the ibm vm cs450/550 section 2 stephen kam. ibm vm - origins originally an experimental os called...
Post on 13-Jan-2016
217 Views
Preview:
TRANSCRIPT
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 CMS MVS SVS CMS
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
End
top related