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From: ROSENBLUM@SIERRA.STANFORD.EDU (David S. Rosenblum)
Newsgroups: comp.lang.ada
Subject: Re: Answer to 'a question'
Message-ID: <12332478709.9.ROSENBLUM@Sierra.Stanford.EDU>
Date: Sun, 6-Sep-87 12:08:51 EDT
Article-I.D.: Sierra.12332478709.9.ROSENBLUM
Posted: Sun Sep  6 12:08:51 1987
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>>>My question is: Are those entry calls queued in the order of calls?
>>>YES. ... The key is that the execution of the
>>
>>Unfortunately, you are assuming a single processor system.  In any distributed
>>or multiprocessor system, where calls are implemented via message exchange,

>Wrong! Not in any OS. In a properly designed real-time, multiprocessor,
>Ada OS, where tasking across machine boundaries is implemented via messages,
>the design of the bus insures that the messages are received in the order
>that they are sent out. A common solution is a ring bus such as the SAE-9B
>token-passing ring. Why would the communication of a rendezvous over a
>interprocessor bus between run-time systems in different kernals be any
>different than passing a message from a task to a run-time kernal
>in the same processor via system services? There are many synchronization
>problems to be reconed with in these implementations and that is why it has
>taken Ada contractors over five years to get even close to a solution.
>As an example all processors are synchronized within nanoseconds by a
>"heartbeat" signal and messages are time-tagged to resolve ambiguities between
>real-time events.

There seems to be a slight misunderstanding in some of these messages.
What is at issue--the order in which calls are issued or the order in
which call messages are sent over a network?

Using both time-tagging and network serialization of messages, the
time order of messages may still NOT be the same as the sending order,
especially using a token ring network.  If time-tagging is used to
resolve the order of real-time events, then the type of network used
is immaterial.  If the network is used to impose a serialization of
real-time events, there is no guarantee that the network order is
the same as the real-time order of occurrence.

Nevertheless, this skirts a more fundamental issue, namely that software
which depends on such implementation characteristics cannot be portable.
Remember that a major goal of the Ada language is portability of Ada
software.

-- David
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