Initial revision

doc/occam/p1

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