Chapter 5: Buses

Introduction to buses: A Bus is a system that moves data from one source to another.
                                       First implementation was in early computing with a System Bus.

Why is it needed:
i) The purpose of buses is to reduce the number of "pathways needed for
   communication between the components, by carrying out all communication
  over a single data channel. This is why the "data highway" is sometimes used.

 ii) Synchronization between components

 iii) HIGH speed transfer between CPU/MEMORY

Characteristic:
Two metrics use to measure data transfer:
i) Total number of bits we can transfer in parallel.
This is called the width of the data

ii) The clock rate or frequency( in Hertz) of the bus.

 Each time data is sent or received is considered one cycle.
                     
Total transfer speed : Bandwidth
A bus with a width of 16 bits and a frequency of 133MHz,
therefore, has a transfer speed equal to:

• 16*133.106 = 2128*106/s
• or 212*106/8 = 266*106 bytes/s
• or 266*106/1000 = 266*103 KB/s
• 259.7*103/1000 = 266 MB/s

Internal bus: sometimes called the front-side bus or FSB for short or the system bus.

Expansion bus : sometimes called the input/output bus or the control bus.

Front Side Bus or System Bus

• A Standard CPU system bus is comprised of a Control bus, and Address bus and a Data bus.
• The FSB can range from speeds of 66MHz, 133Mhz, 100MHz, 266MHz, 400MHz, and up.

Most CPU today add a third bus known as an Expansion bus.

Expansion Bus

• Used to add additional expansion cards into the CPU.
• Comes in internal and external
• Common internal buses are PCI, PCI express and SATA
• Common external buses are USB, CAN and IEE 1394 (Firewire)