Bandwidth issues....

[Lombardaga]

I've read on the web that graphics card with DDR RAM with is set to a speed of 183 MHz would have a bandwidth of 6 GB/s. Now can someone plz explain to me why this is so, and not around 3 GB/s???

 

[Soccerman]

DDR SDRAM doubles the data rate of normal SDRAM. the calculation of SDRAM on a Video card is typically 128bit bus X mhz rating / 8 for bytes.

so a Voodoo 3 3000 has a 128 bit bus, with 166mhz SDRAM. the amount of bandwidth available to it is: 2656 megs, or 2.6 Gigs/second.

a Geforce GTS also has 128 bits, and has 166mhz DDR SDRAM. DDR sends 128 bits on both the rising and falling edges of the clock.

therefor it has 5312 megs/second, or 5.3 gig's/second.

however with a Geforce MX, you have only a 64 bit bus, so in order to get good speeds, you need something like DDR SDRAM. the calculation for this is 64 X 166 / 8... 2656 megs/second, the same as the Voodoo 3 3000.

 

[Lombardaga]

OK, thx. But why is it that the forthcoming DDR II RAM (which will be able to quadruple the transfer rate compared to SDR RAM) with a 150 MHz bus speed will have a bandwidth as high as 6.4GB/s?? Can't u calculate the speed of the system RAM by taking the transfer rates of normal PC100 SDRAM (800MB/s) and multiply that with 6 (150MHz*4=600MHz and 100MHz*6=600Mhz) to get DDR II RAM's transfer rate???

 

[Lombardaga]

well....???

 

[Lombardaga]

plz reply someone that know...

 

[pm]

A common source of confusion is mixing up MHz (which is frequency - so 150MHz DDR2 is still 150MHz) with MB/s.

The thing you are missing is bus width. If the bus width is 64-bits then that's 8bytes per transfer. So you get 150MHz x 4 transfers per clock x 8 bytes per transfer = 4.8GB/s. As to why this doesn't equal 6GB/s - I can't answer that... maybe they are increasing the bus width?

 

[Soccerman]

sorry, I went to sleep!

DDR 2 is just DDR SDRAM running at 200mhz.

trust me on this one k? that's what it is. I read it from some webpage (I do waaayy too much on here to remember trivial things like that!), and it makes perfect sense. it even adds up to the numbers you got (6.4 gig/second).