Virtualizing the VAX architecture
Judith S. Hall, Paul T. Robinson · 1991
This paper describes modifications to the VAX architecture to support virtual machines. The VAX architecture contains several instructions that are sensitive but not privileged. It is also the first architecture with more than two protection rings to support virtual machines. A technique for mapping four virtual rings onto three physical rings, employing both software and microcode, is described. Differences between the modified and standard VAX architectures are presented. along with a description of the virtual VAX computer.