SDRAM, PC100, PC133, and DDR

We all know that, when it comes to memory, that SDRAM is the way to
go. It is faster than EDO RAM, and supports higher bus speeds. EDO RAM
is moving into the older systems, mainly, while even the bargain PC’s
make the move to SDRAM.

But, the world of SDRAM is not cut
and dry. Standard SDRAM is great for “older” boards. Now,
with the release of BX motherboards, and the Super 7 boards, standard
SDRAM begins to cause problems. Why? Because even though it was
originally said that SDRAM could go up to 100MHz, it really couldn’t.
In fact, some SDRAM even got unstable at the 83MHz bus speed.

Enter PC100. Basically, PC100 is
SDRAM which meets a certain specification to work with stability at
100MHz. This SDRAM usually operates at 10ns, although some is created
that is faster. Since the only qualification for PC100 is the ability
to operate at 100MHz, there is no rule as to the access time. 10ns is
the minimum speed for stability at 100MHz. some companies advertise
PC100 faster than this, say 6ns, but, a lot of times you will find
this to be inaccurate.

All PC100 is not equal. While it all
operates at 100MHz, when you get into higher bus speeds than that, the
high-quality stuff starts to stand out. The reason is that the latency
rating of the higher quality stuff is lower. The latency is a
measurement of how long it takes for other hardware to return data to
the RAM. The lower the latency rating, the better the chip, and the
faster it will operate.

The most common, and cheaper, type of
SDRAM chip uses GL or G8 chips. The “GL” or “G8”
will be seen on the actual SDRAM chips on the memory circuit board, so
you will know what you’re looking at. The GL’s use a CAS latency of 3,
which is pretty standard. The better stuff uses “GH” chips,
which uses a CAS latency of 2.

To operate at 100MHz or 112MHz bus
speeds, almost any of this PC100 will work. But, bump it up to 133MHz,
and you’ll need to get the better GH SDRAM with a CAS latency of 2.
Only with this will you get stable operation at such high front
side bus

Along with high quality PC100, one
must take notice of the printed circuit board on which the chips are
mounted. The quality of these boards, for the most part, is measured
in the amount of layers. You can equate this to thickness. Obviously,
the thicker the material of the board, the less chance you have of
damaging it, the longer it will last, and the less electrical noise
you will get. so, the more layers the better. Run-of-the-mill, cheap
SDRAM often used good quality chips, but the manufacturer would cut
corners by using lower quality PCB’s(Printed Circuit Boards). Often
they would use 4-layer PCB’s. Well, part of the PC100 spec is a
minimum of 6-layer PCB. this ensures a higher level of quality. But,
some manufacturers use even better PCB’s, such as 8-layer. Pay
attention to this. The more layers the better.

So, if you find yourself buying a
Super 7 or BX motherboard, you should pick up some PC100 SDRAM with
it. The older stuff will work, but, without PC100, you are stuck with
your new board’s slower bus speeds.

Basically, PC133 SDRAM
is another implementation of the same old SDRAM.  It’s basically
the same SDRAM from the days of the LX Chipset, the Pentium II 333MHz
processor, and the 66MHz bus.  The only difference between PC133
SDRAM and the others, is that the PC133 has a lower latency
than PC100 and PC66 SDRAM, which means it can run on a faster bus.

If you don’t already know,
PC133 SDRAM can run stably on a 133MHz bus, just as PC100 ran stable
on the 100MHz bus, and PC66 ran stable on the 66MHz bus.  PC133
SDRAM increases the total bandwidth available to the processor from
the memory, because it runs faster.  That is because it raises
the speed limit, so to speak, on the road between the Processor and
the RAM.  

Sometimes it easy to think of
the lines data moves between two computer components as roads. 
The road between the SDRAM and a current processor, like the Pentium
III, is 64bit, which can be thought of as a 64 lane highway. 
With older PC100 SDRAM, the speed limit on that road was 100MHz, which
means that during a second, 100 million bits moved though each lane on
the highway.  That’s 6.4 Billion bits, and as we all know, 8 bits
= 1 byte.  That means, that with older PC100 SDRAM, the processor
could get a maximum of 800MB per second.  With PC133 SDRAM, the
speed of that road is increased to 133.33 million bits on each lane
per second.  That translates into 8.533 bill bits.  Using
the same math above, that means the processor could get a maximum of
1060 MB per second (1.06GB) from the SDRAM.

More data, of course, means
better performance.  Your games will run faster, business
applications load faster, and even Windows boots faster.  Only
problem is that the performance increase isn’t all that much, and most
of the time it’s hardly
noticeable.  Possibly with new types of SDRAM which are trying to
compete with RamBus RAM, users will see a
much higher performance increase.

Double Data Rate
Well, there really is not much to say on this topic,  because
the topic is rather cut and dried. DDR RAM is Normal SDRAM that sends data both on the rising of
the clock cycle, and the falling of the clock cycle.  

Twice the sending
of the data, twice the data sent. Where standard 100 MHz SDRAM has an estimated 800 MB/sec data
transfer rate for a theoretical maximum, DDR is, not surprisingly,
twice that. No, we
don’t actually see that much bandwidth, but that is their
theoretical maximum (64 bit X 100 MHz = 800 MB/s). DDR SDRAM would be 1600 MB/s. Its just faster, and it will be cheaper than Rambus RAM, and
its currently supported by quite a few motherboard manufacturers.


  1. Michael Bratcher says:

    I just want to say, Excellent report on the different types of memory, reading this really blew my mind, and thanks, keep up the awesome work, god bless.

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