- Joined
- Dec 10, 2004
- Location
- The CircuitCity FireDog House
heres the link to the info
http://www.hardwaresecrets.com/article/167/4
to highlight the important parts
as we all seem to know Cas (the first timing number) is by far the most important in terms of memory performance. as highlighted here
"If we compare a DDR memory to a DDR2 memory running under the same clock, the one with the lower latency will be the fastest. Thus, if you have a DDR400 CL3 memory and a DDR2-400 CL4 memory, your DDR400 will be faster."
however what isnt common knowledge is this
"Keep in mind that DDR2 memories have an additional parameter called AL (additional latency), which must be added to their nominal latency (CL) in order to get the total latency."
so this means
(CAS + AL) x (Clock Tick Duration) = Main Performance for DDR2
vs
(CAS) x (Clock Tick Duration) = Main Performance for DDR
"The additional latency (AL) is usually found on the memory module technical specs (usually a PDF file for downloading on the manufacturer website)."
now the breakdown of MHZ and how it works with the timings.
"On a DDR400 CL3 memory, this “3” means that the memory delays three clock cycles to start delivering the requested data. Since this memory runs at 200 MHz, each clock tick measures 5 ns (T= 1/f). Thus its latency if of 15 ns.
Now on a DDR2-533 CL3 AL0 memory, this “3” also means that the memory delays three clock cycles to start delivering the request data, but since this memory runs at 266 MHz, each clock tick measures 3.75 ns, so its latency is of 11.25 ns, making this memory faster to data delivery than our DDR400 CL3 memory. So a DDR2-533 CL4 and AL0 memory has the same latency as a DDR400 CL3. Notice that we are assuming the additional latency as zero, or we would need to take it into account, i.e., a DDR2 CL3 AL1 memory has in reality a latency of four clock cycles which would equal 15ns."
the clock tick duration for memory speeds
Memory.........................Clock Tick Duration (each one)
DDR266.........................7.5 ns
DDR333.........................6 ns
DDR400 and DDR2-400.....5 ns
DDR2-533......................3.75 ns
DDR2-667......................3 ns
DDR2-800......................2.5 ns
dont get AL (additional latency) mixed up with the other timings. the other timings mean this
* CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and it returning it.
* tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
* tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the begin of the access the another line of data.
* tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.
* CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not informed. It usually is T1 (1 clock speed) or T2 (2 clock speeds).
ENJOY
http://www.hardwaresecrets.com/article/167/4
to highlight the important parts
as we all seem to know Cas (the first timing number) is by far the most important in terms of memory performance. as highlighted here
"If we compare a DDR memory to a DDR2 memory running under the same clock, the one with the lower latency will be the fastest. Thus, if you have a DDR400 CL3 memory and a DDR2-400 CL4 memory, your DDR400 will be faster."
however what isnt common knowledge is this
"Keep in mind that DDR2 memories have an additional parameter called AL (additional latency), which must be added to their nominal latency (CL) in order to get the total latency."
so this means
(CAS + AL) x (Clock Tick Duration) = Main Performance for DDR2
vs
(CAS) x (Clock Tick Duration) = Main Performance for DDR
"The additional latency (AL) is usually found on the memory module technical specs (usually a PDF file for downloading on the manufacturer website)."
now the breakdown of MHZ and how it works with the timings.
"On a DDR400 CL3 memory, this “3” means that the memory delays three clock cycles to start delivering the requested data. Since this memory runs at 200 MHz, each clock tick measures 5 ns (T= 1/f). Thus its latency if of 15 ns.
Now on a DDR2-533 CL3 AL0 memory, this “3” also means that the memory delays three clock cycles to start delivering the request data, but since this memory runs at 266 MHz, each clock tick measures 3.75 ns, so its latency is of 11.25 ns, making this memory faster to data delivery than our DDR400 CL3 memory. So a DDR2-533 CL4 and AL0 memory has the same latency as a DDR400 CL3. Notice that we are assuming the additional latency as zero, or we would need to take it into account, i.e., a DDR2 CL3 AL1 memory has in reality a latency of four clock cycles which would equal 15ns."
the clock tick duration for memory speeds
Memory.........................Clock Tick Duration (each one)
DDR266.........................7.5 ns
DDR333.........................6 ns
DDR400 and DDR2-400.....5 ns
DDR2-533......................3.75 ns
DDR2-667......................3 ns
DDR2-800......................2.5 ns
dont get AL (additional latency) mixed up with the other timings. the other timings mean this
* CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and it returning it.
* tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
* tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the begin of the access the another line of data.
* tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.
* CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not informed. It usually is T1 (1 clock speed) or T2 (2 clock speeds).
ENJOY
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