Dumb OS question

From: Jerome Fine <jhfine_at_idirect.com>
Date: Sat Apr 17 20:32:06 1999

>Allison J Parent wrote:

> <My dumb question: What is a real time operating system?
> <
> <Hans Olminkhof
>
> Complex question simplified answer.

Jerome Fine replies:

Perhaps an actual example can help.

I one had the responsibility of choosing a CPU, OS and language
(well to be truthful, the choices were made - I needed to figure
out if the choices were OK) to ensure that a READER (scanner
device at one end) connected via a microwave link to a WRITER
(laser plate maker at the other end which produced the actual
plate to print a single newspaper page - about a mile away in the
printing press building). As many plates as were required were
produced by the 6 WRITERs (a computer guided laser printer which
produced metal plates). This was being done back in 1981 and there
was no reasonable device to hold the scanned image from the
READER. Due to the way in which the WRITER functioned,
it had to be pre-charged in some manner prior to the signal being
sent via the microwave link from the READER to the WRITER.
The timing window was about 200 milliseconds between the time
the pre-charge was complete and when the information from the
READER was required to start being sent to the WRITER.
Otherwise, the plate produced by the WRITER would be faulty.

The timing sequence was:
(a) Check to see of the READER was ready to send
(b) Check to see if the WRITER was ready to receive
(c) Tell the WRITER to start the pre-charge
(d) Wait until the WRITER says the pre-charge is complete
(e) Tell the READER to start sending

The real time aspect was that there could be NO MORE than
200 milliseconds between (d) and (e) and in practice, the
program aimed for about 50 milliseconds with a maximum of
100 milliseconds. Incidentally, the communications between
the PDP-11 and the computers in the READERs and WRITERs
used serial lines at about 2400 baud, so the messages (about 10
characters each) could consume a non-trivial portion of the
total time limit.

Since there were 4 READERs and 6 WRITERs (any number of
READERs could be connected to up to 6 WRITERS) and
there were times when all 4 READERs were sending to at
least 4 WRITERs, the co-ordination of 4 READER tasks
and 6 WRITER tasks was not trivial.

In addition, that was the function of the high priority job. In
the low priority job, there were record keeping functions done
of which plates had been made, etc. Sometimes the low
priority job was very slow. It was ONLY a PDP-11/23
running RT-11. The low priority job ran 3 tasks which
allowed personal to keep track of the completed plates
for any issue of the newspaper. We might also have used
TSX-PLUS as the OS, but I doubt that it could have kept
up. Plus, we were restricted to a maximum of 248 KBytes
and the production system only had 192 KBytes if I remember
correctly. Running about 15 separate jobs under TSX-PLUS
might not have been fast enough and they would have needed
more memory (about 24 KBytes each).

Each READER and WRITER was handled by a separate "task"
in the high priority job.

Do you begin to understand why a real time OS must be able to
guarantee that each "task" be scheduled within a specified time
limit so that the command to the READER to start sending could
be completed within the window?

Sincerely yours,

Jerome Fine
RT-11/TSX-PLUS User/Addict
Received on Sat Apr 17 1999 - 20:32:06 BST