Re: how is system geared down?

["Nathan M. Andelin" <nathanma@xxxxxxxxx>]

I'll offer another hypothesis.  This one is based on a little more on
research.  But admitedly, only a theory.

One component of IBM systems is known as the "system clock".  It provides a
signal that defines a base Mhz frequency used by other components, including
the CPU.  Actually, RAM, cache, bus, and I/O processor speed may all be
synchronized with it.

The PowerPC and other components use a circuit known as a "phase-lock-loop"
(PLL) which synchronizes the various components and subsystems with the
system clock.  The CPU has a maximum recommended MHZ frequency, but can
actually be configured to run at some multiple of the system clock.  The
ratio between system clock and CPU speed can range from one-to-one (1:1) up
through ten-to-one (10:1).  Some processors don't support that wide of a
range.  For example, a processor may have a maximum recommended speed of 200
Mhz and support PLL ratios of 1:1 - 4:1.  If the system clock is set at 50
Mhz, and the PLL ratio is set at 1:1, then the CPU runs at 50 Mhz.  If the
system clock is set at 50 Mhz and the PLL ratio is configured at 4:1, then
the CPU runs at 200 Mhz.

Most of this information is a synopsis of my understanding of documents that
can be found at www.chips.ibm.com, but you can also go to www.ibm.com and do
a search on SYSCLK, PLL, system clock, etc.

My theory is that Rochester uses the PPL configuration ratio to "gear down"
the CPU from its maximum recommended Mhz rating.

Nathan.


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