PEARL

	The Realtime- and Multitasking-Programming Language PEARL
Thomas Probol, Stefan Eilers, Torsten Lilge lilge@irt.uni-hannover.de, last modification: 2000-03-6, location: http://www.irt.uni-hannover.de/pearl/pearlein.html

Inhalt Basic data types and language structures Better hardware independency Multitasking-commands Scheduling on events and time instants Task synchronisation to avoid inconsistencies In- and Output Special data types

The name PEARL stands for Process and Experiment Automation Realtime Language and must not be mistaken for Perl, the Practical Extraction and Report Language. PEARL is a higher programming language, which allows a comfortable, secure and almost processor independent programming of multitasking- and realtime problems and has been standardized since 1977 at various stages of its development, the last time 1998 as PEARL-90 (DIN 66253-2 1998, Berlin, Beuth-Verlag, 1998). Besides the simple possibility to map process technical problems, an important principle at the development of PEARL was the easy learning by the programmer. Everyone who already knows a procedural programming language will get acquainted with PEARL in a very short time. All basic data types and language structures of other procedural programming languages exist in PEARL (<). In addition PEARL offers comfortable language elements for the handling of multitasking- and realtime tasks (<). /\ Basic data types and language structures Data types: Basic data types fixed point, floating point characters, character strings bit variables, bit strings Assembled data types structures arrays of various dimensions with user defined lower- and upper limits for the individual dimensions. Functions, procedures (parameter transfer via value, via IDENT and as pointer) Type pointers to all objects, also to functions and procedures Type-free pointers and type-casting Block structure, validity of objects: Object declarations within a BEGIN-END-block Procedure- and function-wide objects Module-wide objects Access to objects of other modules via global specification of variables Announcement of objects for other modules via global declaration Control structures: Conditional commands IF-THEN-ELSE-FIN CASE-ALT-...-ALT-OUT-FIN Repetitions FOR-REPEAT-END WHILE-REPEAT-END Detailed information about the construction of PEARL is contained in DIN 66253-2 1998, Berlin, Beuth-Verlag, 1998 PEARL 90 - Language Report, Version 2.2., GI-Fachgruppe 4.4.2, Bonn, GI Gesellschaft für Informatik e.V., 1998 /\ Better hardware independency To reach a separation from hardware dependent components, like for example in- and output interfaces, to the hardware independent program, a PEARL-module is subdivided into two parts: In the so called system part, beginning after the keyword SYSTEM, the names for the hardware dependent I/O-interfaces are declared and their features defined. Here also interrupt sources can be defined. The so called problem part, beginning withPROBLEM, contains for example variables, constants, tasks and procedures . Tasks and procedures have access to the interfaces defined in the SYSTEM-part. /\ Multitasking-commands The state of tasks known to the operating system can be modified in a PEARL-program. ACTIVATE Taskname immediately makes runable the task with the name 'Taskname'. TERMINATE Taskname the task with the name 'Taskname' is terminated. SUSPEND Taskname the task with the name 'Taskname' is suspended. CONTINUE Taskname the suspended task with the name 'Taskname' will be continued. PREVENT Taskname the task with the name 'Taskname', of which the activation was coupled with an event (see for example (<)), will be prevented. It will not be activated or continued anymore by this event. /\ Scheduling on events and time instants The activation and continuation of tasks can be executed to external events or time instants under certain conditions. Schedules to an exact clock-time, also schedules in case of external events (interrupts), are possible. Examples: ALL 0.00005 SEC ACTIVATE Highspeedcontroller; cyclical activation of a controller with a frequency of 20 kHz AT 12:00 ALL 4 SEC UNTIL 12:30 ACTIVATE lunchhour PRIO 1; cyclical scheduling, every 4 seconds between 12:00 and 13:00 hrs with high priority WHEN fire ACTIVATE extinguish; activation of the task 'extinguish', when interrupt 'fire' occurs. /\ Task synchronisation to avoid inconsistencies A synchronisation of tasks always becomes necessary when they jointly use data. The following three examples show how tasks can be synchronized with the use of 'semaphore-'(<) and 'boltvariables'(<). If a task A intends the access to a data record, which is just being handled by a TASK B, the operating system blocks task A until B releases the data. In connection