_INSIDE THE WINDOWS SCHEDULER_
by Matt Pietrek

Figure 1: 

        //------------------STARTUP SECTION-------------------
        Save registers on stack of outgoing task
        Update task profiles
        If ( TDB signature not OK)
            goto Walk_through_task_list
        Get DirectedYield field in TDB into AX; zero it out in TDB
        If ( DirectedYield hTask == 0 )
            goto Walk_through_task_list
        If ( event count in DirectedYield TDB != 0 )
            goto startup_this_task
        //---------------TOP HALF OF TASK WALK-------------
Walk_through_task_list:
        Point to the first task in the linked list of tasks
Try_next_task:
        if ( another task in list )
            goto Does_this_task_have_an_event?
        //---------------THE IDLE LOOP--------------------
        ShrinkHeap()            // In KRNL386 only
        DiscardFreeBlocks()    // In KRNL386 only
        IsUserIdle()
        if ( fPokeAtSegments != 0 )
            if ( USER is idle )
                Call routine to load a boot time module segment
                goto Walk_through_task_list
        INT 28h
        INT 2Fh, AX = 1689h
        goto Walk_through_task_list
        //-------------BOTTOM HALF OF TASK WALK----------------------
Does_this_task_have_an_event?:
        if ( event count field == 0 )
            point to next TDB
            goto Try_next_task
        //-------------TASK SWITCHING CODE------------------------
        // Found a potential task to switch to
        // Make sure it's OK to switch
        if ( found task == current task )
            goto Reschedule_done
startup_this_task:
        if ( there is a locked task, and it's not the current task )
            goto Reschedule_done
        If ( InDOS flag )
            goto Try_next_task
        // It's OK to switch tasks now
        Increment InScheduler flag
        Delete & re-insert task into TDB list to give it proper priority.
        Lock the global heap
        Save SS:SP in current TDB
        Call function that checks the following
             if ( 80x87 present )
                 save control word in outgoing TDB
             if ( Current disk not set in TDB )
                 Save the current DOS disk/directory in the TDB
        Call routine that sends task switch out notification
        Set new Windows current TDB value
        Set new Windows PDB value from the PDB value in the incoming TDB
        if ( 80x87 present )
            load control word from incoming TDB
        Call routine that sends task switch in notification
        Switch to SS:SP from incoming TDB
        Update the "current TDB" global variable
        Decrement InScheduler flag
        Tell the display driver that we've switched // In KRNL386 only
        Unlock the global heap
Reschedule_done:
        Restore registers from stack