Esterel Overview

1
Esterel Overview

• Roberto Passerone
• ee249 discussion section

2
Classes of applications

• Transformational
• Interactive
• Reactive

3
Model of Time

• Totally ordered sequence of instants
• Instants are also called ticks
• There is nothing in between instants

4
Esterel

• Esterel is a reactive programming language
• Esterel is also a Synchronous language
• Everything proceeds in lock-step

5
You might hear that...

• In Esterel computation takes zero time
• Communication also takes zero time
• In other words, reactions to events are
  instantaneous

6
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

7
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

8
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

9
Checking for presence

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

10
Awaiting an event

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

11
Emitting an event

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

12
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

13
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

14
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

15
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

16
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

17
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

18
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

19
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

20
Esterel supports exceptions

• Useful to jump out of a fragment of code
• Abort when an event occurs
• abort
• code
• code
• when A

21
Seat-belt
If the driver turns on the key, and does not
fasten the seat belt within 5 seconds then an
alarm beeps for 5 seconds, or until the driver
fastens the seat belt, or until the driver turns
off the key
If the driver turns on the key, and does not
fasten the seat belt within 5 seconds then an
alarm beeps for 5 seconds, or until the driver
fastens the seat belt, or until the driver turns
off the key
If the driver turns on the key, and does not
fasten the seat belt within 5 seconds then an
alarm beeps for 5 seconds, or until the driver
fastens the seat belt, or until the driver turns
off the key
If the driver turns on the key, and does not
fasten the seat belt within 5 seconds then an
alarm beeps for 5 seconds, or until the driver
fastens the seat belt, or until the driver turns
off the key
If the driver turns on the key, and does not
fasten the seat belt within 5 seconds then an
alarm beeps for 5 seconds, or until the driver
fastens the seat belt, or until the driver turns
off the key
input KEY_ON
, END_TIMER
, KEY_OFF, BELT_ON
, RESET
output START_TIMER
, BEEP
loop
abort
present KEY_ON then
emit START_TIMER(5) await END_TIMER
emit START_TIMER(5) abort sustain BEEP when
END_TIMER
end
when KEY_OFF or BELT_ON
each RESET
22
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

23
Immediate reactions

• Alternative formulation of await
• It doesnt wait until the next reaction if the
  signal is present
• Often creates causality problems
• loop
• await immediate A
• X X 1
• end loop