Re: A Customer's Request For Open Source Software

["Robert I. Eachus" <rieachus@attbi.com>]

Larry Kilgallen wrote:

> That sounds true for unrestricted access, but the number of occasions
> for checks should have to be the same as any other operating system for
> data protected as "single writer at a time".

I wasn't particularly addressing that, but yeah, in Multics, its "in 
there."  The first and most important feature for supporting multiple 
writers was known as the first reference trap.  When a process accessed 
  a segment for the first time, or wrote to a segment for the first time 
(for that segment) there was a piece of code associated with each action 
that would be run (if non-null).  Sometimes, for example if a program 
had a predefined data segment, it would do copy-on-write so the first 
time the program wrote to its data segment, the process got its own 
copy.  There were a number of "predefined" first reference trap-handlers 
that were used for common purposes.

For example, there was a MRDS (Multics Relational Database System) 
manager.  MRDS ran in ring 2, so if a segment had a write bracket of 2, 
any attempt to write would be passed inward to MRDS.  If you wanted 
transaction protection, you would define a set of MRDS transactions and 
MRDS would guarantee the all or nothing character of each transaction. 
(The first reference trap initialized the database connection.)  Even 
though MRDS supported the whole relational algebra, you could use it for 
concurrency protection with out using the rest of it.

Similarly, the mail system ran in ring 3. Processes sometimes used the 
mail system just to create and manage (in-memory) queues.  No reason not 
to.  Since the mail system didn't have to work through a file system, it 
could be very fast--and low overhead.  Inward ring calls are/were also 
very cheap in Multics, and returns from an inward call wasn't much of a 
problem.  The tricky part of the ring system was the "crawl-out" in the 
login daemon.  The code had to do a lot of checks that were normally 
done by the hardware, then "return" into the newly created outer ring 
environment.

-- 
                                             Robert I. Eachus

"As far as I'm concerned, war always means failure." -- Jacques Chirac, 
President of France
"As far as France is concerned, you're right." -- Rush Limbaugh