CS452 - Real-Time Programming - Spring 2009

Lecture 30 - History : Transputer, Occam

Hardware for Real-Time

First Generation

Second Generation

The Transputer: a Second Generation Offshoot

Use many co-operating, medium capability microCPUs to do a big job.

Problem is communication

Comminication requires either

What about real-time?

The transputer was an early, now vanished, example of the latter

Occam 2

Basic idea

  1. processes (tasks)
  2. channels
  3. time

Combining processes

  1. sequential
  2. conditional
  3. looping
  4. parallel
  5. alternation


  1. input
    channel ? variable
  2. output
    channel ! value // the value of a variable or the result of an procedure
  3. input & output provide synchronization
  4. variant protocols are possible


The Result

You can write a type-safe server, BUT

Program structure is more static than is allowed in your system.

This might be a good thing.

Third Generation

  1. Multi-CPU servers
  2. Micro-computers
  3. Micro-controllers

Ada: A Third Generation Offshoot

In the 1970s we started looking for successors to C/Unix-like environments.

Ada was one such

Over-all structure


task T is
    ....   // public face of task
end T; 
task body T is
    ....   // any code at all
end T;

Tasks are created by the declaration of a task variable

X: T;

Main program is implicitly called as a task.


Tasks can include

  1. Entries in their specifications
    task T is
        entry E( <in & out parameters> );
    end T;
  2. Accept statements that implement the entries
    accept E( <formal parameters> ) do
       ...  //code to be executed
    end E;
  3. Calls to entries in other tasks
    T.E( <actual parameters> );

Accept blocks until the call to the entry occurs.

The call blocks until the accept is executed.

Relationship to S/R/R

How do You Write a Server


    accept Send( request ) do
        // process request
        // fill in return parameters
    end Send;
end loop;

What's wrong here?

Here is one of the work-arounds.

        SendNotifier( data ) do
            // update internal state
        end SendNotifier;
        SendClient( request ) do
            // fulfil request
        end SendClient;
    end select;
end loop

How does this still fall short?

Is it possible to write a server in a programming language with nested scoping?


Pretty easy. Two time primitives

You can build delay from delay until, but not vice versa.


An interrupt handler is a parameterless procedure, that can be attached (either statically, at compile time, or dynamically, at run-time) to the occurrence of an interrupt

`The Ada programming language defines the semantics of interrupt handling as part of the tasking mechanism, making it possible to construct implementation-independent interrupt handlers. However, for the Ada mechanism to be effective, an implementation must provide support not specified by the Ada standard, such as for initializing hardware interrupting devices, handling unexpected interrupts and optimizing for real-time performance constraints. This paper analyses some of the constraints that efficient interrupt support places on an implementation. It develops a model for the interaction between interrupt hardware and Ada tasks and describes optimizations for Ada interrupt handlers. Implementation issues, including task priorities and task termination for interrupt handlers, are discussed in detail.' Real-time interrupt handling in Ada Jørgen Born Rasmussen, Bill Appelbe, Software Practice and Experience.

Fourth Generation

  1. Multi-(multi-core CPUs) called servers
  2. Multi-core microprocessors
  3. Stream processors

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