(no subject)

Brian TAM (btam@cs.ucsd.edu)
Thu, 25 May 2000 08:45:02 -0700 (PDT)

"The Multics Virtual Memory: Concepts and Design"

Contemporary (1972) on-line operating systems permit some
sharing of information among users, usually via I/O from
secondary storage. Multics provides direct hardware addressing
by user and system programs of all information, independent of
physical storage location. This is achieved through a virtual
memory system based on segmentation, the division of memory
into pieces, each of which is potentially sharable and carries
its own independent attributes of size and access privilege.
An implementation is described in which segments are paged and
are accessible through reference to tree table directories.

This paper could have been better organized, for instance
describing the structure of the supervisor BEFORE mentioning
its functionality in the virtual memory system, as was done
with the 645 processor segmentation and paging.

"Machine-Independent Virtual Memory Management for Paged
Uniprocessor and Multiprocessor Architectures"

Mach is an operating system developed at Carnegie Mellon to
be portable between uni- and multiprocessors. This is
accomplished by separating software virtual memory management
from hardware support. The machine-dependent portion of Mach's
virtual memory subsystem consists of a single code module and
its related header file. All information important to virtual
memory management is maintained in machine-independent data
structures. Machine-dependent data structures contain only
those mappings necessary to running the current mix of programs.

Mach is one of those few systems that seems truly to live up
to the claims of its inventors. The authors say it is portable,
and have not had to modify the system fundamentally to make it so.
That is, it was possible to run Mach on machines with particular
features (like the SUN 3) by changing only the machine-dependent
component of Mach.