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Re: Newer laptops & CPU speed

Hello Joerg,

On Sat, Mar 19, 2005 at 10:29:28PM +0000, Joerg Sommer wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> > On Mon, 2005-03-14 at 16:01 +0000, Jochen Voss wrote:
> >
> >> From other people's posts I conclude that bogomips should approximately
> >> equal clock?
> >
> > It depends on the CPU, but on a 7447A, yes.
> Do you know why it is for i386 nearly the double of cpu clock? Or reverse,
> why is it only equal the clock on ppc?

I am no expert on this, but the delay loop in question is

	    cmpwi   0,r3,0
	    mtctr   r3
    1:      bdnz    1b

in file "arch/ppc/kernel/misc.S".  I do not yet speak powerpc
assembler, but I read bdnz as decrease (register r3?) and branch
backwards to label 1 if the result is not zero.  Probably a 7447A CPU
can do this within 1 CPU clock cycle?  I didn't check the loop for
i386 but maybe it consists of two instructions or of one which takes
two CPU clock cycles?

I hope this helps,

PS.: Probably the "MPC7450 RISC Microprocessor Family Reference
Manual" (downloadable from [1]) contains the timing information for
the MPC7447A processor, but it is hard stuff to understand after

[1] http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPC7447A&nodeId=018rH3bTdG8653

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