Virtual memory, processes, and sharing in multics

Review by Thorsten von Eicken, Jan 1998

Goal

• Allow direct sharing of code and data between applications, while maintaining protection
  • processor can access all on-line information directly
  • protection can be enforced at each point of reference
• Multics goal: computing "utility"

Benefits

• eliminate copies for I/O (file->memory->file)
• eliminate off-line linking of programs and program storage as "memory image"
• propagate changes to persistent store immediately
• not mentioned in this paper:
  • Eliminate explicit movement of data between different levels of storage hierarchy.
  • Independent of various devices.
  • Call procedures by symbolic name.
  • Obviate need to know all procedures/data referenced a priori.
  • Allow recompilation of a segment without recompiling anything else .

Additional info

• protection rings
• software development problems
  • typically, specifications representing less-important features were found to be introducing much of the complexity
  • the initial choice of modularity and interfacing between modules was sometimes awkward
  • it was rediscovered that the most important property of algorithms is simplicity rather than special mechanisms for unusual cases
• status, oct 69
  • 500 users
  • 50 at a time
  • 2 processors, 384 Kwords memory
  • 1-2 crashes per day

Calling convention

• Intersegment linking
  • Requirements
    • Pure procedure segments (i.e. no modification of procedure segments)
    • Call by symbolic name without any prearrangement
    • Segments must be invariant to recompilation of other segments
  • Implementation
    • for each segment, have a linkage section
    • calls are indirect via descriptor in linkage section
    • linkage section entries have: global address and indirection word
    • indirect call traps on first use, trap sets up indirection word
    • the link pointer (lp) points to the beginning of the linkage section.
    • external references are relative to link pointer

Segmentation vs. paging

• segmentation
  • access rights check
  • logical entities
  • address space management
• paging
  • physical memory management

Uses of segmentation

• to implement and organize the supervisor
• to map files and directories directly onto segments
• to implement dynamic linking
• by a called procedure to find its linkage (static) pointer
• to find per-process data (kept at the base of the stack)
• to control access to data and programs
• to assign data (segments) to rings
• for temporary storage
• for stacks, linkage (static), and areas (heaps)
• and so on

Successful concepts

• symbolic links (links)
• hierarchical directory structure (files=branches)
• shared libraries (segments)
• page sweep algorithm
• pageable kernel (supervisor)
• access control lists (segment attributes)

Random comments

• segmentation and paging -> x86
• "core" memory -> ferrite cores
• supervisor = kernel
• emacs

RDZ

• at MIT 81-85, used Multics
• systems with 100’s of people
• "proc" directory

Questions

• how do the implementations of today’s shared libraries (DLLs) differ from Multics?
• which features of multics have not survived to today?