Q. What do all of these numbers people in the forums keep mentioning, and how do they affect performance?
A.
1) The most famous of these is the CAS Latency, which stands for 'Column Address Strobe'. It is important in determining how fast memory modules will respond to a request for data. It is normally called the CAS delay, and has a value of 2 or 3 in SDR SDRAM. For DDR RAM however, the typical values are 2 or 2.5. Some 845 boards allow a CAS of 1.5 to be selected but the performance increase is next to zero, despite a considerable reduction in system stability. These numbers correspond to a number of clock cycles. Lower is better, and in SDR SDRAM the difference between CAS 2 and 3 can affect performance considerably. In DDR however the difference is not so pronounced and is less important than others.
2) An important setting in most Athlon boards is the Bank Interleaving setting. Because you have to wait for all these latencies to occur before you can actually do anything with the RAM, it didn’t take long before there was more waiting going on than actually reading or writing. To get around this, memory is often interlaced. Consider the simplified memory cycle of “CAS -> CMD -> RAS -> output”. Imagine if while half your RAM is charging the column (in the CAS step), the other half is already charging the row (in the RAS step). As you could imagine, this will mean the output step will occur twice as often. The default for most RAM is no interlacing. To improve performance, set it to 2-way, or even 4way, and setting your ram to 4-bank will considerably improve performance.
3) Command Rate determines whether a chip select can be executed in a single clock or whether it needs two or more clocks. In DDR the CMD rate offers a bigger boost in performance than CAS latency, with 1T command being the best currently available.
4) RAS-to-CAS Delay (tRCD) is becoming an increasingly important limiting factor in determining overall memory performance. It can usually be set to 4, 3 or 2, with 2 being the fastest. Because RAS Precharge (tRP) is also set as 4, 3, or 2, the two settings are often grouped together or confused. However, the former setting has a significant impact on system performance where as the latter does, in general, not.
5) Bank cycle time (tRAS) is the number of cycles necessary to develop the full charge differential between bit and reference lines to restore the data in the memory cells. Lower is faster, with most motherboards having settings for 7, 6 and 5.
Q. Woah, woah, thats too technical. Just tell me what the common timings do and what 6-2-2 etc refers to!
A:
CAS (2.5 or 2) = Lower gives significant performance increase
tRAS (7,6 or 5) = Gives very small performance increase
tRCD (2, 3 or 4) = Lower gives significant performance increase
tRP (2, 3 or 4) = Gives very small performance increase
CMD (1T, 2T or 3T) = Lower gives very significant performance increase
Bank Interleave (None, 2B or 4B) = Higher gives very significant performance increase.
