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I don't get it.

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John Jr

Member
Joined
Apr 30, 2002
Location
Florida
I am kinda new to the understanding of the various types of DDR memory. I have read alot of the posts here but fail to understand the total picture. Please excuse my ignorance for a moment.
What is DDR 333 or DDR 400? There are so many PC3200 or PC 2700 or PC 2100. How do I know which is 333 or 266 or 400? The PC xxxx value is for performance? :eek:
 
The pc xxxx corresponds to the rated speed. Pc 3200 (ddr400) will run at 200mhz front side bus. Pc 2100 will run at 133. Pc 2700 will run at 166.
Higher memory (pc 3200 and up) Is only good if you are overclocking. Putting pc3200 memory in a system running 133 front side bus will give no performance advantage over pc2100.
 
pip said:
you are talking about athlon systems though, p4 systems can easily run RAM rated at up to an effective 533 MHZ if it's a B

The 533 in reference to P4's is the front side bus, not the ram. The fsb is quad pumped, and operates at 133MHz. Since it is doing four things per clock cycle this is an effective 533MHz data rate. This tells us absolutely nothing about the ram, just the bus that the cpu uses to interface with the rest of the system.

DDR employs the same concept though. It does two things per clock cycle. It transfers data on both the rise and fall of the clock cycle. DDR, or dual data rate, thus effectively doubles the throuput at a given data rate. At least in theory, nothing is perfectly efficient. But it comes very close.

PCxxxx ratings are a somewhat vague description of the speed of the ram. Here is what they translate to in more usefull terms:

PC1600-100MHz DDR (200MHz effective data rate)
PC2100-133MHz DDR (266MHz EDR)
PC2700-166MHz DDR (333MHz EDR)
PC3200-200MHz DDR (400MHz EDR)
PC3500-216.5MHz DDR (433MHz EDR)

The PC number is supposed to indicate throughput, but this is more marketing psuedo information than reality. The only thing fixed is the speed the ram is capable of operating at, not the realized throughput. The actual throughput yielded is dependant on a dyzzying array of factors, inlcuing (but not limited to) clock rate and the efficiency and latency of the memory subsystem as a whole, not simply the type of module being utilized. The PC number is like calling a given gasoline 500HP because it is possible to power a 500HP engine with it. The potential for the PC speed is there, but practical realities erode the realized performance to a level beneath it. While 3200MB/s is the theoretical throuput of 200/400MHz DDR, rest assured the actual realized thougput is substantially less.

SDR sdram was rated in a more truthful manner. PC100 runs at 100MHz, PC133 at 133. When RDRAM was introduced it was called PC800, so the SDRAM faction had to invent its own big numbers.

Note that only the 1600-2700 grades are officially defined standards. The PC3200 grade's official status was declared dead, but is now being revived since Intel has announced it will be using 200/400MHz DDR for its upcoming 200/800MHz fsb chipsets. No company moves the market by the weight of its decisions like Intel does.
 
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