Computer Architecture and the FetchExecute Cycle Buses CPU

Computer Architecture and the Fetch-Execute Cycle Buses, CPU Clock & Ports

Learning Objectives Show understanding of how data are transferred between various components of the computer system using the address bus, data bus and control bus. Show understanding of how the bus width and clock speed are factors that contribute to the performance of the computer system. Show understanding of the need for ports, for example Universal Serial Bus (USB), to provide the connection to peripheral devices.

What's a bus? Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any transmission path. n A wire basically.

Types of Buses 1. Address 2. Data 3. Control

Control Address Control Data 1 Address 3 Control 2 Data Single headed arrow = Uni-directional Double headed arrow = Bi-directional Data 4

http: //en. wikipedia. org/wiki/File: Computer_system_bus. svg

Types of Buses 1. Address n Carries the address of a memory location. e. g. Address of location in memory from MAR. 2. Data n To carry data from one location to another in processor. e. g. from MDR to CIR 3. Control n Carries control signals around processor to synchronise the operation of the processor components. e. g. Memory read/write completed. n In reality there are many lines/wires and each line carries a different signal.

CPU Clock: n A timing device connected to the processor that synchronises when the fetch, decode execute cycle starts. Determined by the frequency of an oscillator crystal. n Measured in Hertz, which means 'per second'. e. g. 3. 4 GHz means it might be capable of processing 3, 400, 000 instructions per second! However, some processors can perform more than one calculation on each clock cycle, and processors from different manufacturers and using different architecture are often difficult to compare. Clock speed: n The number of Fetch-Execute-Decode-Reset cycles that are performed by the CPU per second. Note that this is not directly defined by the “number of clock ticks” per second (Hertz), as a processor may not be able to “keep up” i. e. execute that number of Fetch-Execute-Decode-Reset cycles per second. Also “heat” due to increased current is a restricting factor. n See “Increasing Processor Performance” next for more details.

Increasing Processor Performance Increasing the clock speed: n Limited by the time for the CPU to settle after each pulse, and by the extra heat created by the increased current flow. e. g. smarter chip designs, heat sinks, fans, and even liquid cooling. Increasing bus widths: n Increase the number of bits of information a bus can carry at one time (the number of wires in a bus). This can only be effective if the processor can process this number of bits at one time e. g. increase the “Word Size”.

I/O ports Allow I/O devices to be connected to the CPU without having to have specialist hardware for each one. Think about the USB port on your computer, you can connect Keyboards, Mice, Game pads, Cameras, Phones, etc. and they all connect using the same port. Control Bus: n When you plug in your USB key, after a few moments a screen pops up asking you what you want to do with it.

Plenary What's a bus?

What's a bus? Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any transmission path. n A wire basically.

Plenary Name the types of buses.

Types of Buses 1. Address 2. Data 3. Control

Plenary What do the different types of buses carry?

Types of Buses 1. Address n Carries the address of a memory location. e. g. Address of location in memory from MAR. 2. Data n To carry data from one location to another in processor. e. g. from MDR to CIR 3. Control n Carries control signals around processor to synchronise the operation of the processor components. e. g. Memory read/write completed. n In reality there are many lines/wires and each line carries a different signal.