IT-606 Embedded Systems (Software)

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IT-606Embedded Systems(Software)

• S. Ramesh
• Kavi Arya
• Krithi Ramamritham
• KReSIT/ IIT Bombay

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Esterel Basic Features and Constructs S.
Ramesh
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Layered OrganizationConventional View
Application Tasks
Scheduling, IP Communication
OS
I/O Handlers
Hardware
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Layered OrganizationEsterel View
Esterel Program Data Handler
Esterel Application
I/O Handlers
Bare Machine
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Layer Interaction
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An Esterel program

• Describes the behavior of the reactive kernel
• Has rich set of constructs for programming the
  kernel
• Kernel is typically finite state
• Interacts with its environment through an
  abstract interface
• Signals and Sensors are the means of
  communication
• Input, Output and Local signals
• Sensors are inputs only

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An Esterel program (contd.)

• Has minimal data processing functions
• Uses the data handling part for major data
  processing
• Functions and Tasks are the means of
  communication.
• Global and Local variables are used for
  communication
• Host language support - C,C, Ada

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Signals and sensors

• Signals are the novel means of communication
• idea from hardware systems
• software abstractions of the interface
• Signals can be pure or valued
• pure signals have two status 'presence' or
  'absence
• valued signals when present carry values
• values are typed, like integer, boolean,
  string,float
• Signals are transient! - reset at the end of a
  reaction

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Signals and sensors

• environment communicates by setting input signals
• program communicates back via output signals
• local signals are used for communication between
  concurrent modules
• has a no. of constructs for handling signals
• emit S, await S, present S then
• tick is a special signal always present
• sensors are special signals used as input only

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Variables and Expressions

• Esterel is an imperative language and hence uses
  variables
• variables can store different types of values
• integer, boolean, string, float
• variables retain values until updated (across
  reactions)
• variables can be local to a block of statements,
  a procedure or function or global
• no sharing of variables with the environment

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Variables and Expressions

• No sharing of variables between concurrent
  threads
• Variables are means of communication along a
  single sequential thread
• The 'race problem' is absent!
• Expressions can be formed out of variables

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Types and Functions

• Esterel is meant for controller applications
• Has minimal number of types
• Integer, boolean, float and string
• All other types used should be defined in the
  host language
• Functions and Procedures called during execution

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Functions and Procedures

• Their type specifications given in the program
• their definition is written in the host language
• value and reference parameters (like Pascal)

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Modules

• Basic programming unit
• Declarations
• types, variables, functions and procedures
  (Pascal syntax)
• input and output signals
• relation constraints
• Body of a module
• the statement executed
• sequential and concurrent flow of control
• preemption and exceptions

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Declaration

• module TIMER
• declaration
• type TIME
• var t0integer
• procedure dec(TIME)()
• function zorn()(TIME)boolean
• input SECOND, SET(TIME), RESET
• output ALARM
• relation SECOND RESET

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Modules

• loop abort await SET(t) trap T in
  loop if zorn(t) then exit T
  else nothing
  await SECOND call dec(t)
  end end emit ALARM
  when RESET
• end
• end module.

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Execution Model

• execution is a series of reactions
• invoked from an external 'main' program
  repeatedly at discrete points of time
• one reaction per invocation
• control returns after each reaction

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Reaction

• Considered instantaneous!
• Control flows from one statement to its next
• Concurrent control flows
• Input signals do not change in status nor in
  their values.
• Output and local signals may change
• Signal presence tested and variables updated
• Reaction proceeds until pause is encountered

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Reaction

• Reaction stops when pause is encountered in all
  active threads
• Next reaction starts from the next statement
• Status and values of input signals are reset at
  the end of reaction
• New values are set by the environment

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Statements

• Rich set of high level constructs
• Basic Statements
• Derived Statements
• Basic statements
• Nothing
• does nothing, terminates instantaneously
• Pause
• special control statement
• stops the current reaction
• does not terminate in the current reaction
• terminates in the next reaction

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Basic Statements

• xexpr
• classical assignment statement
• terminates instantaneously
• emit S
• terminates instantaneously generating a pure
  signal S

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Basic Statements (contd.)

• emit S(exp)
• evaluate exp' and emit S with the expression
  value
• sustain S
• sustains the signal S, i.e. emits the signal in
  each instant

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Classical control structures

• stat1 stat2
• when stat1 terminates stat2 start instantaneously
• if expr then stat1 else stat2
• evaluation of the expression and the execution of
  the branch done in the same instance

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Classical control structures (contd.)

