The Origin of the VM 370 Time-Sharing System

source: https://cseweb.ucsd.edu/classes/fa25/cse221-a/papers/vm370.pdf

Question

What was VM/370’s motivation for virtualizing the hardware? Trying to make a personal computer dedicated to a user so by extending time-sharing of a dedicated machine to with more isolation, to the user it seems like they own the computer. This lead to the idea of a virtual machine.

Introduction

VM/370 is an OS which provides its multiple users with seemingly separate and independent IBM System/370 computing systems.

• Contains three different OSes:
  • Control Program (CP)
    • OS that uses a computing machine to simulate multiple copies of the machine itself
  • Conversational Monitor System (CMS)
    • OS that supports the interactive use of a computing machine by one person, the typical OS used within each virtual machine (VM)
  • Remote Spooling and Communications Subsystem (RSCS)
    • OS used to provide information transfer among machines linked with communication facilities Key idea: perfect hardware compatibility at the ISA boundary
• privileged operations trap to CP
• problem-state code runs natively
• Multiple different OSes run concurrently, isolated from each other

Design Goals

• Production of VM identical to its real counterpart
• Meant to be a learning experience
• Modularity: split “resource control” (CP) from “user experience/tools” (CMS)
• VM cannot be compromised by the operation of any other VM
  • private secure and reliable computing environment for its users

Control Program

General multiprogramming system that uses virtual machines to organize independent job streams

• Each VM is identified by an entry in the CP directory

• basically the hypervisor:

  • created illusion of private CPU(s), memory, disks, and devices for every user
  • S/370’s hardware Dynamic Address Translation (DAT) to map guest physical memory to real physical memory
  • Trapped privileged instructions (I/O) and emulated their effects
  • Scheduled multiple VMs concurrently, switching context between them’

• Privileged instruction trapping

  • Any s/370 privileged instruction caused a trap (exception), and CP emulated its effect safely

• Device virtualization
  Virtual devices mapped to either:

  • real hardware (dedicated), or
  • spool files (simulated card reader/printer/tape).

• Scheduler & dispatcher
  CP’s scheduler gave each VM CPU time slices, switching among them.

• Shared disks and communication
  CP allowed controlled sharing via “minidisks” (sub-volumes of real disks) and message passing.

Key feature of CP/CMS design: independence of each virtual machines

• all connections between VMs are explicitly defined and controlled by CP

Conversational Monitor System

Disk-file-oriented OS to support personal use of a dedicated computer

• Guest operating system to run in side VM