- 1 - Embedded Systems - SDL Some general properties of languages 1. Synchronous vs. asynchronous languages Description of several processes in many languages

- 1 -Embedded Systems - SDL

Some general properties of languages1. Synchronous vs. asynchronous languages

Description of several processes in many languages non-deterministic:The order in which executable tasks are executed is not specified (may affect result).

Synchronous languages: based on automata models.

“Synchronous languages aim at providing high level, modular constructs, to make the design of such an automaton easier” [Halbwachs].

Synchronous languages describe concurrently operating automata... “when automata are composed in parallel, a transition of the product is made of the "simultaneous" transitions of all of them“.

Description of several processes in many languages non-deterministic:The order in which executable tasks are executed is not specified (may affect result).

Synchronous languages: based on automata models.

“Synchronous languages aim at providing high level, modular constructs, to make the design of such an automaton easier” [Halbwachs].

Synchronous languages describe concurrently operating automata... “when automata are composed in parallel, a transition of the product is made of the "simultaneous" transitions of all of them“.


Synchronous languages implicitly assume the presence of a (global) clock. Each clock tick, all inputs are considered, new outputs and states are calculated and then the transitions are made.

This requires a broadcast mechanism for all parts of the model.

Idealistic view of concurrency.

Has the advantage of guaranteeing deterministic behavior.

StateCharts is a synchronous language.

Synchronous languages implicitly assume the presence of a (global) clock. Each clock tick, all inputs are considered, new outputs and states are calculated and then the transitions are made.

This requires a broadcast mechanism for all parts of the model.

Idealistic view of concurrency.

Has the advantage of guaranteeing deterministic behavior.

StateCharts is a synchronous language.

Some general properties of languages1. Synchronous vs. asynchronous languages


Some general properties of languages2. Properties of processes

• Number of processesstatic; dynamic (dynamically changed hardware architecture?)

• Nested declaration of processesor all declared at the same level

• Different techniques for process creationElaboration in the source code,explicit fork and join,process creation calls

StateCharts comprises a static number of processes, nested declaration of processes, and process creation through elaboration in the source code.

• Number of processesstatic; dynamic (dynamically changed hardware architecture?)

• Nested declaration of processesor all declared at the same level

• Different techniques for process creationElaboration in the source code,explicit fork and join,process creation calls

StateCharts comprises a static number of processes, nested declaration of processes, and process creation through elaboration in the source code.


Some general properties of languages3. Communication paradigms

• 1. Message passing:– Non-blocking communication

Sender does not have to wait until message has arrived; potential problem: buffer overflow

– Blocking communication, rendez-vous-based communicationSender will wait until receiver has received message

– Extended rendez-vousExplicit acknowledge from receiver required. Receiver can do checking before sending acknowledgement.

• 1. Message passing:– Non-blocking communication

Sender does not have to wait until message has arrived; potential problem: buffer overflow

– Blocking communication, rendez-vous-based communicationSender will wait until receiver has received message

– Extended rendez-vousExplicit acknowledge from receiver required. Receiver can do checking before sending acknowledgement.


Some general properties of languages3. Communication paradigms

• 2. Shared memoryVariables accessible to several tasks– Critical sections = sections at which exclusive access

to some resource r must be guaranteed.

• 2. Shared memoryVariables accessible to several tasks– Critical sections = sections at which exclusive access

to some resource r must be guaranteed.

StateCharts uses shared memory for communication between processes.

StateCharts uses shared memory for communication between processes.


Some general properties of languages 4. Specifying timing

4 types of timing specs required [Burns, 1990]:• Measure elapsed time

Check, how much time has elapsed since last call• Means for delaying processes• Possibility to specify timeouts

We would like to be in a certain state only a certain maximum amount of time.

• Methods for specifying deadlinesWith current languages not available or specified in separate control file.

4 types of timing specs required [Burns, 1990]:• Measure elapsed time

Check, how much time has elapsed since last call• Means for delaying processes• Possibility to specify timeouts

We would like to be in a certain state only a certain maximum amount of time.

• Methods for specifying deadlinesWith current languages not available or specified in separate control file.

StateCharts comprises a mechanism for specifying timeouts. Other types of timing specs are not supported.

StateCharts comprises a mechanism for specifying timeouts. Other types of timing specs are not supported.


Properties of specification languages5. Using non-standard I/O devices -

Direct access to switches, displays etc;

No protection required; OS can be much faster than for operating system with protection.

