Internal components, buses and von Neumann and Harvard architectures

Overview

Recall questions on the processor, main memory and I/O controllers, the address, data and control buses, addressable memory, and the von Neumann and Harvard architectures and the von Neumann bottleneck.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Name the basic internal components of a computer system.

   Model answer

   The processor, main memory, the address bus, the data bus, the control bus, and the input/output controllers that connect peripherals to the system. All of these are mounted on or attached to the motherboard, and the buses are the wires that carry signals between the rest.

2. 2

   What is the role of the processor?

   Model answer

   The processor is the component that actually runs programs. It repeatedly fetches machine code instructions from main memory, decodes them and executes them, performing arithmetic and logical operations on data and generating the control signals that direct every other component. Everything else in the machine exists to supply it with instructions and data or to accept its results.

3. 3

   What is the role of main memory, and how does it relate to the processor?

   Model answer

   Main memory holds the machine code instructions and the data of the programs currently running, in numbered locations that the processor can reach directly and quickly. The processor cannot execute a program from secondary storage: the program must first be loaded into main memory. The processor supplies an address on the address bus and the instruction or datum travels back along the data bus. Main memory is volatile, so its contents are lost when power is removed.

4. 4

   Understand the need for communication between components: why is a bus needed?

   Model answer

   The processor, memory and input/output controllers are physically separate, so signals have to travel between them. Wiring every component individually to every other would need an unmanageable number of connections and could not be extended. A bus solves this by providing one shared set of parallel lines that every component connects to, so any component can send to any other over the same wires, and a new device can be added simply by attaching it to the bus.

5. 5

   What is a bus?

   Model answer

   A bus is a set of parallel wires, or lines, that carries signals between the components of a computer. The number of lines is the width of the bus, and each line carries one bit at a time, so all the bits of an address or a data word travel simultaneously. Only one transfer may take place on a bus at any moment, so access to it has to be controlled.

6. 6

   Describe the address bus and what its width determines.

   Model answer

   The address bus carries the address of the memory location or input/output port that the processor wants to read from or write to. It is unidirectional: addresses travel out of the processor only. Its width determines how many different locations can be distinguished, because n lines give 2 to the power n distinct addresses. A 16-line address bus can therefore address 65,536 locations, and adding one line doubles the addressable memory.

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