What is the bus interconnection?

Introduction:

We know that a computer consists of a CPU, Main Memory MM, and I/O unit. For data to flow between these components we need some kind of interconnections, which is another very important component of the overall computer architecture.

These components are interconnected by using a set of parallel lines to transmit the data from one location to another. Each of these lines can be used to transfer a sequence of bits from one component to the other component for data transfer.

Definition:

Each of these lines can be used to transfer a sequence of bits for communication from one component of the computer to the other component. This set of parallel lines is called a BUS. The system interconnection is used to communicate channel.

System Bus:

The bus used to connect the main components of a computer to the other component is called the system bus. Generally, the purpose of computers has a 70-100 line system bus. 

The system bus is divided into three main categories which are as follows.

Control bus

These lines are used to transmit different commands from one component to the other components for sending and receiving data. For example, if the CPU wants to read data from the main memory; it will use the control bus to send the memory rea command to the main memory of the computer. The control bus is also used to transmit any kind of data other control signals like ACKS(Acknowledgement signals) For example when the CPU gives a command to the main memory for writing data,  the memory sends an acknowledgment signal to the CPU after writing the data successfully so that the CPU can move forward and perform some more actions. A few commonly used commands and their purpose are given as follows:

MEMORY WRITE: This command is used to write some data to a given location in the main memory.

MEMORY READ: This command is used to read some data from a given location in the main memory for transfer data

I/O WRITE: This command is used to write some data to a given output device.

I/O READ: This command is used to read some data to a given output device

BUS REQUEST: This command is used to request control on the bus so that the requesting device can use it to transmit data

BUS GRANT: This command is used by the bus controller to indicate the grant of the bus to a device for transfer data

TRANSFER ASK: This command is used to deliver information that the data was read by the devices.

Data bus

On the system bus, thirty-two(32) or sixty-four(64) lines are reserved to transfer data from one component to the other. These lines are commonly said to be data bus. A sixty-four(64)-line data bus can transfer 64 bits of data simultaneously so it is not difficult to see that the width of the data bus has a direct impact on the performance of the computer for transmitting all kinds of data.

Address bus

As we know that many components are connected to one another through the system bus so it is important to assign a uniqueID to each component that wants to communicate with another. it uses a few of the system bus lines to specify the destination component by using its address to transfer the data from one location to another. These lines are commonly said to be the address bus. Not only the address is used to identify different components of a system but it is also used to specify the different memory locations within the main memory.

For example; if the CPU wants to write some data at a location 9872 in the main memory it places the address of the main memory and location on the address bus. When the main memory sees its address on the address bus for working of communication it reads the data from the data bus and writes it to the specified location for communicating within the main memory to transfer data from one location to another location

As the number of components connected to the system bus increase more components will be trying to use the system bus simultaneously. This will slow down the computer as components will have to wait longer to get access to the bus to communicate and transfer data from one location to another location. . To solve this problem only the major components of the computer are connected to the system bus and the remaining components are connected to another bus usually known as the expansion bus. The expansion bus is connected to the system bus for communication.

The I/O Unit

The I/O Unit is another very important component of a computer. Nowadays we have many input/output devices like keyboard, mouse, disks, etc. All these devices are very different from one location to another in their organization. Also, these devices can handle different data transfer rates and support different data formats from one location to another. Because of all these differences, it is impractical to connect all these devices to the system bus. It is not sensible to require the Central Processing Unit to control these devices directly as they will take a lot of time and  Central Processing Unit will fill the system bus capacity to transfer data from one location to another.

To avoid these difficulties stem, a special hardware component Input or Output unit is used. Only the Input or Output unit is connected to the bus and the processor and all other devices are connected.

The I/O unit is responsible for keeping the track of states of different devices attached to it. It is also responsible for compensating the speed difference between the processor and the Input or Output devices for communicating device I/O devices. 

INTERRUPTS

Two main ways of transferring data from the peripherals devices into the computer.

Interrupts: In this I/O devices scheme, the processor transfer issues the command to the Input or Output devices.  When the devices get ready to communicate, they generate an interrupt signal for the processor working. On sensing this signal, the processor suspends all other processing and performs the I/O operation for communication.

The disadvantage of this scheme is that it reduces the overall performance of the processor.

DMA:

The second scheme is DMA. In this scheme, the processor issues the Input or Output command and then gets busy in some other useful task. The special hardware gets the data from the Input /Output devices and uses the system bus to place it in the main memory.