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diff between SDRAM and DDRAM

yes I know that this is repeat question. I have search EE but I am not satisfied with other answers. I know so much so far
DDR = Dual/Double Data Rate
SDRAM = Synchronous Dynamic RAM (in sync with CPU clock)
I know DDR SDRAM exists because it is in my machine right now. is there a "DDR DDRAM"? I believe there should exist one. what does the first 'D' stand for in DDRAM? I know it is not Dual Data Rate again because last four letters are for Dynamic Random Access Memory.

how do you calculate throughput by looking at the chip's number (PC100, PC133, PC3200, etc.)?

thank you
jaydutt
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jhshukla
Asked:
jhshukla
2 Solutions
 
f-kingCommented:
Hi,actually DDR SDRAM is also called DDRAM.

http://www.webopedia.com/TERM/D/DDR_SDRAM.html

In terms of the troughput question I hope this short description helps.

The PC133 SDRAM is designed for systems equipped with a 133 MHz frontside bus. It is used in many Pentium III B, AMD Athlon and Power Mac G4 systems.

The PC1600 DDR memory is designed for systems equipped with a 100 MHz (200 MHz in DDRmode) frontside bus. '1600' refers to the module's bandwidth (i.e. the quantity of data that it transfers in one second), of 1.6 Gb per second. The PC1600 memory has been largely replaced by the fully compatible PC2100 memory.

The PC2100 DDR memory is designed for systems equipped with a 133 MHz (266 MHz in DDR mode) frontside bus. '2100' refers to the module's bandwidth (i.e. the quantity of data that it transfers in one second), of 2.1 Gb per second. The PC2100 memory is used mostly in AMD Athlon, Pentium III and Pentium IV systems.

The PC2700 DDR memory is designed for systems equipped with a 166 MHz (333 MHz in DDR mode) frontside bus. '2700' refers to the module's bandwidth (i.e. the quantity of data that it transfers in one second), of 2.7 Gb per second.

The PC3200 DDR memory is designed for systems equipped with a 200 MHz (400 MHz in DDR mode) frontside bus. '3200' refers to the module's bandwidth (i.e. the quantity of data that it transfers in one second), of 3.2 Gb per second.


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WatzmanCommented:

DRAM = Dynamic Random Access Memory

SDRAM = Synchronous Dynamic Random Access Memory

DDR = Double Data Rate

Double data rate means that the data is transferred between the memory and the memory controller on both edges of the clock pulse.  Standard SDRAM (single data rate) transferrs data on only one edge of the clock pulse.  Thus, at the same clock speed, DDR SDRAM is twice as fast as SDRAM [SDRAM without the DDR prefix implies single data rate]

All DDR memory is also still SDRAM, even if, for brevity, it's not described as DDR SDRAM.  I've never seen DDRAM used before your post.

The bandwidth number (such as PC3200) is calculated by multiplying the number of transfers per second by the memory bus width (e.g. memory that is 32 bits wide is transferring 4 (8-bit) bytes per transfer).
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Glen A.IT Project ManagerCommented:
from a previous post of mine....

Well, first you should understand that DDR IS SDRAM.  Regardless of what anyone will tell you, or wants to argue, DDR is SDRAM.

However . . .  

DDR does stand for double data rate, as has been mentioned.  that's how it differs from standard SDRAM.   It also comes on a 184pin module whereas standard SDRAM comes on a 168pin module.  These modules are not interchangeable - ie: you cannot put 168pin SDRAM into a 184 DDR-SDRAM slot, or vice-versa.

Regular 168 pin SDRAM is typically available in PC66 (66Mhz), PC100 (100MHz) and PC133 (133MHz) varieties.

184 pin DDR-SDRAM is available in different configurations:  PC1600 (200MHz), PC2100 (266MHz), PC2700 (333MHz), PC3200 (400MHz), PC3500 (433MHz), PC3700 (466MHz) , PC4000 (500MHz) etc.

To explain further:

Double data rate (DDR) SDRAM does exactly what it sounds like, it doubles the rate of speed at which standard SDRAM can process data. This means DDR memory is roughly twice as fast as standard SDRAM.  The big difference between DDR SDRAM and standard SDRAM is that DDR reads data on both the rising and falling edges of the clock signal.  Standard SDRAM, or single data rate (SDR) SDRAM, only carries information on the rising edge of a signal.  Basically this allows the DDR module to transfer data twice as fast as standard SDRAM.  For example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz.

Regarding the PCxxxx spec:  (from cluboverclocker.com) The guys in suits who sit in their plush offices and do nothing but think up of ways to make us purchase their new technology have come up with a new way of naming DDR memory to make it "sound faster".  Don't be confused, DDR memory has not changed.  The suits have renamed the specs of DDR memory after the peak bandwidth, not the bus frequency.  Hang on, this is where it gets confusing!  PC1600 DDR is really PC200 DDR or PC100 DDR and PC2100 is actually PC266 DDR or PC133 DDR!  Why the change?  It's all because the 100MHz bus transfers two bits of data per wire per clock (double data rate), which yields 1600MB (1.6GB) per second over a 64 bit bus.  The same goes for PC2100 (PC133), 133MHz multiplied by two bits per clock multiplied by eight bytes equals 2100MB (2.1GB) per second, and so on...  Don't be fooled, DDR memory has not changed.  This all boils down to a sad attempt to make DDR memory sound more impressive when it actually runs on the same bus speeds as before.

And Watzman, although you haven't seen DDRAM used previously, it is a correct term, though seldom used by only some people in the industry.  You're quite correct though that MOST people, especially in North America, simply refer to it as DDR, or as DDR-SDRAM.  It's not commonly referred to as DDRAM.
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jhshuklaAuthor Commented:
30 points :  Hi,actually DDR SDRAM is also called DDRAM.
well, all three of you said the same thing but he got there first.

30 points : 184 pin DDR-SDRAM is available in different configurations:  PC1600 (200MHz), PC2100 (266MHz), PC2700 (333MHz), PC3200 (400MHz), PC3500 (433MHz), PC3700 (466MHz) , PC4000 (500MHz) etc.... and more

final quick question: Alhtough memories are rated for particular bus rates, can they operate at slower bus rates?

thank you all.
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WatzmanCommented:

Yes, they can operate slower without problems.  They can operate faster also, but faster operation (overclocking) is not guaranteed and may fail to work proerly, it may also result in overheating (possibly to the point of physical damage).

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Glen A.IT Project ManagerCommented:
Just to add to what Watzman said -  I concur that USUALLY you can operate memory spec'd for one frequency at another, but it depends on the system.  In some older systems spec'd for 168pin SDRAM of PC66 or PC100 configuration, I do find that some of the newer PC133 doesn't operate in the system properly.  This is due to several differing factors which I can go into further if you'd like, but suffice it to say that some of the newer higher density modules aren't compatible with some older systems.

Regarding running faster - overclocking the memory -  RAM doesn't generally overheat, (unless you've increased the voltage to the memory itself, in which case heat spreaders may be necessary) but it does tend to cause system instabilities when overclocked.

And glad we could help you,

AlbertaBeef
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