64-bit vs. 128-bit DDR

[SocrPlyr]

i just so happened to be browsing throught he geforce ti article and i noticed that it uses 64-bit DDR while the older cards and the ati cards use 128-bit DDR...
my question is Is 64-bit DDR memory cheaper? i know that DDR for main memory is 64-bit as well... so could we see that scale all the way up to those speeds we are seeing(somewhat ignorng DDR-II)? Does it cost more to make 128-bit or is it justa different setup?

i don't know really what i am trying to say here but i just wanted to know ur guys' input on this... hopefully you can figure out what i am asking...

thanks

Josh

 

[aka1nas]

The memory is the same, it's the bus that is 64 or 128 bit. A 64 bit bus with DDR cuts the bandwith in half, effectively making it Sdram as far as performance. Even worse, DDR has just a tad more latency that SDRAM, so a 64 bit DDR bus may actually be a little slower than 128 bit Sdram. These were mainly on the Geforce2 MX cards. The DDR basically ends up being a marketing gimmick on these cards.

 

[SocrPlyr]

that is what i thought... i was just wondering if there is a higher cost w/ the 128-bit... cuz it just seems to be a little more complex... and really what i was trying to get at was the fact that the same type of ram is used as system memory change the cost at all (since it is the same tech)

Josh

 

[Guilty]

128 has a higher pin count and therefore is more expensive to do on a board. 64bit DDR may be castrated but it cuts costs.

 

[RaynorWolfcastle]

I think what Nvidia does is use two 64-bit DDR channels to the crossbar memory controller. This has advantages though I'm not sure what they are . I recall something about increased efficiency but don't hold me to that.

-Ice

 

[Mears]

Wait, I'm confused. Main memory uses 64bit busses, except the NFORCE which uses two 64 bit busses. So why is DDR faster than SDR as far as main memory is concerned?

 

[RaynorWolfcastle]

DDR means it sends data on both the rising edge and the falling edge of the clock signal, it is completely separate from the RAM's "bitness".

For example:
-a 100 MHz SDR 64-bit chip of RAM will send 64 bits of data every rising edge of the clock (100,000,000 times / second) (64*100,000,000 bits/sec)

-a 100 MHz DDR 64-bit chip of RAM will send 64 bits of data every rising edge and 64 bits every falling edge (100,000,000 times / second). This means that 64-bit DDR will have twice the bandwidth of SDR RAM (128*100,000,000 bits/sec). DDR *almost* acts as if you had SDR that's twice as fast.

-So a 200 MHz 128-Bit DDR chip will send (2*128*200,000,000 bits/sec)
- Two independant 200 MHz 64-bit DDR chips will send (2*2*64*200,000,000 bits/sec)

As you can see a 128-bit ~= 2*64-bit RAM chips. The difference is that Nvidia uses their crossbar memory controller to somehow increasy the 2*64-bit chips' efficiency above the 128-bit chip's efficiency. (I'm talking GF3 technology vs. Radeon technology here, the clockspeeds I gave here are lower than actual clockspeeds, they are only to give you an idea of what's going on)

A dual channel DDR-based memory subsystem will give you 4 times more bandwidth than a single-channel SDR based memory subsystem because throughput is doubled by the DDR nature then is oubled again because there are 2 independant channels.

I hope this helps you better understand what is actually going on

-Ice

 

[anthrax]

A graphix board made on a 64 bit DDR is cheaper than a 128bit DDR board for a simple reasons...
you need less connection on the board to each of the chips...(trace)...this reduced the amount of space needed on the PCB (Printed Circuit Board)..thus the board is cheaper to make.....
Of course, lowering the bus width halfs the perfomance........since DDR prices are very close the SDR prices....in budget cards, manfactures use 64 bit DDR instead of 128 bit SD ram...
u need less space on the PCS......

 

[Shudder]

If The GF line of budget cards used 128 bit DDR, the only thing holding you back from a full-out GF3 would be the clock speed, I imagine.

Somewhat crippling, but you either make it worse for the budget market or you don't have a budget market at all. Better to sell to the lower end and the higher end than just the higher end while your competition beats you silly on the low end.

I will agree though that having 64bit ddr opposed to 128 bit sdr is a marketing gimmick more than anything.