# Correlation between PCxxxx and xxxMhz

G

#### Guest

##### Guest

A friend and I were talking about RAM, and we were trying to figure
something out. With the DDR RAM, what exactly is the correlation
between the PC #'s (ie PC2700) and the Mhz (ie 333 Mhz) Why not just
refer to them by Mhz like before? Just to confuse us ? I noticed
that if you divide the PC # by the Mhz it is in the vicinity of 8, but
it's not always the same number.
Any help to lessen the confusion would be appreciated.
Thanx
-joltman

#### Bass

##### Distinguished

On 14 Jan 2005 12:20:04 -0800, "joltman" <joltman@geocities.com>
wrote:

>A friend and I were talking about RAM, and we were trying to figure
>something out. With the DDR RAM, what exactly is the correlation
>between the PC #'s (ie PC2700) and the Mhz (ie 333 Mhz) Why not just
>refer to them by Mhz like before? Just to confuse us ? I noticed
>that if you divide the PC # by the Mhz it is in the vicinity of 8, but
>it's not always the same number.
>Any help to lessen the confusion would be appreciated.
>Thanx
>-joltman

It refers to bandwidth and not speed.

G

#### Guest

##### Guest

On 14 Jan 2005 12:20:04 -0800, "joltman" <joltman@geocities.com>
wrote:

>A friend and I were talking about RAM, and we were trying to figure
>something out. With the DDR RAM, what exactly is the correlation
>between the PC #'s (ie PC2700) and the Mhz (ie 333 Mhz)

It depends on what type of memory you're talking about. For DDR you
simply divide by 16, ie PC3200 memory runs at 200MHz. Of course,
there are some approximations in the nomenclature, like if you divide
2700 by 16 you end up with a result of 168.75MHz instead of the actual
166.67MHz that PC2700 memory runs at.

> Why not just
>refer to them by Mhz like before? Just to confuse us ? I noticed
>that if you divide the PC # by the Mhz it is in the vicinity of 8, but
>it's not always the same number.

Why not use MHz? Perhaps because MHz isn't really accurate? PC3200
memory runs at 200MHz, but it uses double data rate clocking (hence
the name "DDR SDRAM") for 400MT/s. Given that the data path is
64-bits wide, that works out to 3.2GB/s of bandwidth.

With DDR2 things get even more confusing as some things are run as DDR
and others as QDR (more or less).

When you get right down to it though, a lot of it has to do with
marketing and a bit of history. See, when SDRAM was common Intel came
up with some specifications like PC100 that specified that the memory
could work with a 100MHz bus speed. However a bit later Rambus came
out with their RDRAM memory that used a totally different design
running at much higher clock speeds (but narrower buses), and they
called them PC600 and PC800. So, when it came time for DDR, a
competing technology to Rambus' RDRAM, the memory manufacturers (who
have no love for Rambus) decided to do a bit of one-upmanship and
called their memory designed for 133MHz DDR buses "PC1600".

Bigger is better right? Who cares if Rambus' PC800 and DDR PC1600
memory had the exact same bandwidth, the bigger number must mean that
DDR was twice as good?! Well, at least that's what the marketing
people wanted us to believe. In reality there WERE plenty of good
reasons to opt for DDR instead of RDRAM, but extra bandwidth was not
one of them and clock speed DEFINITELY wasn't one either.

-------------
Tony Hill
hilla <underscore> 20 <at> yahoo <dot> ca