31. What kind of buses does the System Bus include?

A system bus consists of from 50 to 100 separate lines, which can be classified into three functional groups: data, address and control lines (power lines are usually omitted ).

The data lines provide a path for moving data between system modules. Number of lines is referred as WIDTH of the data bus

Address Bus width determines maximum memory capacity of the system. Identify the source or destination of data. Used to address as the Main Memory, so I/O ports .

Control Bus. Is used to control the access to and the use of the data and address lines. Control and timing information (indicate validity of data and address information)

• Memory read/write signal

• Interrupt request

Typical control lines include:

• Memory Write

• Memory Read

• I/O Write

• I/O Read

• Transfer ACK

• Bus Request

• Bus Grant

• Interrupt request

• Interrupt ACK

• Clock

• Reset

31. What do we call the width of a bus?

Width of the data bus (the number of lines determines how many bits can be transferred at a time). Width determines maximum memory capacity of the system.

31. What operation does the control signal “I/o read” set?

I/O Read: Causes data from the addressed I/O port to be placed on the bus

The operation of any bus is as follows:

If one of the modules “wishes” to send data to another, it must do two things:

1. Obtain the use of the bus;

2. Transfer data through the bus.

If one of the modules “wishes” to receive data from the other module it must do:

1. Obtain the use of the bus;

2. Send request to the other module, by putting the corresponding code on the address lines after formation signals on the certain control lines.

31. What problems may arise, when only one (single) bus is used in a computer system?

    • Lots of devices on one bus leads to:

• Propagation delays (задержки распространения).

• Long data paths mean that co-ordination of bus use can adversely affect (неблагоприятно сказываться) performance (dynamic characteristics become worse).

• If aggregate data transfer approaches bus capacity the system’s work may become unreliable.

• Most systems use multiple buses organised by hierarchy principle to overcome these problems

Give examples of using multiple bus structures in computer systems and explain necessity of including each…

The use of multiple buses, each of which may connect only some certain modules (Expansion Bus, High-Speed Bus)

31. List and describe main generic types of buses.

Dedicated

• Separate data & address lines

Multiplexed

• Shared Lines

• Address valid or Data valid control lines

• Advantage – fewer lines

• Disadvantages

• More complex control

• Reduction in performance

Physically Dedicated

• The use of multiple buses, each of which may connect only some certain modules (Expansion Bus, High-Speed Bus)

• Advantage – high throughput (there is less bus connection)

Disadvantage – increased size and cost of the system.