thread.h

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// thread.h 
//      Data structures for managing threads.  A thread represents
//      sequential execution of code within a program.
//      So the state of a thread includes the program counter,
//      the processor registers, and the execution stack.
//      
//      Note that because we allocate a fixed size stack for each
//      thread, it is possible to overflow the stack -- for instance,
//      by recursing to too deep a level.  The most common reason
//      for this occuring is allocating large data structures
//      on the stack.  For instance, this will cause problems:
//
//              void foo() { int buf[1000]; ...}
//
//      Instead, you should allocate all data structures dynamically:
//
//              void foo() { int *buf = new int[1000]; ...}
//
//
//      Bad things happen if you overflow the stack, and in the worst 
//      case, the problem may not be caught explicitly.  Instead,
//      the only symptom may be bizarre segmentation faults.  (Of course,
//      other problems can cause seg faults, so that isn't a sure sign
//      that your thread stacks are too small.)
//      
//      One thing to try if you find yourself with seg faults is to
//      increase the size of thread stack -- ThreadStackSize.
//
//      In this interface, forking a thread takes two steps.
//      We must first allocate a data structure for it: "t = new Thread".
//      Only then can we do the fork: "t->fork(f, arg)".
//
// Copyright (c) 1992-1996 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.

#ifndef THREAD_H
#define THREAD_H

#include "copyright.h"
#include "utility.h"
#include "sysdep.h"

#include "machine.h"
#include "addrspace.h"

// CPU register state to be saved on context switch.  
// The x86 needs to save only a few registers, 
// SPARC and MIPS needs to save 10 registers, 
// the Snake needs 18,
// and the RS6000 needs to save 75 (!)
// For simplicity, I just take the maximum over all architectures.

#define MachineStateSize 75 


// Size of the thread's private execution stack.
// WATCH OUT IF THIS ISN'T BIG ENOUGH!!!!!
const int StackSize = (8 * 1024);       // in words


// Thread state
enum ThreadStatus { JUST_CREATED, RUNNING, READY, BLOCKED };


// The following class defines a "thread control block" -- which
// represents a single thread of execution.
//
//  Every thread has:
//     an execution stack for activation records ("stackTop" and "stack")
//     space to save CPU registers while not running ("machineState")
//     a "status" (running/ready/blocked)
//    
//  Some threads also belong to a user address space; threads
//  that only run in the kernel have a NULL address space.

class Thread {
  private:
    // NOTE: DO NOT CHANGE the order of these first two members.
    // THEY MUST be in this position for SWITCH to work.
    int *stackTop;                       // the current stack pointer
    void *machineState[MachineStateSize];  // all registers except for stackTop

  public:
    Thread(char* debugName);            // initialize a Thread 
    ~Thread();                          // deallocate a Thread
                                        // NOTE -- thread being deleted
                                        // must not be running when delete 
                                        // is called

    // basic thread operations

    void Fork(VoidFunctionPtr func, void *arg); 
                                // Make thread run (*func)(arg)
    void Yield();               // Relinquish the CPU if any 
                                // other thread is runnable
    void Sleep(bool finishing); // Put the thread to sleep and 
                                // relinquish the processor
    void Begin();               // Startup code for the thread  
    void Finish();              // The thread is done executing
    
    void CheckOverflow();       // Check if thread stack has overflowed
    void setStatus(ThreadStatus st) { status = st; }
    char* getName() { return (name); }
    void Print() { cout << name; }
    void SelfTest();            // test whether thread impl is working

  private:
    // some of the private data for this class is listed above
    
    int *stack;                 // Bottom of the stack 
                                // NULL if this is the main thread
                                // (If NULL, don't deallocate stack)
    ThreadStatus status;        // ready, running or blocked
    char* name;

    void StackAllocate(VoidFunctionPtr func, void *arg);
                                // Allocate a stack for thread.
                                // Used internally by Fork()

// A thread running a user program actually has *two* sets of CPU registers -- 
// one for its state while executing user code, one for its state 
// while executing kernel code.

    int userRegisters[NumTotalRegs];    // user-level CPU register state

  public:
    void SaveUserState();               // save user-level register state
    void RestoreUserState();            // restore user-level register state

    AddrSpace *space;                   // User code this thread is running.
};

// external function, dummy routine whose sole job is to call Thread::Print
extern void ThreadPrint(Thread *thread);         

// Magical machine-dependent routines, defined in switch.s

extern "C" {
// First frame on thread execution stack; 
//      call ThreadBegin
//      call "func"
//      (when func returns, if ever) call ThreadFinish()
void ThreadRoot();

// Stop running oldThread and start running newThread
void SWITCH(Thread *oldThread, Thread *newThread);
}

#endif // THREAD_H

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