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					What is a bus?
 • Buses are defined not only by their
   placement but by their width.
 • The larger the bus width, the more
   information that can be transferred in a
   specific time period.
 • Example: A 32 bit data bus can transmit
   32 bits of data across the bus at any one
   time, whereas an 8 bit data bus can only
   transmit ¼ of the data.
Bus Width

 Bus width refers to the number of wires in the
  bus over which data can travel.
Buses
• There are two buses on every motherboard:
  - internal bus and
  - external bus.
Bus Speed
 • A computer or device's bus speed or
   throughput is always measured in bits per
   second or megabytes per second.
Internal Bus
• The Internal bus (also called the system bus)
  is the wiring that connects the CPU,
  memory, and all the other components on
  the motherboard.
• The internal bus is represented by the
  wiring on the bottom of the motherboard
Parts of the Internal Bus
  Part                      Function
Power     The power bus is the wires that provide
          electrical power to every part of the
          motherboard.
Control   The control bus sends out timing signals so that
          other components on the motherboard can stay
          in time with the CPU.
Address   The address bus is used for sending information
          on memory addressing. This information tells
          the components on the motherboard where to
          find instructions and data in memory.
Data      The data bus does exactly what it sounds
          like. It is responsible for transmitting the actual
          data between the system components.
External Bus
• The External bus (or expansion bus) connects all
  the peripherals and attachments to the
  motherboard.
• The most common form of external bus connector
  is the expansion slot.
• Types of expansion slots include:
  - 8-Bit, ISA, MSA,
       EISA, IDE, VLB,
       PCI, AGP


                            4 PCI Express bus card slots
• Every external bus slot carries a certain number
  of signals through it's copper pathways to the
  external bus.
• Some of these signals are used for power,
  some for IRQ requests, and some connect
  directly to the data and address bus.
Some important connections to
the external bus include:
•   Clock Signal
•   Interrupt Request (IRQ)
•   Direct Memory Access (DMA)
•   Input/Output Address (I/O)
•   Bus Mastering
Clock Signal
• The clock signal is used to provide timing
  information in order for the bus clock to
  synchronize it's transmissions with the system
  buses.
Interrupt Request (IRQ)
• Sometimes the external bus must carry a
  request to the CPU that needs urgent or special
  attention. This is called an Interrupt Request,
  or IRQ.
• Each external bus device is given an IRQ value
  so that the CPU knows which IRQ's are coming
  from which devices.
• IRQ's can be shared between devices, but only
  if the two devices NEVER attempt to use the
  Interrupt at the same time.
Direct Memory Access (DMA)
• DMA allows certain types of instructions to
  totally bypass the processor and access main
  memory themselves.
• Unlike IRQ's, two devices can not share a DMA
  channel, although current system buses provide
  more DMA channels than were previously
  available.
• Hard drives and video cards use DMA
  extensively, mainly because they have on-
  board processors that don't necessarily require
  CPU access.
Input/Output Address (I/O)
• As opposed to an IRQ sending an interrupt to
  the CPU, I/O addresses allow the processor to
  directly access the device.
• Each expansion port is identified by a unique
  I/O address, which lets the CPU send
  instructions directly to the device.
• The device then responds through it's DMA
  channel or the data bus.
• No two devices can share the same I/O
  address.
Bus Mastering
• Sometimes a device requires the ability to talk
  directly to another device.
• This is called bus mastering, and is controlled
  by a bus mastering chip installed directly on the
  expansion device.

				
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posted:6/23/2011
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