Chapter10Rev05 - Chapter 10 (Revision number 5)...

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1 Chapter 10 (Revision number 5) Input/Output This chapter deals with interfacing peripherals to the CPU. In Chapter 4, we discussed interrupts, a mechanism for I/O devices to grab the attention of the processor. In this chapter, we will present more details on the interaction between processor and I/O devices, as well as different methods of transferring data among the processor, memory, and I/O devices. 10.1 Communication between the CPU and the I/O devices Although we started our exploration of the computer system with the processor, most users of computers may not even be aware of the processor that is inside the gadget they are using! Take for example the cell phone or the i-pod . It is the functionality provided by the i-pod that attracts a teenager to it. Where does the i-pod or the cell phone get its utility? It is from the input/output devices that the i-pod provides for interacting with it. Thus, knowing how I/O devices interact with the rest of the computer system is a key component to unraveling the “box”. Although there are a wide variety of I/O devices, their connection to the rest of the system is quite similar. As we have seen so far, the processor executes instructions in its instruction set repertoire. LC-2200 has no special instruction that would make it communicate with a CD player or a speaker directly. Then how does the i-pod play music? A special piece of hardware known as a device controller acts as an intermediary between an I/O device and the computer. This controller knows how to communicate with both the I/O device and with the computer. Figure 10.1 Relationship of device controller to other components Device Controller Input/Output Device Computer
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2 10.1.1 Device controller What is in the device controller? To make this discussion concrete, let us consider a very simple device, the keyboard . The device itself has circuitry inside it to map the mechanical action of tapping on a key to a binary encoding of the character that the key represents. This binary encoding, usually in a format called ASCII ( American Standard Code for Information Interchange) , has to be conveyed to the computer. For this information exchange to happen, two things are necessary. First, we need temporary space to hold the character that was typed. Second, we have to grab the attention of the processor to give it this character. This is where the device controller comes in. Let us consider the minimal smarts needed in the keyboard device controller. It has two registers: data register , and status register . The data register is the storage space for the character typed on the keyboard. What is the purpose of the status register? As the name suggests, it is an aggregation of the current state of information exchange of the device with the computer. With respect to the keyboard, the state includes:
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This note was uploaded on 11/25/2010 for the course CENG 100 taught by Professor Ceng during the Spring '10 term at Universidad Europea de Madrid.

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Chapter10Rev05 - Chapter 10 (Revision number 5)...

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