Karbosguide.com - Module 2b2.

About the System Bus


In this module, you can read about the following subjects, which add to our tour of the PC:

  • PC buses, an intro
  • The system bus
  • 66 MHz bus
  • 100 MHz bus

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  • Introduction to the PC buses

    The PC receives and sends its data from and to buses. They can be divided into:

  • The system bus, which connects the CPU with RAM
  • I/O buses, which connect the CPU with other components.

    The point is, that the system bus is the central bus. Actually, it connects to the I/O buses, as you can see in this illustration. It is not completely correct, since the architecture is much more complex, but it shows the important point, that the I/O-buses usually derive from the system bus:

    You see the central system bus, which connects the CPU with RAM. A bridge connects the I/O buses with the system bus and on to RAM. The bridge is part of the PC chip set, which will be covered in module 2c.


    3 different I/O buses

    The I/O buses move data. They connect all I/O devices with the CPU and RAM. I/O devices are those components, which can receive or send data (disk drives, monitor, keyboard, etc. ). In a modern Pentium driven PC, there are two or three different I/O buses:

  • The ISA bus, which is oldest, simplest, and slowest bus.
  • The PCI bus, which is the fastest and most powerful bus.
  • The USB bus, which is the newest bus. It may in the long run replace the ISA bus.

    The three I/O buses will be described later. Here, we will take a closer look at the PC's fundamental bus, from which the others are branches from.


    The system bus

    The system bus connects the CPU with RAM and maybe a buffer memory (L2-cache). The system bus is the central bus. Other buses branch off from it.

    The system bus is on the motherboard. It is designed to match a specific type of CPU. Processor technology determines dimensioning of the system bus. At the same time, it has taken much technological development to speed up "traffic" on the motherboard. The faster the system bus gets, the faster the remainder of the electronic components must be..

    The following three tables show different CPUs and their system buses:

    Older CPUs
    System bus width
    System bus speed
    8088
    8 bit
    4.77 MHz
    8086
    16 bit
    8 MHz
    80286-12
    16 bit
    12 MHz
    80386SX-16
    16 bit
    16 MHz
    80386DX-25
    32 bit
    25 MHz

    We see, that system bus speed follows the CPU's speed limitation. First at the fourth generation CPU 80486DX2-50 are doubled clock speeds utilized. That gives the CPU a higher internal clock frequency. The external clock frequency, used in the system bus, is only half of the internal frequency:

    CPUs in the 80486 family
    System bus width
    System bus speed
    80486SX-25
    32 bit
    25 MHz
    80486DX-33
    32 bit
    33 MHz
    80486DX2-50
    32 bit
    25 MHz
    80486DX-50
    32 bit
    50 MHz
    80486DX2-66
    32 bit
    33 MHz
    80486DX4-100
    32 bit
    40 MHz
    5X86-133
    32 bit
    33 MHz


    66 MHz bus

    For a long time all Pentium based computers ran at 60 or 66 MHz on the system bus, which is 64 bit wide:

    CPUs in the
    Pentium family
    System bus width
    System bus speed
    Intel P60
    64 bit
    60 MHz
    Intel P100
    64 bit
    66 MHz
    Cyrix 6X86 P133+
    64 bit
    55 MHz
    AMD K5-133
    64 bit
    66 MHz
    Intel P150
    64 bit
    60 MHz
    Intel P166
    64 bit
    66 MHz
    Cyrix 6X86 P166+
    64 bit
    66 MHz
    Pentium Pro 200
    64 bit
    66 MHz
    Cyrix 6X86 P200+
    64 bit
    75 MHz
    Pentium II
    64 bit
    66 MHz


    100 MHz bus

    The speed of the system bus has increased in 1998. Using PC100 SDRAM a speed of 100 MHz is well proven and the use of RDRAM will give us much higher speeds.

    However the rise from 66 MHz to 100 MHz has the greatest impact on Socket 7 CPUs and boards. In the Pentium-II modules 70-80% of the traffic is inside the SEC module, holding both L1 and L2 cache. And the module has its own speed independent of the system bus.

    With the K6 the increase of system bus speed gives a vastly improved performance since the traffic between L1 and L2 cache crosses the system bus.

    133 MHz

    Intel's 820 and 815 chipsets to be used with Pentium III work with 133 MHz RAM as well as several VIA chipsets do.

    In AMD's Athlon the system bus architecture was changed; it is not really a system bus any longer. Hence Athlon chipsets may work with many types of RAM.

    Processor
    Chip set
    System bus speed
    CPU speed
    Intel Pentium II
    82440BX
    82440GX
    100 MHz
    350, 400, 450 MHz
    AMD K6-2
    Via MVP3ALi Aladdin V
    100 MHz
    250, 300, 400 MHz
    Intel Pentium II Xeon
    82450NX
    100 MHz
    450, 500 MHz
    Intel Pentium III
    i815
    i820
    133 MHz
    600, 667 MHz and up
    AMD Athlon
    VIA KT133 and others
    200 MHz
    600 - 1000 MHz

    With the 100 MHz bus, we dicovered that motherboards have to be well constructed with good power supply and many capacitors.

    Newer buses

    As mentioned under AMD Athlon, "system bus" is not that relevant a term looking at modern motherboards. The bus to RAM becomes separated from the other buses and this design opens up for better bandwidth between the CPU and the RAM.

    Intels use of Rambus RAM working at 400 MHz as well as PC2100 RAM on non-Intel boards follows this trend.

    The DDRAM operates with interfaces working at 200, 266 and 333 MHz.


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    Learn more

    Read more about the motherboards chip set in module 2d

    Read more about RAM in module 2e

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