Cas Latency

[Spoda17]

What is the difference between cas 4 and cas 5...?

 

[firewolfsm]

Basically...4 is better by a few %.

 

[JustaGeek]

http://en.wikipedia.org/wiki/CAS_latency

Bottom line - lower CAS latency means faster memory for the same clock speed.

DDR2 800 at 4-4-4-12 is faster than DDR2 800 at 5-5-5-15.

 

[lopri]

Access latency. 4 is faster than 5 other things being equal, but often times other things are not equal and CL5 will let your memory scale somewhat better than CL4. Same for CL3 vs CL4. In theory, however, the scaling is kinda linear:

800MHz/CL3 = 1066MHz/CL4 = 1333MHz/CL

I personally prefer low latency to high bandwidth.

 

[boglwe]

time like 5.5.5.15 is (CAS-TRCD-TRP-TRAS)

This was all cut and pasted but well worth the read.

CAS (tCL):
This stands for Column Address Strobe or Column Address Select and that essentially means the number of cycles for the memory to get information and then process that information. Also keep in mind that a lower CAS is much better than a higher CAS, so try and keep CAS as low as possible when overclocking.

tRCD:
This is the delay from the time a row is activated to when the cell (or column) is activated via the CAS signal and data can be written to or read from a memory cell. When memory is accessed sequentially, the row is already active and tRCD will not have much impact. However, if memory is not accessed in a linear fashion, the current active row must be deactivated and then a new row selected/activated. In such an example, low tRCD's can improve performance. However, like any other memory timing, putting this too low for the module can cause instability.

tRP:
RAS Precharge; the time required to switch from one row to the next row. For example: switching internal memory banks.

tRAS:
Memory architecture is like a spreadsheet with row upon row and column upon column, with each row being one bank. For the CPU to access memory, it first must determine which row or bank in the memory is to be accessed and then activate that row with the RAS signal. Once activated, the row can be accessed over and over, until the data is exhausted. This is why tRAS has little effect on overall system performance but could impact system stability if set incorrectly.

tCLK:
This is simply the clock used for the memory. Note that because frequency is 1/t, if memory were running at 100Mhz, the timing of the memory would be 1/100Mhz, or 10nS.

EPP:
This stands for Enhanced Performance Profiles and is a new approach that simplifies overclocking and ensures memory and platform compatibility. This technology was defined to utilize the extra space in the EEPROM to store memory configuration settings that are sufficient to optimize and maximize the performance of a memory module. It basically auto detects your motherboard's chipset and sets the memory timing accordingly. While EEP is nice, many overclockers will be manually adjusting their memory in the motherboard's BIOS.