An Operating Systems Vade Mecum

Virtual machines 21A second use of virtual machine operating systems is to integrate batch andinteractive modes by letting them occupy different virtual machines. This scheme,shown in Figure 1.10, can be a fast way to piece together two fundamentally differentoperating systems for the same machine.The ability to run several operating systems at once on the same machine has otheradvantages as well.It can alleviate the trauma of new operating system releases, since the old releasemay be used on one of the virtual machines until users have switched over to thenew release, which is running on a different virtual machine under the control ofthe same virtualizing kernel.It can permit students to write real operating systems without interfering with theother users of the machine.It can enhance software reliability by isolating software components in differentvirtual machines.It can enhance security by isolating sensitive programs and data to their own virtualmachine.It can test network facilities, such as those discussed in Chapter 9, by simulatingmachine-machine communication between several virtual machines on one physicalmachine.It can provide each user with a separate virtual machine in which a simple singleuseroperating system runs. Designing and implementing this simple operatingsystem can be much easier than designing a multi-user operating system, since itcan ignore protection and multiprogramming issues. The CMS (ConversationalMonitor System) operating system for the IBM 370 computer, for example, is usuallyrun in a virtual machine controlled by the VM/ 370 virtualizing kernel. CMSprovides only one process and very little protection. It assumes that the entiremachine is at the disposal of the user. Under VM/ 370, a new virtual machine isbatch operating systeminteractive operating systemkernelkernelvirtual devicesvirtual devicesP1 2Pvirtualizing kernelVphysical devicesFigure 1.10 Integrating two operating systems