Threading eyebrows 1.1
n important task of an operating system is the allocation of the machines resources to the processes in the system. If a process needs to print its results, it must request that the operating system give it access to the printer's device driver. At this point, the operating system must decide whether to grant this request, depending on whether the printer is already being used by another process. If it is not, the operating system should grant the request and allow the process to continue; otherwise, the operating system should deny the request and perhaps classify the process as a waiting process until the printer becomes available. After all, if two processes were given simultaneous access to the computer's printer, the results would be worthless to both. To control access to the printer, the operating system must keep track of whether the printer has been allocated.
One approach to this task would be to use a flag, which in this context refers to a bit in memory whose states are often referred to as set and clear, rather than 1 and 0. A clear flag (value 0) indicates that the printer is available and a set flag (value 1) indicates that the printer is currently allocated. On the surface, this approach seems well founded. The operating system merely checks the flag each time a request for printer access is made. If it is clear, the request is granted and the operating system sets the flag. If the flag is set, the operating system makes the requesting process wait. Each time a process finishes with the printer, the operating system either allocates the printer to a waiting process or, if no process is waiting, merely clears the flag.
However, this simple flag system has a problem. The task of testing and possibly setting the flag may require several machine instructions. The solution to this problem is to insist that the task of testing and possibly setting the flag be completed without interruption. One approach is to use the interrupt disable and interrupt
