From: "Alexei A. Frounze" <dummy_addressee AT hotmail DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Reading MSR (Athlon multiplier)
Date: Thu, 18 Jan 2001 12:57:17 -0500
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

oh man, you're not understanding the issue at all. do you?
okay, you suuggest that I use the following code in order to compute the
overhead involved by "push eax/edx":
-------8<--------
mov ebp,128
l1:
rdtsc
push eax
push edx
rdtsc
pop eax
pop edx
add ebx,eax
adc ecx,edx
dec ebp
jnz l1:
shrd ebx,ecx,7
-------8<--------

now notice that i have this overhead every time the loop is executed (see my
source). e.g. i always have a value given by RDTSC a little bit
greater/lower/whatever, but this "overhead" is always constant in the loop.
now, if i subtract two values I got from RDTSC withing 2 consecutive loops,
I get a rid of this overhead. I do not have to take in account this
overhead.
okay, if you still don't get me, let's go for a simple example...
let's imaging we have a ruler and a pen, we want to measure (length of a
pen):
0   1   2   3   4   5   6    7    8   9   10 11 12 13 14 15
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|  <-- ruler
<------------------------------------>                       <-- pen
from this you can see that pen length is 11 units (cms, I guess)
let's imaging a new picture:
0   1   2   3   4   5   6    7    8   9   10 11 12 13 14 15
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|  <-- ruler
           <------------------------------------>            <-- pen
same pen shifted 2 units left. the pen length remains the same, though now
we should calculate 13-2=11 units (as opposed to 11-0=11 units)
this is what I'm talking about. the analogy is direct. and if you still
don't get it, just quit or come up when you finally get what I'm talking
about.

Good Luck
--
Alexei A. Frounze
alexfru [AT] chat [DOT] ru
frounze [AT] ece [DOT] rochester [DOT] edu
http://alexfru.chat.ru
http://members.xoom.com/alexfru/
http://welcome.to/pmode/

"Tom St Denis" <stdenis AT compmore DOT net> wrote in message
news:9471af$7r4$1 AT nnrp1 DOT deja DOT com...
> In article <946rg8$c61d6$1 AT ID-57378 DOT news DOT dfncis DOT de>,
>   "Alexei A. Frounze" <dummy_addressee AT hotmail DOT com> wrote:
> > it does not really matter. you know why? because the only thing we want
is
> > having that value from RDTSC after a certain period of time. it doesn't
> > matter what CPU does meanwhile (messes with kernel code or user task or
with
> > the stack you meantion). and even if you talk about the overhead, this
> > overhead simply shifts everything in time (e.g. adds the same constant
value
> > to the value from RDTSC). and when we compute the difference this
"overhead"
> > disappears. do you care about value of (say) (5-2) more than about value
of
> > (7-4)???
> > btw, if there was something like that overhead, it would have less
> > contribution to the measured frequency rather than contribution of timer
> > accuracy and BIOS code which increments the counter at 40h:6ch.
>
> well on my Athlon it takes about 10~15 cycles (I think) todo the following
>
> rdtsc
> mov [start],eax
> mov [start+4],edx
> call myfunc
> rdtsc
>
> myfunc:
>  ret
>
>
> So there is obviously some overhead in the code.  By subtracting out the
10
> cycles you get zero based RDTSC counts.
>
> Tom
>
>
> Sent via Deja.com
> http://www.deja.com/