Summary of 11/VAX Architecture

This memorandum, dated September 13, 1974, outlines the proposed "11/VAX" architecture. Authored by Craig Mudge, the document details the design goals for extending the PDP-11 architecture, primarily focusing on increasing virtual address space while maintaining strict compatibility with existing PDP-11 software.

Key highlights include:

• Design Goals: The architecture aimed for scalability, an expanded 32-bit virtual address space, and backward compatibility, allowing existing user and system code to run without modification.
• Architecture: The proposal introduces an extended 32-bit address consisting of a "chapter" number and a displacement within the chapter, mirroring the structure of existing 64K byte PDP-11 spaces.
• Mapping: To maintain performance, the architecture suggests using a "DAT box" (dynamic address translation) to cache map tables, minimizing the performance penalty of translation.
• Compatibility Mechanisms: The document describes the use of spare Processor Status (PS) bits to manage X-mode (extended) vs. non-X-mode program execution and handle stack operations during interrupts.
• Implementation: Appendices discuss how these architectural changes could be implemented on a medium-scale machine (specifically referencing the "DRAGON" project), noting that the required hardware "hooks" would be relatively inexpensive.