# Lecture 20 - Short moves; Anthropomorphic Programming.

## Public Service Annoucements

1. On Friday the 17th we decided that the final exam will start at 12.30 on Thursday 6 April, 12.30 and end on Friday 7 April at 15.00.
2. First Train Control Milestone: Tuesday 7 March.

# Calibration I

## 2. Calibrating Constant Velocity

### How Long does it Take to Stop?

Try the following exercise.

1. Choose a sensor.
2. Put the train on a course that will cross the sensor.
3. Run the train up to a constant speed.
4. Give the speed zero command at a location that stops the train with its contact on the sensor
5. Calculate the time between when you gave the command and when the sensor triggered.
6. Look for regularities.

## 3. Short Moves.

Trains often must travel short distance, starting with the train stopped, and finishing with it stopped. When doing so the train spends its whole time either accelerating or decelerating. Your constant speed calibration is useless because the train doesn't travel at constant speed. Simmilarly your measured stopping distances are not useful.

Creating a perfect calibration of the train's position while it is accelerating is hard. But there is an easy and precise calibration that covers most of the moves the train makes where you need a good calibration It's the subject of this section.

Most of the your train project can get away with ignoring acceleration and decelleration. The one place you can't is when you are doing a short move, giving a speed command followed by a stop command before it gets up to speed. How far will the train go? How long will it be before the train is fully stopped?

Short moves are common when the train is changing direction, which you need to increase the number of possible paths from one point to another.

The general idea is to give the train a carefully timed series of commands knowing how far and for how long the train moves during the series of commands.

#### A procedure to calibrate short moves.

Write a small application that performs the following sequence of actions.

1. Place the train on the track in the sort of location where you expect to make short moves.
2. Give the train a `speed n` command, where n is big enough to get the train moving reliably.
3. Wait `t` seconds.
4. Give the train a `speed 0` command.
5. Measure how far the train travelled from its initial location.
6. You how far the train will travel for the chosen values of `n` and `t`.
Experiment with different values of `t` and `n` until you have a reasonable set of distances you can travel.

You now know how far the train moves for a given sequence of commands.

1. Position the train that distance ahead of a sensor.
2. Read the time and give a `speed n` command.
3. After `t` seconds give a `speed 0` command.
4. When the train triggers the sensor read the time again.
The distance between the two readings is the time it takes to make that short move.

Together with knowing when and where the train will stop if given the speed 0 command when running at a constant velocity, this will provide most projects with all the calibration they need. But you can do better.

# Anthropomorphic Programming

We all, even most programmers (!), have effective intuitions about human relations

• We use them to `understand' pets, which means attributing to them
• goals
• knowledge
• capabilities
• emotions
• Why not programs?
• apply them to intertask relationships

• Understand them by thinking about them as if they have capabilities and goals.
• When you are developing something like the train application you are defining roles and relationships

Why do servers need attendant tasks?

• What happens if a server calls AwaitEvent?

## 1. Proprietor with a Notifier

Proprietor `owns' a service, which usually means a resource.

• Think of the owner at the counter of an old-fashioned store
• `store' means where things are stored;
• a proprietor is a store-keeper;
• The goods are behind the counter and only the proprietor can access them.
• Clients come to the front and are processed one by one.
• Comment. The modern `store' is considered by many to be the most important innovation of the 20th century. (Yes, including the transistor, the computer, quantum mechanics, antibiotics, etc.) A whole lot of work that was previously done by store personnel is now done by the client. This is possible only because extensive codes of conduct have been internalized by clients. (That is, a large collection of new behaviour norms have been created and propagated.)
• Somebody has to sit out back waiting for the truck and bringing it to the proprietor

#### Notes

1. Notifier is usually of higher priority than server
• Notice the early reply in the proprietor
2. The server buffers both clients and data from the notifier. In this implementation client and data buffering are duals of each other. Our server code should
• pack input into (unpack output from) logical units,
• exhibit duality explicitly,
• be easy to break into parts, and
• be easy to extend.

## 2. Using a Courier

We can handle two interrupts coming very close together using a courier. Simplest is best, so we shouldn't go beyond a courier unless we expect more than two.

#### Transmit Notifier Code

• Initialize
```      Receive( &courierTid, ... );
```
• Work
```      FOREVER {
store( UART..., byte )
data = AwaitEvent( eventid );
}
```

#### Transmit Courier Code

• Initialize
```      Receive( &serverTid, notifierTid );
Send( notifierTid, ... );
```
• Work
```      FOREVER {
Send( notifierTid, {data} );
Send( serverTid, {req}, {data} );
}
```

#### Transmit Proprietor Code

• Initialize
```      // queues & fifos
notifierTid = Create( notifier );
courierTid = Create( courier );
Send( courierTid, notifierTid, ... ); // On return courier & notifier are known to be okay
RegisterAs( );                        //On return client requests will begin.
```
• Work
```      FOREVER {
requesterTid = Receive( request, {request-type, data} );
switch ( request-type ) {
case NOT_XMIT:
enqueue( requesterTid, xmitQ )
if ( ! empty( xmitFifo ) ) Reply( dequeue( xmitQ ), dequeue( xmitFifo ) );
break;
case CLIENT_XMIT:
enqueue ( xmitFifo, data );
if ( ! empty( xmitQ ) ) Reply( dequeue( xmitQ ), dequeue( xmitFifo ) );
break;
default:
ASSERT( "..." );
}
}
```

#### Notes

This gets you through a bottleneck where at most than two events come too fast.

Remember that all the calls provide error returns. You can/should use them for error recovery

• static error recovery: debugging
• dynamic error recovery: at run time

Another possible arrangement for task creation

• Server creates the courier
• Couier creates the notifier

Another possible arrangement for initialization

• Courier sends to its parentTid
• Notifier sends to its parentTid

Distributed gating

I am showing you collections of tasks implemented together because sets of related tasks is a level of organization above the individual task.

E.g., the decision to add a courier requires revision of code within the group, but not outside it.

## 3. The Warehouse

Add a warehouse between the courier and the notifier.

• In structure the warehouse is a typical server.
• The warehouse has only a single client, the courier, so its sendQ is never longer than one.
• The warehouse put together multiple inputs into a package, or break a package into multiple outputs.
• The sooner you package, and the later you unpackage, the fewer messages you need to pass.

#### Initialization

The initialization given for the courier, above, generalizes to include the warehouse, essentially without change.

#### Forever

The notifier is now talking directly to a server and has the shape given above for the proprietor. The warehouse cannot talk directly to the proprietor because both are servers.

• A courier is inserted between them.
• If there is to be two way traffic between the proprietor and the warehouse two servers are needed.

#### Note

This structure clears up most problems when a burst of requests to the server would leave the notifier waiting in a long sendQ.

• Warehouse and proprietor share the work.
• Server's Tid is public; Warehouse's Tid is private.
• This is far from the only way to share the work. For example,
• The server could be guarded by a receptionist (assistant) who ensures that another client request occurs only when the previous request is complete. Then the warehouse is unnecessary.

Note:

1. Handles bottlenecks of all sizes. Give a precise and quantitative definition of `bottleneck'.

What this amounts to is that a server should be lean and hungry

• do as little as possible