INPUT OUTPUT MANAGEMENT

Input and output management: 

• So far we have studied how resources like processor and main memory are managed. We shall now examine the I/O management. Humans interact with machines by providing information through IO devices. Also, much of whatever a computer system provides as on-line services is essentially made available through specialized devices such as screen displays, printers, keyboards, mouse, etc. Clearly, management of all these devices can affect the throughput of a system. For this reason, input output management also becomes one of the primary responsibilities of an operating system.  In this chapter we shall examine the role of operating systems in managing IO devices. In particular, we shall examine how the end use of the devices determines the way they are regulated for communication with either humans or with systems.

Buffering concept:

• Single buffer: The device first fills out a buffer. Next the device driver hands in its control to the kernel to input the data in the buffer. Once the buffer has been used up, the device fills it up again for input.

• Double buffer: In this case there are two buffers. The device fills up one of the two buffers, say buffer-0. The device driver hands in buffer-0 to the kernel to be emptied and the device starts filling up buffer-1 while  kernel is using up buffer-0. The roles are switched when the buffer-1 is filled up.

• Circular buffer: One can say that the double buffer is a circular queue of size two. We can extend this notion to have several buffers in the circular queue. These buffers are filled up in sequence. The kernel accesses the filled up buffers in the same sequence as these are filled up. The buffers  are organized as a circular queue data structure, i.e. in case of output, the buffer is filled up from the CPU(or memory) end and used up by the output device, i.e. buffer n = buffer 0.

Spooling techniques:

• Spooling: Suppose we have a printer connected to a machine. Many users may seek to use the printer. To avoid print clashes, it is important to be able to queue up all the print requests. This is achieved by spooling. The OS maintains all print requests and schedules each users' print requests. In other words, all output commands to print are intercepted by the OS kernel. An area is used to spool the output so that a users' job does not have to wait for the printer to be available. One can examine a print queue status by using lpq and lpstat commands in Unix