Virtual machines first appeared commercially on IBM mainframes in 1972. Virtualization was provided by the IBM VM operating system. This system has evolved and is still available. In addition, many of its original concepts are found in other systems, making it worth exploring.

IBM VM/370 divided a mainframe into multiple virtual machines, each running its own operating system. A major difficulty with the VM approach involved disk systems. Suppose that the physical machine had three disk drives but wanted to support seven virtual machines. Clearly, it could not allocate a disk drive to each virtual machine. The solution was to provide virtual disks—termed minidisks in IBM’s VM operating system. The mini disks were identical to the system’s hard disks in all respects except size. The system implemented each minidisk by allocating as many tracks on the physical disks as the minidisk needed.

Once the virtual machines were created, users could run any of the operating systems or software packages that were available on the underlying machine. For the IBM VM system, a user normally ran CMS—a single-user interactive operating system.

For many years after IBM introduced this technology, virtualization remained in its domain. Most systems could not support virtualization. However, a formal definition of virtualization helped to establish system requirements and a target for functionality. The virtualization requirements called for:

• Fidelity. A VMM provides an environment for programs that is essentially identical to the original machine.

• Performance. Programs running within that environment show only minor performance decreases.

• Safety. The VMM is in complete control of system resources.

These requirements still guide virtualization efforts today. By the late 1990s, Intel 80x86 CPUs had become common, fast, and rich

in features. Accordingly, developers launched multiple efforts to implement virtualization on that platform. Both Xen and VMware created technologies, still used today, to allow guest operating systems to run on the 80x86. Since that time, virtualization has expanded to include all common CPUs, many commercial and open-source tools, and many operating systems. For exam- ple, the open-source VirtualBox project (http://www.virtualbox.org) provides a program that runs on Intel x86 and AMD 64 CPUs and on Windows, Linux, macOS, and Solaris host operating systems. Possible guest operating systems include many versions of Windows, Linux, Solaris, and BSD, including even MS-DOS and IBM OS/2.