esterel overview roberto passerone ee249 discussion section

Esterel Overview

Roberto Passerone

ee249 discussion section

April 18, 2023 Esterel notes 2

Classes of applications

• Transformational

• Interactive

• Reactive


Model of Time

• Totally ordered sequence of instants

• Instants are also called ticks

• There is nothing in between instants


Esterel

• Esterel is a reactive programming language

• Esterel is also a Synchronous language

• Everything proceeds in lock-step


You might hear that...

• In Esterel computation takes zero time

• Communication also takes zero time

• In other words, reactions to events are instantaneous


Model of Execution

• There is a set of events E

• We have a sequence of instants

• For each instant a subset of the events in E is present

• Hence the execution is a totally ordered sequence of events


Model of a System

• The System implicitly represents the execution

• The System is a collection of processes executing concurrently

• Processes are placed in parallel using the parallel operator ||


Interaction of Processes

• Processes interact by through signals that carry events

• Events in signals can be either Present or Absent

• Processes do NOT interact through shared variables


Checking for presence

• Processes can test for the presence or absence of events in signals– present A then … else ...


Awaiting an event

• Processes can stop their execution until an event is present in a signal– await A;– await [ A or B ]


Emitting an event

• Processes can emit an event to a signal to make it present– emit A– emit A, 10


Execution of the System

• Input events are posted for the current instant

• The System reacts to the input event in the current instant

• The System posts the output events for the current instant

• Only at this point does the instant increment


When things occur

• present tests the presence of signals in the current reaction

• await stops the execution for the current reaction, until at least the next instant

• emit emits the event during the current reaction


Important

• There is no ordering of events within a reaction• All events present during a reaction occur at the

same exact time (instant)• Hence when executing a present:

– if the signal is present continue with the “then”

– if absent, must wait until it either becomes present or can prove it will not be emitted during the current reaction


Atomicity of reaction• The triple (input, reaction, output) is executed

atomically– Atomic means that others cannot observe the

intermediate steps. It is not necessarily instantaneous

– Intermediate steps are only necessary to compute the overall reaction

– All goes as if the computation took zero time

– There are micro-steps: they converge to a fixed point


How to execute

• Start with the input events present• Activate all processes awaiting a present event• If events are emitted, activate processes that may

be waiting• If presence is tested, wait until you know the

outcome• Stop a process when it reaches an await


Compiling Esterel

• The interpreter does the previous procedure– might have to look ahead

• The compiler computes the reaction for all possible patterns of events– it constructs a finite automaton– construction similar to the derivative

construction


Causality Problems

• A reaction ends when all processes arrive at an await

• What if you never hit one?– Loop

– emit A;

– X = X + 1;

– end loop

• Same as combinational loops in hardware


The fixed point may not exist

• There are no solutions– present X then nothing else emit X;

• There are too many solutions– present X then emit X end;


Esterel supports exceptions

• Useful to jump out of a fragment of code

• Abort when an event occurs– abort– code– code– when A;


Seat-beltIf the driver turns on the key, and

does not fasten the seat belt within 5 seconds

then an alarm beeps for 5 seconds, oruntil the driver fastens the seat belt, or

until the driver turns off the key

If the driver turns on the key, anddoes not fasten the seat belt within 5 seconds



present KEY_ON then

input KEY_ON




emit START_TIMER(5); await END_TIMER;

, END_TIMER

output START_TIMER

emit START_TIMER(5); abort sustain BEEP when END_TIMER;

end

abort

when KEY_OFF or BELT_ON







, KEY_OFF, BELT_ON

, BEEP;

loop

each RESET

, RESET;


You can run examples

• Compile with -simul to run textual simulation

• Use xes for graphical debugging

• If we have time we can look at a simple example


Immediate reactions

• Alternative formulation of await• It doesn’t wait until the next reaction if the signal

is present• Often creates causality problems

– loop

– await immediate A

– X = X + 1;

– end loop

esterel overview roberto passerone ee249 discussion section

Documents

esterel notes4 esterel

events events

esterel computation

esterel notes13

esterel notes11

esterel notes9

esterel notes10

esterel notes5