previous | contents | next

THE PDP-11 FAMILY 239

VIRTUAL ADDRESS EXTENSION OF THE PDP-11

The latest member of the PDP-11 family, the Virtual Address Extension 11 or VAX-11, is described in Chapter 17. This paper, by the architect of VAX-11, discusses the new architecture and its first implementation, the VAX-l1 /780.


VAX-11 extends the PDP-11 to provide a large, 32-bit virtual address for each user process. The architecture includes a compatibility mode that allows PDP-l1 programs written for the RSX-11M program environment to run unchanged. In this way, PDP-11 programs can be moved among VAX and PDP-l 1 computers, depending on the user's address size and computational and generality needs.


Chapter 17 provides a clean, somewhat terse, yet comprehensive description of the VAX-11 architecture. Because the VAX part of the architecture is so complete in terms of data-types, operators, addressing and memory management, it can also serve as a textbook model and case study for architecture in general. Goals, constraints, and various design choices are given, although explanations of what was traded away in the design choices are not detailed.

previous | contents | next