory are kept in the physical main memory by using a Least Recently Used (LRU) page replacement algorithm

Virtual Memory Address Translation

Conclusion VM equally divides the resources and processing power of a mainframe among multiple users Provides advantages of running several distinct operating systems simultaneously on different virtual machines Still runs at hundreds of sites today, more than 3 decades later

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