Virtual machine: A virtual machine is the one which works exactly like a real machine. Many virtual machines can be set up in a real machine. Different operating systems and softwares can be run in different virtual machines. Thus, a user can install different operating systems and softwares in a single physical machine by using the concept of a virtual machine.
VM/370 (Virtual Machine Facility/370) is an operating system based on CP/CMS (Control Program/Cambridge Monitor System). Here is the architecture of VM/370:
1) Virtual machine hypervisor: It is used for multiprogramming and runs on the hardware. It contains many virtual machines which have copies of hardware including kernel mode, user mode, Input and Output interrupts, etc.
2) CMS (Conversational Monitor System): The virtual machines run a single user, interactive operating system known as CMS.
A system call executed by the CMS is trapped by the operating system in the virtual machine. Then, I/O instructions are sent from CMS and are trapped by VM/370.