with semaphores PEARL allows instead of a blockade also the "testing with entry when free" (keyword TRY in example (<)). Critical paths Critical paths arise, when 2 tasks intend to access shared objects at the same time. With the help of 'semaphore-variables'(<) a started access can be terminated unchecked, without interruption by other tasks, which also intend an access. PROBLEM; DCL A CHAR(255); ! character string: 255 Bytes DCL SEMVAR SEMA PRESET(1); ! reservation: access allowed DCL FAILED FIXED INIT(0); T1: TASK; DCL B1 CHAR(255); REQUEST SEMVAR; A=B1; RELEASE SEMVAR; END; T2: TASK; DCL B2(CHAR255); REQUEST SEMVAR; A=B2; RELEASE SEMVAR; END; TEST: TASK; IF TRY SEMVAR THEN RELEASE SEMVAR; !release semaphore again ELSE FAILED=FAILED+1; FIN; AFTER 10 SEC ACTIVATE TEST; ! self activation after 10 seconds END; Producer-consumer-schemes If a task A produces data, which are provided for subsequent treatment by one or several other tasks, one talks about a producer-consumer-scheme. Following is the example of a ring storage with 1024 characters, of which the reading task always needs 2 at the same time. PROBLEM; DCL PLACE_AVAILABLE SEMA PRESET(1024); ! init: 1024 places free DCL CHARACTER_AVAILABLE SEMA PERSET(0); ! init: empty DATAFETCH: TASK; REQUEST CHARACTER_AVAILABLE; REQUEST CHARACTER_AVAILABLE; FETCH_TWO_CHARACTERS_FROM_THE_RINGBUFFER; RELEASE PLACE_AVAILABLE; RELEASE PLACE_AVAILABLE; END; DATAPUT: TASK; REQUEST PLACE_AVAILABLE; PUT_ONE_CHARACTER_INTO_RINGBUFFER; RELEASE CHARACTER_AVAILABLE; END; Producer-consumer-schemes also occur at the digital measuring and the automation of production processes. Data base management When the programmer wants to allow the reading of a data record by several tasks simultaneously (in the example the tasks READER1 and READER2), but only one task can write without another task reading at the same time, BOLT-variables are the choice. PROBLEM; DCL A CHAR(255); ! character string: 255 Bytes DCL BOLTVAR BOLT; ! default initialisation: free READER1: TASK; DCL L1 CHAR(255); !ENTER allows, that all other tasks with ENTER !can continue to run ENTER BOLTVAR; L1=A; LEAVE BOLTVAR; END; READER2: TASK; DCL L2 CHAR(255); ENTER BOLTVAR; L2=A; LEAVE BOLTVAR; END; WRITER1: TASK; DCL S1 CHAR(255); !After a RESERVE a task only continues to run if no other !reader or writer has executed an ENTER/RESERVE. RESERVE BOLTVAR; A=S1; FREE BOLTVAR; END; WRITER2: TASK; DCL S2 CHAR(255); RESERVE BOLTVAR; A=S2; FREE BOLTVAR; END; /\ In- and Output PEARL offers an own keyword for each of the different in- and output forms. In the following examples alphic_dation stands for a harddisk, terminal, LCD-display and serial or parallel interface e. g., a basic_dation stands for an analog or digital process I/O e. g. Output formatted output PUT object1, ... TO alphic_dation BY format list; binary output WRITE object1, ... TO alphic_dation; output to process periphery SEND object1, ... TO basic_dation; Input formatted read-in GET object1, ... FROM alphic_dation BY format list; binary read in READ object1, ... FROM alphic_dation; read from process periphery TAKE object1, ... FROM basic_dation; The sizes in the individual format lists can be fixed by constants and also by variables and function calls. They don't need to be known already at time of compilation.. /\ Special data types CLOCK, DURATION Both these data types describe dates and periodics. With the time schedules these data types also appear. In the mentioned examples (<) 12:00 is a constant of the type CLOCK and 0.00005 SEC a constant of the type DURATION. Arithmetic operations between these data types are also possible, as the following two examples show. durationvar=clockvar-clockvar clockvar=clockvar+durationvar INTERRUPT These data types must be connected in the system part of a module with a hardware interrupt entry. In the problem part they can be used with the above described means (<) (WHEN ..) for changing a task state. SEMA Semaphore-variables are used for the synchronisation between different tasks, (<) mainly for the protection of compound data objects, which are used by several tasks at the same time. Used with critical paths (<) or producer consumer-schemes(<). A semaphore emulation is not possible without support of the operating system, as the usual high language operations 'allocate, resp. block' resp. 'release or continue blocked tasks' must not be divisible in a multitasking system. BOLT Contrary to semaphores, boltvariables allow a reading access to data of several tasks and at the same time block writing tasks, when another task has started a reading- or writing access (<). Regarding the emulation of these variables without support of the operating system, the explanations to semaphores (<) apply analogously. /\