118 lines
2.2 KiB
Modula-2
118 lines
2.2 KiB
Modula-2
(*
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(c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.
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See the copyright notice in the ACK home directory, in the file "Copyright".
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*)
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(*$R-*)
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IMPLEMENTATION MODULE Semaphores [1];
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(*
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Module: Processes with semaphores
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Author: Ceriel J.H. Jacobs
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Version: $Header$
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Quasi-concurrency implementation
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*)
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FROM SYSTEM IMPORT ADDRESS, NEWPROCESS, TRANSFER;
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FROM Storage IMPORT Allocate;
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FROM random IMPORT Uniform;
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FROM Traps IMPORT Message;
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TYPE Sema = POINTER TO Semaphore;
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Processes = POINTER TO Process;
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Semaphore =
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RECORD
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level: CARDINAL;
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END;
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Process =
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RECORD next: Processes;
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proc: ADDRESS;
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waiting: Sema;
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END;
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VAR cp: Processes; (* current process *)
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PROCEDURE StartProcess(P: PROC; n: CARDINAL);
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VAR s0: Processes;
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wsp: ADDRESS;
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BEGIN
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s0 := cp;
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Allocate(wsp, n);
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Allocate(cp, SIZE(Process));
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WITH cp^ DO
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next := s0^.next;
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s0^.next := cp;
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waiting := NIL;
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END;
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NEWPROCESS(P, wsp, n, cp^.proc);
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TRANSFER(s0^.proc, cp^.proc);
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END StartProcess;
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PROCEDURE Up(VAR s: Sema);
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BEGIN
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s^.level := s^.level + 1;
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ReSchedule;
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END Up;
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PROCEDURE Down(VAR s: Sema);
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BEGIN
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IF s^.level = 0 THEN
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cp^.waiting := s;
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ELSE
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s^.level := s^.level - 1;
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END;
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ReSchedule;
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END Down;
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PROCEDURE NewSema(n: CARDINAL): Sema;
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VAR s: Sema;
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BEGIN
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Allocate(s, SIZE(Semaphore));
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s^.level := n;
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RETURN s;
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END NewSema;
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PROCEDURE Level(s: Sema): CARDINAL;
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BEGIN
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RETURN s^.level;
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END Level;
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PROCEDURE ReSchedule;
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VAR s0: Processes;
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i, j: CARDINAL;
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BEGIN
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s0 := cp;
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i := Uniform(1, 5);
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j := i;
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LOOP
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cp := cp^.next;
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IF Runnable(cp) THEN
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DEC(i);
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IF i = 0 THEN EXIT END;
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END;
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IF (cp = s0) AND (j = i) THEN
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(* deadlock *)
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Message("deadlock");
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HALT
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END;
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END;
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IF cp # s0 THEN TRANSFER(s0^.proc, cp^.proc); END;
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END ReSchedule;
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PROCEDURE Runnable(p: Processes): BOOLEAN;
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BEGIN
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IF p^.waiting = NIL THEN RETURN TRUE; END;
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IF p^.waiting^.level > 0 THEN
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p^.waiting^.level := p^.waiting^.level - 1;
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p^.waiting := NIL;
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RETURN TRUE;
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END;
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RETURN FALSE;
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END Runnable;
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BEGIN
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Allocate(cp, SIZE(Process));
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WITH cp^ DO
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next := cp;
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waiting := NIL;
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END
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END Semaphores.
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