On 29 Mar 98 at 8:34, Wilson B. Vicentini wrote:

> I need information on bus arbitrations in Pentium. How is the
> relationship between the CPU clock   and the bus clock ? Who management
> that ? Is the chipset ? How does he make that?
> At once I thank the released attention.
> Wilson

  You can download .PDF files from Intel's web site that will tell
you more than you ever wanted to know about the various bus and
timing signals of the Pentium.

  I'll guess that maybe your question is a lot simpler than that, and
try to give you enough information to get by:

  There is a master timing signal on the motherboard that I'll call
CLOCK.  Most Pentium systems have this at 66MHz, but the P75 used 50
MHz and some earlier Pentiums used 60 MHz.  And some motherboards
support 75 Mhz, 83 MHz, and by now you may have heard talk from both
AMD and Intel about chipset and CPU support for 100 MHz motherboards.
  Generally, RAM and motherboard L2 cache memory are use timings
derived directly from the CLOCK signal.  It appears that video
buffers on AGP video cards may also be accessible at this speed, if
both card and motherboard support that.

  Pentium CPU speeds are determined by applying a multiplier to the
CLOCK frequency, anywhere from 1.0 to (so far) 5.0, typically in
steps of .5.  [No CPU implements all of the values in this range.]
L1 cache runs at this CPU speed; the PPro runs its on-chip L2 cache
at this speed as well -- the PII runs its at half this speed.
  Note:  AMD K5s and Cyrix CPUs may provide two speeds; the actual
CPU speed is typically de-emphasized in favour of a rating of
"equivalent Pentium speed".  For instance, my Cyrix 6x86-L is running
at 2.0x75MHz, which is obviously 150 MHz -- but except for the FPU,
this produces execution benchmarks similar to an Intel Pentium at
3.0x66MHz, or 200MHz, and so Cyrix sells it as "PR200+".

  The ISA bus works the other way:  the CLOCK is divided to produce
timings between 8MHz and 10MHz.  66/8 = 8.25 MHz is pretty typical.

  On many systems, PCI bus timings are also generated by division;
66/2 = 33MHz is typical.  When the CLOCK is set to 75 or 83 MHz, this
gives PCI speeds of 37.5 or 41.5 MHz, and some PCI boards don't like
that.  A *few* motherboards offer "asynchronous PCI", where the PCI
bus is locked at 33 MHz even if CLOCK is increased.
  It appears that the first generation of 100MHz motherboards will
run the PCI bus at CLOCK/3 = 33 MHz.

  If you need more detail, you should probably explore Intel's
"Developer's" site.  [Intel used to produce a quarterly CD of
documents, but decided to move them all to the web and discontinued
that program.]

David G