Queries regarding Latency of RAM

[sainit]

Hello Everyone!!!

I have few queries like:

How decide about MHz. mentioned on RAM - like 512 MB 533 MHz, 256 MB 667 MHz.

What about Latency time like 4:4:4:8. What r these four figure here and how they effect any decision making.

Which one is best efficient Latency, MHz and Size of RAM suitable for Vista. In case I want to play games and also Graphical work on my PC.


Thanks

[deathvirus_me]

RAM Latency : A delay in transmitting data between a computer's RAM and its processor.

4-4-4-8 -> CAS-RCD-RP-RAS.

RAM timing are measured in CAS, RCD, RP, and RAS.

CAS refers to the amount of clock cycles to reach the correct column of memory,
RCD refers to the amount of cycles between RAS to CAS,
RP refers to the amount of cycles needed to close a row and open the next row for reading, and
RAS refers to the smallest number of clock cycles a row must be actively accessed.

To simplify that explanation, remember that RAM timings are measured in FSB clock cycles, so the lower the number, the faster your system is.

Now , here the twist ... ddr modules can push real low latencies , while ddr2 can't push to those unless ofcourse u can afford really good modules ... for normal ddr400 modules , 2.5,3,3,8 should suffice ... while for ddr2-667 modules , it should easily do 4,4,4,12 ... the later definitely offers better bandwidth at the cost of mem. latency .

Overall , u should be looking for ddr2-667 with 4,4,4,12 or ddr2-800 with 5,5,5,15 ... but ur CPU and graphics card , and the amount of RAM will actually make a more better impact on performance , than just the latency ...

[kooabs03]

i too didn't know that. thnx @deathvirus..

[janitha]

What about Command rate, 2T,1T etc?

[kooabs03]

and i was abt to think that now i know everything about RAM

[abhijangda]

this 4:4:4:8 are the delays of seconds for the decision making.

[deathvirus_me]

Originally Posted by janitha

What about Command rate, 2T,1T etc?

Here something nice i found googling around that gives a very good explanation :

When the MC (Memory Controller) first tries to access memory, it has to latch onto a memory bank, known as CS (Chip Select). Then it proceeds to find the column (CAS), the Row (RAS), and then return the data to the CPU. Now, 1T means it takes 1 clock cycle to "find" a memory bank, vs. 2T where it takes 2 clock cycles to "find" the memory bank.

But there's a sorta quirk, this only happens the first time data is attempted to be fetched from memory, and all subsequent accesses to that chip are done w/o delay, making the command rate null after the initial chip fetch.

Whether the chip select can be executed in a single clock or whether it needs two clocks, depends on a variety of factors. Among the most crucial contributing factors appears to be the number of banks populated within the system from which the correct bank has to be selected. In a single bank configuration, the system already knows that all data have to be within this bank. If more banks are populated, there is an additional decision involved. Translated, that means that the number of chips within the entire pool of system memory plays an important role in how fast the DRAM command can be executed. This is highly oversimplified but to spell it out, it means that a single DIMM with only 8 chips (single bank) is easier and faster decoded within the entire possible memory space than 2 DIMMs with 2 banks each.

Other factors involve the distance of the DIMM slot from the memory controller and, most importantly, the quality of the DIMM's PCB. If a 4-layer PCB with its relatively high noise level is used, there is a fat chance of ever hitting a 1T command rate. A 6 layer PCB on the other hand has a better signal to noise ratio and can greatly speed up the decoding (on the level of the PCB). It is, therefore, not surprising that, even if the same chips are used on different DIMMs, the differences in the PCB will dictate whether the module is able to perform at a command rate of 1T or 2T.

Now , for raw mem. benchmark performance , 1T will definitely be better than 2T at all times , with an diff. of almost 10% at times ... but in actual real life performance , the performance diff. is hardly 2-3% ... plus , 2T can esure much better stability at times , particularly if u oc ur RAM ...

[janitha]

Thanks.