DDR RAM questions

[Guest]

I swear, these are my last questions. Thanks for the patience toward an ignorant (but humble) newbie. People have told me repeatedly that it doesn't matter if the RAM is DDR, it has to do with the processor. However, other people have told me that the motherboard has to support DDR RAM also. What are the differences between motherboards which do and don't support DDR? Second, I have learned that PC133 RAM doesn't neccesarily have to go at 133 Mhz. So, is the 133 just give an indication of the maximum bandwith (in a less obvious way than, say, PC2100 RAM)? Thanks guys. You've been a big help so far.

 

[FUGGER]

PC100 SDRAM
PC133 SDRAM
PC1600 DDRAM 200Mhz FSB (basicly double PC100)
PC2100 DDRAM 266Mhz FSB (basicly double PC133)
RDRAM PC800 800Mhz (100Mhz FSB x8)
RDRAM PC1066 1066Mhz (133Mhz FSB x8)

This is laid out from slowest to fastest. Do not get confused with the "PCxxx" that is not always accrate to bandwidth or operating Mhz.

Your motherboard must support DDR. the processor does not care what RAM is used.

 

[juin]

Pc 1066 when it will be support.

 

[Crashman]

Here goes:
AMD has a DDR PROCESSOR bus, a.k.a "Front Side Bus" or FSB. This is the bus that goes from the processor to the northbridge of the CHIPSET. The chipset is the part on the motherboard that controls ALL devices attached to the motherboard. ALL AMD Athlon (including T-Bird and Duron) motherboards have this DDR FSB.

OK, so now you know that all these boards are running the FSB at DDR. Sometimes they refer to the actual clock rate (100 or 133), sometimes they refer to the "DDR clock equivalent", 200 or 266. It is important to remember that for the processor bus, 100 and 200 are interchangable, 133 and 266 are also interchangable.

Now on to memory. The pathway from the chipset to the memory, the memory bus, ALWAYS runs at either 100 or 133MHz. Newer chipset support DDR SDRAM. The rest support normal SDRAM. This has no affect on the processor bus, it is a separate bus. It is not connected to the processor directly, but to the chipset. As previously stated, the processor bus ALWAYS runs at DDR, but now you know that the MEMORY bus can run at either DDR or normal (SDR) rate.

DDR memory can support almost twice the bandwidth as SDR memory, but it is limmited by the chipset and the processor, it can only pass as much data as is fed to it or requested from it.

This leads to the PC133 question. PC133 has an maximum bnadwidth of 1066 MB/s. Half of PC2100. But 133 is a measurement of clock rate. Real clock rate. It is the maximum clock rate that the memory is garunteed to run at by the manufacturer. But it can also be used at slower speeds, such as 100 or even 66. If your Lamboarghini Diablo can go 204MPh, do you have to drive 204MPh?
Memory speed is set by the chipset. You can use PC133 on a PC100 motherboard or even a PC66 motherboard. It simply runs at whatever rate it is set at. It NEVER exceeds the speed it is set at. So PC133 MUST run at 100MHz if it is set at that speed.

Example: if you were in tight traffic, you would drive the same speed as everyone else. If they were driving 66km/h, you would drive 66km/h. If they were driving 100km/h, you would drive 100km/h. If they were driving 133km/h, you would drive 133km/h. It doesn't matter if your car COULD go 300km/h, if you can't pass trafic, you go the same speed as them.

Cast not thine pearls before the swine

 

[Guest]

"PC133 has an maximum bnadwidth of 1050 bits per second. "

Wow... hold on, even a V90 modem can handle 56.000 bits per second. Imagine your memory bus being even slower ;-)

Lets see.. 133 Mhz is 133.000.000 clock ticks / sec. Each tick, 64 bits can be transferred, right ? That gives us 133.000.000 x 64 bits /second or 8.512.000.000 bits. or about 1 Gigabyte per second. 266 DDR memory would double that to 2 Gbyte.

Now, its not only about bandwith. Latency is also important. Whenever a cpu requests a memory segment, there is some delay before the data can be read. SDRAM has low latency (especially at CAS 2). DDR increases this latency, effectively making a PC1600 (100/200 Mhz DDR memory) about as fast as PC133 SDR in most apps. PC2100 is faster though. If you want to know by how much, go and read some articles. In my opinion, anything below 1 Ghz doesnt require DDR Ram. The performance delta is too small, the price too high. It would be cheaper to buy a cpu that is 10% faster. Once you exceed 1.2 Ghz, PC2100 starts making sense. Especially if you want to upgrade later on. PC1600 makes no sense whatsoever.

---- Owner of the only Dell computer with a AMD chip

 

[Crashman]

I meant Mega bits per second, which would be 1 Gigabit per second. I did't do the math, I just divided PC2100 by 2. But my math shows it's actually 1066.66...MegaBytes per second, so other than a typo (I forgot the Maga), I should have said BYTES, for that I will apologies. It's 133+1/3 BTW, I know all about Cas Latency, and I could probably write a book on it. Thanks for catching the typo (Mega missing) and the mistake (byte vs. bit, I just wasn't concentrating). As far as the rest goes, I don't think there's much you could tell me.
As for the math, it's (133+1/3)x64/8 = (1066+2/3).

Cast not thine pearls before the swine