On 26 Oct 99, at 15:31, Bill J McMilleon wrote:

> To all,
>
> Many motherboards allow you to set the External Clock in the Bios
> settings so that you can have 66, 75, 85 100 and 133 bus speeds.
> Is this the bus speed for the cpu only, or does this also set bus
> speed for PCI and ISA slots too?
>
> In order for a motherboard to have PCI and ISA slots running at
> 100 mhz doesn't it have to have what is called 100mhz  "front side
> bus" capabilities?
>
> What exactly is meant by the term "front side bus"?

  The FSB is the base speed that the CPU multiplier is applied to to
determine the CPU speed.  It is also usually the cock speed used on
accesses to motherboard RAM, although this may not be true when using
SIMMs on some "Super 7" motherboards.

  The clock speeds of PCI are typically derived from the FSB, by
dividing it by 2 or 3.  66/2 = 33 and 100/3 = 33, and 33 MHz is the
designed speed of PCI slots.
  On many systems that provide these intermediate settings, 75 MHz
FSB gives 37.5 MHz PCI, and 83 MHz FSB gives 41.5 PCI.  Some
peripherals -- especially some Adaptec SCSI controllers -- do not
like these faster PCI speeds.
  133 MHz FSB is still very new.  I expect sch machines to produce a
33 MHz PCI clock by dividing the FSB by 4.
  Some boards support "asynchronous" PCI timing, which keeps the PCI
bus at 33 MHz even when this is not a simple fration of the FSB.

  ISA speed is generally derived from PCI speed by dividing by 3, 4
or 5, to yield a frequency in the range of 8-10 MHz.  This will
typically appear in the CMOS setup as something like "PCI/4".

David G

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