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synch.cc

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// synch.cc 
//      Routines for synchronizing threads.  Three kinds of
//      synchronization routines are defined here: semaphores, locks 
//      and condition variables (the implementation of the last two
//      are left to the reader).
//
// Any implementation of a synchronization routine needs some
// primitive atomic operation.  We assume Nachos is running on
// a uniprocessor, and thus atomicity can be provided by
// turning off interrupts.  While interrupts are disabled, no
// context switch can occur, and thus the current thread is guaranteed
// to hold the CPU throughout, until interrupts are reenabled.
//
// Because some of these routines might be called with interrupts
// already disabled (Semaphore::V for one), instead of turning
// on interrupts at the end of the atomic operation, we always simply
// re-set the interrupt state back to its original value (whether
// that be disabled or enabled).
//
// Copyright (c) 1992-1993 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.

#include "copyright.h"
#include "synch.h"
#include "system.h"

//----------------------------------------------------------------------
// Semaphore::Semaphore
//      Initialize a semaphore, so that it can be used for synchronization.
//
//      "debugName" is an arbitrary name, useful for debugging.
//      "initialValue" is the initial value of the semaphore.
//----------------------------------------------------------------------

Semaphore::Semaphore(char* debugName, int initialValue)
{
    name = debugName;
    value = initialValue;
    queue = new List;
}

//----------------------------------------------------------------------
// Semaphore::Semaphore
//      De-allocate semaphore, when no longer needed.  Assume no one
//      is still waiting on the semaphore!
//----------------------------------------------------------------------

Semaphore::~Semaphore()
{
    delete queue;
}

//----------------------------------------------------------------------
// Semaphore::P
//      Wait until semaphore value > 0, then decrement.  Checking the
//      value and decrementing must be done atomically, so we
//      need to disable interrupts before checking the value.
//
//      Note that Thread::Sleep assumes that interrupts are disabled
//      when it is called.
//----------------------------------------------------------------------

void
Semaphore::P()
{
    IntStatus oldLevel = interrupt->SetLevel(IntOff);   // disable interrupts
    
    while (value == 0) {                        // semaphore not available
        queue->Append((void *)currentThread);   // so go to sleep
        currentThread->Sleep();
    } 
    value--;                                    // semaphore available, 
                                                // consume its value
    
    (void) interrupt->SetLevel(oldLevel);       // re-enable interrupts
}

//----------------------------------------------------------------------
// Semaphore::V
//      Increment semaphore value, waking up a waiter if necessary.
//      As with P(), this operation must be atomic, so we need to disable
//      interrupts.  Scheduler::ReadyToRun() assumes that threads
//      are disabled when it is called.
//----------------------------------------------------------------------

void
Semaphore::V()
{
    Thread *thread;
    IntStatus oldLevel = interrupt->SetLevel(IntOff);

    thread = (Thread *)queue->Remove();
    if (thread != NULL)    // make thread ready, consuming the V immediately
        scheduler->ReadyToRun(thread);
    value++;
    (void) interrupt->SetLevel(oldLevel);
}

// Dummy functions -- so we can compile our later assignments 
// Note -- without a correct implementation of Condition::Wait(), 
// the test case in the network assignment won't work!
Lock::Lock(char* debugName) {}
Lock::~Lock() {}
void Lock::Acquire() {}
void Lock::Release() {}

Condition::Condition(char* debugName) { }
Condition::~Condition() { }
void Condition::Wait(Lock* conditionLock) { ASSERT(FALSE); }
void Condition::Signal(Lock* conditionLock) { }
void Condition::Broadcast(Lock* conditionLock) { }

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