87 lines
3.1 KiB
Text
87 lines
3.1 KiB
Text
.NH
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Introduction
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.PP
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Occam [1] is a programming language which is based on the concepts of
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concurrency and communication. These concepts enable today's applications of
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microprocessors and computers to be implemented more effectively.
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.PP
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An Occam program consists of a (dynamically determined) number
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of processes communicating through channels.
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To communicate with the outside world some predefined channels are needed.
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A channel has only one writer and one reader; it carries machine words and
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bytes, at the reader/writer's discretion. The process with its communication
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in Occam replaces the procedure with parameters in other languages (there are
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no procedures in Occam).
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.PP
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In addition to the normal assignment statement, Occam has two more
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information-transfer statements, the input and the output:
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.DS
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.ft 5
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chan1 ? x -- reads a value from chan1 into x
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chan2 ! x -- writes the value of x onto chan2
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.ft
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.DE
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Both the outputting and the inputting processes wait until the other is there.
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Channels are declared and given names. Arrays of channels are possible.
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.PP
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Processes come in 5 varieties: sequential, parallel, alternative,
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conditional and repetitive. A process starts with a reserved word telling
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its nature, followed by an indented list of other processes. (Indentation
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is used to indicate block structure.) It may be preceded by declarations.
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The processes in a sequential/parallel process are executed sequentially/in
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parallel. The processes in an alternative process have guards based on the
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availability of input; the first to be ready is executed (this is waiting
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for multiple input). The conditional and repetitive processes are normal
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\fBIF\fPs and \fBWHILE\fPs.
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.PP
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\fIProducer-consumer example:\fP
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.DS
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.ft 5
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.nf
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CHAN buffer: -- declares the channel buffer
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PAR
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WHILE TRUE -- the producer
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VAR x: -- a local variable
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SEQ
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produce(x) -- in some way
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buffer ! x -- and send it
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WHILE TRUE -- the consumer
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VAR x:
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SEQ
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buffer ? x -- get a value
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consume(x) -- in some way
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.ft
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.fi
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.DE
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.bp
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.PP
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Processes can be replicated from a given template; this combines
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with arrays of variables and/or channels.
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.PP
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\fIExample: 20 window-sorters in series:\fP
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.DS
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.ft 5
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.nf
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CHAN s[20]: -- 20 channels
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PAR i = [ 0 FOR 19 ] -- 19 processes
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WHILE TRUE
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VAR v1, v2:
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SEQ
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s[i] ? v1; v2 -- wait for 2 variables from s[i]
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IF
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v1 <= v2 -- ok
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s[i+1] ! v1; v2
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v1 > v2 -- reorder
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s[i+1] ! v2; v1
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.fi
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.ft
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.DE
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.PP
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A process may wait for a condition, which must include a comparison
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with \fBNOW\fP, the present clock value.
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.PP
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Processes may be distributed over several processors; all processes
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under a \fBVAR\fP declaration must run on the same processor. Concurrency can be
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improved by avoiding \fBVAR\fP declarations, and replacing them by \fBCHAN\fP
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declarations. Processes can be allocated explicitly on named processors and
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channels can be connected to physical ports.
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