Intro to COA

Types Of Computer Buses

Different components of computers are connected through communication channels, known as computer buses, to transfer data between each other. Busses contain a set of wires where each wire can carry one bit. So, 8 wires can carry 8 bits at a time, and 64 wires can carry 64 bits in the bus at a time. Let’s explain the types of computer buses.

Types of Computer Buses

There are two types of computer buses

1. System Buses

The System bus connects the CPU and the main memory i.e. (RAM). System buses are also known as memory. System buses are of three types

  1. Address Bus: It carries memory addresses from the CPU to other components such as main memory and I/O devices. The address bus is unidirectional.
  2. Data Bus: It carries the data between the CPU and other components such as CPU, RAM, and I/O devices. The data bus is bidirectional.
  3. Control Bus: It carries control signals from the CPU to other components such as RAM and I/O devices. The control bus also holds the clock’s pulses. The control bus is unidirectional.

2. I/O Buses

I/O buses connect the CPU to peripheral devices such as USBs, Printers, etc. The working of buses is fully explained in the following diagram

Types of computer buses

When a user hits any keyword from the keyboard, the CPU generates an address of that particular key. The address of that key moves towards the input unit through the address bus, and then data of the corresponding address will move toward the monitor through the data bus.

Size of Computer Buses

If there are 224 memory locations and the size of Word is 2 bytes (16bits), then the Size of memory can be calculated as 

Size of memory = Total no. of locations x word size
Size of memory =  224 x 24 bits = 228 bits = 256MB

As there are 224 memory locations so, the size of the address register will be 24 bits because 24 bits can represent the 224 memory locations. As the size of the word is 16 bits, then at least 16 bits are required for data Bus.

If the size of the data bus or address bus is greater than its requirement, then the most significant bits will become zero. For example, As in the above example, at least 24 bits are required for the address bus, but if the size of the address bus is fixed to 32 bits, then the last 8 (most significant ) bits will become zero. In the way, if the size of the data bus is 32 bits, then, in the above example, the last 16 (most significant) bits will become zero.

North Bridge and South Bridges Design

The system bus and Input/output buses are connected through North and South bridges. These bridges usually exist on the motherboard. There are two main types

1. Northbridge

Northbridge is the chip that connects a CPU to those components where high-speed communication is required i.e. RAM, ROM, PCI bus, cache, and AGP activities. The Northbridge chip communicates with the CPU through the front-side bus (FSB). It is also known as the Memory Controller Hub.

2. Southbridge

It is the chip that connects a CPU to those components where no need for high-speed communication is required, such as audio, USB, PCI, ISA, IDE, BIOS, Ethernet, etc.  In simple words, all functions of a processor for I/O devices are taking place through Southbridge. The Northbridge ties the Southbridge to the CPU through an internal bus. The Southbridge is also known as I/O Controller Hub (ICH), and in AMD, it is known as Fusion Controller Hub (FCH).

Northbridge-Southbridge chipset architecture designs are given below

Northbridge Southbridge
Northbridge is connected directly to the CPU via the front-side bus (FSB)  Northbridge ties the Southbridge to the CPU through an internal bus. 
It facilitates the high-speed path to the CPU for communication with other components It facilitates the slow-speed path to the CPU for communication with I/O devices
Memory and Hard disk data go to the CPU using the Northbridge. That’s why the copying speed of data within the system is faster (Hard disk to hard disk) External data from (USB, CD ROM, Mouse, keyboard, etc. ) flows to the CPU using the Southbridge. That’s why the copying speed of data outside the system is slower (USB to hard disk)
It is usually larger than the Southbridge and closer to the CPU and memory. It is usually smaller than the Southbridge and a little bit away from the CPU and memory.

History of Computer Buses (Northbridge and Southbridge)

1. Later, the Intel “Sandy Bridge” CPU placed all functions of Northbridge onto the CPU in 2011.

2. Nowadays, modern motherboards are replacing the Northbridge and the Southbridge with IHA. And some chipsets are combining the Southbridge and Super I/O chips into a single chip. This combined chip is known as the Super Southbridge chip.

3. Some manufacturers i.e. SiS and NVIDIA, have combined all chips, such as the

  • The Northbridge,
  • The Southbridge
  • and the Super I/O

 Within a single chip.

In the next lecture, we will see Digital Computers