Some general properties of languages 1. Synchronous vs. asynchronous languages

1. 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“.

2. Some general properties of languages1. Synchronous vs. asynchronous languages • 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.

3. 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.

4. Some general properties of languages3. Communication paradigms • 1. Message passing: • Non-blocking communicationSender 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.

5. 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.  StateCharts uses shared memory for communication between processes.

6. Some general properties of languages 4. Specifying timing • 4 types of timing specs required [Burns, 1990]: • Measure elapsed timeCheck, how much time has elapsed since last call • Means for delaying processes • Possibility to specify timeoutsWe 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.

7. 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.

8. 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

9. 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.

10. SDL-representation of FSMs/processes state input output

11. 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:

12. Communication among SDL-FSMs • 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).

13. 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: , (channel)

14. Counter TO OFFSPRING CounterVia Sw1 Designation of signal recipients • Through process identifiers:Example: OFFSPRING represents identifiers of processes generated dynamically. • Explicitly:By including the channel name. • Implicitly:If signal names imply channel names (B  Sw1)

15. Hierarchy in SDL • Process interaction diagrams can be included in blocks. The root block is called system. Processes cannot contain other processes, unlike in StateCharts.

16. Timers • Timers can be declared locally. Elapsed timers put signal into queue (not necessarily processed immediately). • RESET also removes timer signal from queue.

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

18. 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]

19. Overall view of vending machine

20. p DecodeRequests

21. ChipHandler