12.3.2.4 explain how data is transferred between different components of a computer system through the address bus, data bus, and control bus

12.3.2.6 explain how the clock speed, word length and bus width affect the performance  

System bus (extended)

System bus (grade 11)

System bus is the main highway (a set of parallel wires) connecting the processor, main memory and I/O controllers.

The processor is connected to the main memory by three separate buses. When the CPU wishes to access a particular main memory location, it sends this address to memory on the address bus. The data in that location is then returned to the CPU on the data bus. Control signals are sent along the control bus.

Control bus

The control bus is a bi-directional bus (signals can be carried in both directions). The data and address buses are shared by all components of the system. Control lines must therefore be provided to ensure that access to and use of the data and address buses by the different components of the system does not lead to conflict.

The purpose of the control bus is to transmit command, timing, and specific status information between system components.

Control lines include:

• Memory Write: causes data on the data bus to be written into the addressed location:
• Memory Read: causes data from the addressed location to be placed on the data bus;
• Interrupt request: indicates that the device is requesting access to the CPU;
• Bus Request: indicates that a device is requesting the use of the data bus;
• Bus Grant: indicates that the CPU has granted access to the data bus;
• Clock: used to synchronize operations;
• Reset: initializes all components.

Data bus

The data bus, typically consisting of 8, 16, 32, 64 separate lines provides a bi-directional path for moving data and instructions between system components. The width of the data bus is a key factor in determining overall system performance. For example, if the data bus is 16 bits wide, and each instruction is 32 bits long, then the processor must access the main memory twice just to fetch the instruction.

Address bus

Memory is divided up internally into units called words. A word is a fixed-size group of digits, typically 8, 16, 32, or 64 bits, which a handled as a unit by the processor, and different types of processors have different word sizes. Each word in memory has its own specific address. 

When the processor wishes to read a word of data from memory, it first puts the address of the desired word on the address bus. The width of the address bus determines the maximum possible memory capacity of the system.

For example, if the address bus consisted of only 8 lines, then the maximum address it could transmit would be 255 or 11111111 (in binary), giving a maximum memory capacity of 256 (including address 0). A system with a 32-bit address bus can address 2³² (4 294 967 296) memory locations giving an addressable memory space of 4Gb.

The address bus is also used to address I/O ports during input/output operations.

I/O Controllers

An I/O controller is a device that interfaces between an input or output device and the processor. Each device has a separate controller that connects to the control bus. I/O controllers receive input and output requests from the processor and then send device-specific control signals to the device they control. They also manage the data flow to and from the device.

The controller is an electronic circuit board consisting of three parts:

• an interface that allows connection of the controller to the system or I/O bus
• a set of data, command, and status registers
• an interface that enables the connection of the controller to the cable connecting the device to the computer

An interface is a standardized form of connection defining such things as signals, the number of connecting pins/sockets, and voltage levels that appear at the interface. An example is a Universal Serial Bus (USB) connection, which can be used with many different peripherals.

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Ex. 1

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