Shared code/libraries

[hamlet at umcp-cs.UUCP]

In seeking models for a clean implementation of shared code, the Burroughs
B5000/6000/7000 systems are important.  Although there is a very
large assist from the descriptor architecture, these systems have all
programs shared, and all share a library, even across languages.  On
the B5500 this was a necessity, since there was a maximum of 32K of real
memory, and the operating system often took up as much as 4K of that.
(It had to arrange for all that sharing, among other things.)

When the shared-library code was added to the B5500 (by a new-hire programmer
who took several weeks at the job, and made only one not large mistake--
it was a nice system to work on), it was found that the sharing was not
being used very much, except for things like long-running utilities and
their i-o library routines.  The trouble may have been the severly limited
memory, which didn't allow enough users to remain resident long enough to
share.  Perhaps it would be a good idea to do some measurements on what
sharing would buy before putting a lot of work into it.

When it comes time for multiple-processor systems, the Burroughs
machines did that too, but all in the architecture, so there isn't any
good to be had by studying the code.