Re: [TCLUG:9929] Adding RAM for slow machine

[Chris Kesler <chris@pconline.com>]

On Tue, 9 Nov 1999, Thomas T. Veldhouse wrote:

> 
> > It really has nothing to do with your processor speed.  Your computer
> > slows down to do the swapping.  ...
> 
> That is not exactly true.  It is true for the older SIMMS and EDO, but SDRAM
> is faster with faster processor speeds.   Although, I can not give you
> quoted numbers, I believe on modern SDRAM, you will see improvements from
> 8ns to 6ns on SDRAM100 based upon processor speed. 2ns may not seem like
> much, but look at it in terms of percentage and you will see that it is a
> 25% improvement in access time.
> 
> Tom Veldhouse
> veldy@visi.com

Actually, RAM speed is independent of processor speed.  When you say that
you have 8 ns RAM, that means that, after you request a word from memory,
the data will be available to the bus within 8 ns.  It's not always the
case, but usually, the RAM bus operates at the same speed as your
front-side bus. So, if you're running at 100MHz FSB, your processor will
make a request for data, and it will be available on the bus within 8 ns. 
But, your processor will wait 10 ns before reading it. The 10 ns comes in
because 100 MHz == 100 million cycles/second.  If you calculate it out,
you see that every cycle, or every clock tick lasts 10 ns.

To go from MHz to ns, you take the reciprocal.

  1             1                     1
------- * --------------- s == 10 * ------ s == 10 ns
100 MHz     100 * 10^6               10^9

From that, you'd expect that you couldget away with using 10 ns RAM if
your bus operates at 100 MHz.  But, it requires an extra 2 ns for bus
overhead.  i.e. the RAM doesn't get the read request at exactly the same
time the processor puts the request on the bus.  It takes some time.

Now, if you simply put 6 ns RAM into your machine, it still takes 10 ns
for your processor to read it.  If you want to take advantage of the
faster RAM, you'll need to run at a faster bus speed.  For example, if you
set your bus speed to 133 MHz, the processor will read the bus 7.5 ns
after making the request.  That's where you get your 25 % performance
boost.

Chris