No support in standard StateCharts.

No particular OS support anyhow.

Direct access to switches, displays etc;

No protection required; OS can be much faster than for operating system with protection.

No support in standard StateCharts.

No particular OS support anyhow.


Specification and Description Language (SDL)

Language designed for specification of distributed systems.

• Dates back to early 70s,

• Formal semantics defined in the late 80s,

• Defined by ITU (International Telecommunication Union): Z.100 recommendation in 1980Updates in 1984, 1988, 1992, 1996 and 1999

Language designed for specification of distributed systems.

• Dates back to early 70s,

• Formal semantics defined in the late 80s,

• Defined by ITU (International Telecommunication Union): Z.100 recommendation in 1980Updates in 1984, 1988, 1992, 1996 and 1999


SDL

• Designed for distributed applications, based on asynchronous message passing.

• Provides textual and graphical formats to please all users (graphical and textual formats).

• Just like StateCharts, it is based on the CFSM model of computation; each FSM is called a process,

• However, it uses message passing instead of shared memory for communications,

• SDL supports operations on data.

• Designed for distributed applications, based on asynchronous message passing.

• Provides textual and graphical formats to please all users (graphical and textual formats).

• Just like StateCharts, it is based on the CFSM model of computation; each FSM is called a process,

• However, it uses message passing instead of shared memory for communications,

• SDL supports operations on data.


SDL-representation of FSMs/processes

output

input

state


Operations on data

Variables can be declared locally for processes.Their type can be predefined or defined in SDL itself.SDL supports abstract data types (ADTs). Examples:

Variables can be declared locally for processes.Their type can be predefined or defined in SDL itself.SDL supports abstract data types (ADTs). Examples:


Communication among SDL-FSMs

Communication between FSMs (or “processes”) is based on asynchronous message-passing, assuming a potentially indefinitely large FIFO-queue.

Communication between FSMs (or “processes”) is based on asynchronous message-passing, assuming a potentially indefinitely large FIFO-queue.

• Each process fetches next entry from FIFO,

• checks if input enables transition,

• if yes: transition takes place,

• if no: input is ignored (exception: SAVE-mechanism).

• Each process fetches next entry from FIFO,

• checks if input enables transition,

• if yes: transition takes place,

• if no: input is ignored (exception: SAVE-mechanism).


Process interaction diagrams

Interaction between processes can be described in process interaction diagrams (special case of block diagrams).

In addition to processes, these diagrams contain channels and declarations of local signals.

Example:

Interaction between processes can be described in process interaction diagrams (special case of block diagrams).

In addition to processes, these diagrams contain channels and declarations of local signals.

Example:

, (channel)


Designation of signal recipients

1. Through process identifiers:Example: OFFSPRING represents identifiers of processes generated dynamically.

2. Explicitly:By including the channel name.

3. Implicitly:If signal names imply channel names (B Sw1)

1. Through process identifiers:Example: OFFSPRING represents identifiers of processes generated dynamically.

2. Explicitly:By including the channel name.

3. Implicitly:If signal names imply channel names (B Sw1)

CounterVia Sw1

CounterTO OFFSPRING


Hierarchy in SDL

Process interaction diagrams can be included in blocks. The root block is called system.

Process interaction diagrams can be included in blocks. The root block is called system.

Processes cannot contain other processes, unlike in StateCharts.


Timers

Timers can be declared locally. Elapsed timers put signal into queue (not necessarily processed immediately).

RESET also removes timer signal from queue.

Timers can be declared locally. Elapsed timers put signal into queue (not necessarily processed immediately).

RESET also removes timer signal from queue.


Additional language elements

SDL includes a number of additional language elements, like• procedures• creation and termination of processes• advanced description of data

SDL includes a number of additional language elements, like• procedures• creation and termination of processes• advanced description of data


Example: vending machine

Machine° selling pretzels, (potato) chips, cookies, and doughnuts:

accepts nickels, dime, quarters, and half-dollar coins.

Not a distributed application.

° [J.M. Bergé, O. Levia, J. Roullard: High-Level System Modeling, Kluwer Academic Publishers, 1995]


Overall view of vending machine

DecodeRequests

p


ChipHandler

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- 1 - Embedded Systems - SDL Some general properties of languages 1. Synchronous vs. asynchronous languages Description of several processes in many languages