• call A(arg1)(arg2)
• procedure call statement
• transfer of control to the procedure, execution
  of the body and the return all done
  instantaneously!
• var x in stat
• block statement
• x is local in this block

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Loop statement

• loop stat end
• repeated execution of stat
• when stat terminates it is restarted
• stat should not terminate instantaneously
• one or more pause should be there
• Consider
• loop pause end
• What is the behavior of this?
• halt is a derived statement that stands for this

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Signal testing

• present S then stat1 else stat2
• Similar to conditional statement
• tests the presence of a signal at the current
  reaction
• testing, branching and executing are
  instantaneous
• one of the branches could be absent

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Synchronous Parallelism

• stat1 stat2 stat3
• simultaneous (not concurrent) execution of all
  the statements
• signals are used for communication
• signal emitted by one thread is broadcast to all
  other threads
• terminates when every stati terminates
• no sharing of variables
• compare with asynchronous parallelism

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Synchronous Parallelism

• Example
• emit S
• present S then emit O1 else emit O2
• present S then emit O3 else emit O4
• What is the behaviour of this program?

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Preemption Statements

• Strong abort primitive - watchdog
• abort
• stat
• when S
• The body stat is executed only when S is not
  present
• Presence of S instantaneously kills the body
• No statement in stat is executed when S is
  present
• terminates either when either stat terminates or
  when S is present

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Example
abort pause emit S1 pause
emit S2 when S

• emits S1 in the second instant and S2 in third
  instant if S is not present during these
  instants.
• if S is present in second instant then nothing
  happens the whole statement exits.

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Example

• if S is not present in the second instant but
  present in third instant then
• S1 is emitted in the second instant, terminates
  in the third instant no S2 is emitted in the
  third instant
• S in the first instant is ignored
• S in the first instant is not ignored if you
  write
• abort stat when immediate S

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Await statements
Consider abort halt when
S This can be abbreviated as await S

• await tick
• waits for the special signal tick
• tick is present in every instant
• equivalent to pause

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A generalized await statement

• awaitcase S1 do stat1case S2 do stat2case S3
  do stat3
• end

• waits for one of the signals to be present
• selects one of stati for execution
• selects stati only if Si is present
• selection is deterministic

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Nesting of aborts
Consider abort abort
stat1 when S1 stat2
when S2

• when S1 is present, stat1 is killed and stat2 is
  started
• when S2 is present, what happens?
• when both S1,S2 are present, the outer abort
  statement is exited

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Weak Abort
weak abort stat when S

• A weaker form of watchdog
• The strong abort statement prevented the
  execution of body in the instant when it was
  aborted
• many time the body would like to write the last
  will at the time of aborting-some book keeping
  activity
• weak abort statement allows computation of the
  body at the instant of aborting

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Example

• weak abort
• pause
• emit S1
• pause
• emit S2
• when S

• What is the difference? 
• Weak abort statements can be nested. 
• weak and strong statements can be nested

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Traps and exits

• trap T in
• stat1
• handle T do
• stat2
• end trap

• Another weak preemption primitive
• The body stat1 may contain exit statement
• exit T

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Traps and exits

• execution starts with execution of stat1 
• when exit T is encountered the control jumps to
  the handle statement
• handle statement is optional - control then
  returns to the statement following the trap
  statement
• if stat1 is terminated then the whole trap
  statement is exited - stat2 is not executed

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Traps and exits (contd.)

• Concurrent traps
• trap T,U,V in
• stat1
• handle T do
• stat2
• handle U do
• stat3
• handle V do
• stat4
• end trap

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Traps and exits (contd.)

• Nested traps
•  trap T in
• trap U in
• stat1
• handle U do
• stat2
• end trap U
• stat3
• handle T do
• stat4
• end trap T

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Process Suspension

• Abort statements are like ctrl-C of Unix
• Suspension inspired by ctrl-Z
• suspend
• stat
• when S
• behaves like stat so long as S is not present if
  stat terminates then the whole terminates

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Process Suspension (contd.)

• stat is not executed in the instants when S is
  present
• execution is resumed at the suspended point,
  when S is present
• S in the first instant is ignored use immediate
  S to avoid this

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Local Signal Declarations

• signal S in
• stat
• end signal

• signal S is local in stat
• stat does not react to any external S
• S emitted in stat not visible outside

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Module Instantiation

• A program is a collection of modules
• Any module can be main module, defined by the
  user at the time of compilation
• modules can be instantiated in other modules
• module instantiation is a macro expansion

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Module Instantiation (contd.)

• run M
• is the simplest instantiation.
• during compilation, this statement is replaced by
  the body
• all signal declarations discarded
• data declarations exported to the parent module

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A More General Instantiation

• run MX1/Y1, X2/Y2, . . . , Xn/Yn
• X/Y means that X renames Y'
• X can be a type, constant, function
• X can be a variable or a signal
• X should be declared in the module