// We enter as oldThread, but we return as newThread.
// Returns with newThread's registers and stack.
void thread_switch(oldThreadTCB, newThreadTCB) {
    pushad;                  // Push general register values onto the old stack.
    oldThreadTCB->sp = %esp; // Save the old thread's stack pointer.
    %esp = newThreadTCB->sp; // Switch to the new stack.
    popad;		     // Pop register values from the new stack.
    return;
}

void thread_yield() {
    TCB *chosenTCB, *finishedTCB; 

    // Prevent an interrupt from stopping us in the middle of a switch.
    disableInterrupts();

    // Choose another TCB from the ready list.
    chosenTCB = readyList.getNextThread();
    if (chosenTCB == NULL) {
        // Nothing else to run, so go back to running the original thread.
    } else {
        // Move running thread onto the ready list.
        runningThread->state = #\readyThreadState#;
        readyList.add(runningThread); 
        thread_switch(runningThread, chosenTCB); // Switch to the new thread.
        runningThread->state = #\runningThreadState#;
    } 

    // Delete any threads on the finished list.
    while ((finishedTCB = finishedList->getNextThread()) != NULL) {
        delete finishedTCB->stack;
        delete finishedTCB; 
    }
    enableInterrupts();
}

// thread_create must put a dummy frame at the top of its stack: 
// the return PC and space for pushad to have stored a copy of the registers.
// This way, when someone switches to a newly created thread,
// the last two lines of thread_switch work correctly.
void thread_dummySwitchFrame(newThread) {
    *(tcb->sp) = stub;      // Return to the beginning of stub.
    tcb->sp--;
    tcb->sp -= SizeOfPopad;
}