System Bus
The system bus is a set of wires that act as the main pathway for data moving between all components on the computer mainboard.

The system bus is a wired connection within the mainboard that provides connections between all the components on the motherboard (CPU, Cache, RAM, I/O controller, adapter slots and drive controllers) and the I/O ports. The bus in most modern computers is actually composed of several other busses of different types specialized to specific functions.

  • Host Bus (Processor and RAM bus)
  • Video Bus (AGP, PCI-X)
  • I/O Bus (PCI and PCI-X)

I/O Bus Types

  • Industry Standard Architecture (ISA)
  • Extended ISA Bus (EISA)
  • Local Bus
    • VESA Bus - Video Electronics Standards Association
    • PCI Bus - Personal Computer Interconnect
      • PCI 1.0
      • PCI 2.0
      • PCI 2.1
      • PCI 2.2
      • PCI-X
      • Mini-PCI
      • PCI 2.3
    • AGP Bus
  • Hypertransport

Bus Speed

A simplified way of calculating bus speed is as follows:

Bus speed = Bandwidth = Clock Speed * Width

The system bus has a limited capacity to carry data between components on the mainboard. Clock speed determines how many transfer cycles occur in one second and the bus' width determines how many bits get sent during each transfer. The bus' clock speed is rated in hertz, where one hertz is one transfer cycle per second. The Commodor 64 transfers at the rate of 1 Hertz. A Pentium IV system with an 800Mhz bus has more than 800 million transfers per second. More transfers means more data is moved faster. Bus width is the number of bits the system bus can move simultaneously on each clock cycle.

The width of the bus determines how many bits get transfered in each cycle.

Accessing the Bus

  • Host Bridge
  • Primary and Secondary bus
  • Bus Arbitration
  • Interrupt Requests
  • IRQ Numbers


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