´╗┐ RAM timings knowledge base.
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A guide to overclocking your RAM timings is as follows: start with Trfc, then Trc and then Trrd, Twr, Twtr, and Trtw values. I wouldn't go lower than 2 for Trrd and Twr if you ram can handle it.

Here's a chart about the different RAM timings. I will cite sources for info wherever I can.
Timing Expanded description Detailed description
tCL CAS# Latency CAS is Column Address Strobe or Column Address Select. CAS controls the amount of time (in cycles (2, 2.5,& 3) between receiving a command and acting on that command. Since CAS primarily controls the location of HEX addresses, or memory columns, within the memory matrix, this is the most important timing to set as low as your system will stably accept it. There are both rows and columns inside a memory matrix. When the request is first electronically set on the memory pins, the first triggered response is tRAS (Active to Precharge Delay). Data requested electronically is precharge, and the memory actually going to initiate RAS is activation. Once tRAS is active, RAS, or Row Address Strobe begins to find one half of the address for the required data. Once the row is located, tRCD is initiated, cycles out, and then the exact HEX location of the data required is accessed via CAS. The time between CAS start and CAS end is the CAS latency. Since CAS is the last stage in actually finding the proper data, it's the most important step of memory timing.

From: http://www.cooltechzone.com/Reviews/Memory_Reviews/Memory_Timings_Explained_20040725377/2/
-tRTC
-tRCD
RAS# to CAS# Delay Delay between row access and column access strobe - Important for multi-tasking. (Unknown source)

Also known as RAS to CAS Delay, in addition to Column Address Strobe, there is Row Address Strobe. CAS and RAS combined allow for the exact location of memory blocks. There is an interval between RAS (activated when data is first requested) and CAS (activated when RAS is complete), as memory can't locate a block precisely in a single stage. tRCD is the cycle time between the first stage in memory access, the row strobe, and the second stage. However, the performance impact of this setting is often neglible, as memory tries to store data from programs in sequential order. It tries to keep the same row for a single program, and ordered columns to reduce the time for tRCD.

From: http://www.cooltechzone.com/Reviews/Memory_Reviews/Memory_Timings_Explained_20040725377/2/
tRP RAS# Precharge Switches active row - Important for memory copy operations, and apps that use large amounts of memory in different memory "rows". (Unknown source)

Also known as RAS Precharge, this is the amount of time it takes for memory to terminate the access in one row and begin another. To put it simply, after data is set to the pins and activates tRAS, then RAS, tRCD, and CAS; the memory needs to terminate its current row and start all over at tRAS. This is the very basic function of how memory works. This is only an important setting when you're doing massive shifting in data, for example - working with large virtual buffers or video rendering. At that point, several rows are being consumed by a single program, and its advantageous for the program to be able to switch quickly between these rows.

From: http://www.cooltechzone.com/Reviews/Memory_Reviews/Memory_Timings_Explained_20040725377/2/
tRAS -Active to Precharge Delay
-Precharge Delay
Formula for setting tRAS = tCl (CAS latency) + tRtc (ras to CAS delay) + 2 (Well-known formula)

Also known as Active to Precharge Delay, this is the time between receiving a request for data electronically on the pins of a memory module and then initiating RAS to start the actual retrieval of data. This command seems important, but really it isn't. Memory access is a very dynamic thing. Sometimes memory is being hit hard, and other times very sporadically. Though at all times, memory access is at constant, therefore, it is rare that the tRAS command is received to access new data (such as a substantial change, like opening a new program).

From: http://www.cooltechzone.com/Reviews/Memory_Reviews/Memory_Timings_Explained_20040725377/2/
CpC -Command per Clock
-Command Rate
CMD rate is generally used to describe the time from a chip select until a Row Activate Command can be given. The chip select defines the physical bank in which the row is located. In a system running a single, single-sided memory module, there is never a question which bank will be selected since there is only one.

More generally, the CMD Rate is a chipset latency that is not determined by the memory but by the time it takes the chipset to translate the virtual address space into physical memory addresses. Needless to say that higher density system memory with its more addresses will take longer to decode than a single low density module, even if it is double-sided.

Intel has taken care of this problem by simply limiting the number of banks supported per memory channel to four. This, in turn allows them to run all their chipsets on a fixed CMD rate of 1T, regardless of how much memory is installed.

Rating a module as 1T is actually somewhat misleading advertising because all unbuffered modules are capable of a 1T CMD rate up to four banks per channel, beyond which chipset limitations become a factor.

From http://www.cooltechzone.com/Reviews/Memory_Reviews/Memory_Timings_Explained_20040725377/1/

This describes the amount of time it takes for the RAS to be executed after the memory chip has been selected.

From: http://www.tomshardware.com/2004/01/19/ups_and_downs/page3.html
tRC Row Cycle Time Formula for setting tRC = tRp (RAS precharge) + tRas (precharge delay) (Well-known formula)

Row Cycle Time. The minimum time in cycles it takes a row to complete a full cycle. If this is set too short it can cause corruption of data and if it is to high, it will cause a loss in performance, but increase stability.

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
tRFC Refresh Cycle Time Row Refresh Cycle Timing. This determines the amount of cycles to refresh a row on a memory bank. If this is set too short it can cause corruption of data and if it is too high, it will cause a loss in performance, but increased stability.

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
tREF   The amount of time it takes before a charge is refreshed so it does not lose its charge and corrupt. Measured in micro-seconds (┬Ásec).

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
-tWR
-tRW
Write Recovery Time Write Recovery Time. The amount of cycles that are required after a valid write operation and precharge. This is to insure that data is written properly.

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
-tWTR
-tWTW
Write to Real Delay Write to Read Delay. The amount of cycles required between a valid write command and the next read command. Lower is better performance, but can cause instability.

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
tRRD RAS# to RAS# Delay Row to Row Delay or RAS to RAS Delay. The amount of cycles that it takes to activate the next bank of memory. It is the opposite of tRAS. The lower the timing, the better the performance, but it can cause instability.

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
RMS Refresh Mode Select  
-tRTW
-tRWT
Read to Write Delay Read to Write Delay. When a write command is received, this is the amount of cycles for the command to be executed.

From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2
tRRD Read to Read Delay  
tWRWR Write to Write Delay  
tRD -MCH Read Delay
-Common Performance Level
Means for adjusting the timing of each individual memory phase associated with the selected memory divider.

a.k.a. "MCH Read Delay" a.k.a. "Common Performance Level" = means for adjusting the timing of each individual memory phase associated with the selected memory divider.

Each individual phase can be "pulled-in" or left as is. "Pulling-in" a phase reduces just that phase's associated tRD value (performance level) by one. Like memory timings, lower values are tighter and thus provide better memory read performance and lower latencies. If all phases are pulled-in, this is the equivalent of selecting the next lower common performance level and performs identically to this new setting. Thus, pulling-in particular phases can allow the user to affect a minor performance improvement if selecting the next lower common performance level is not possible.

From: http://www.anandtech.com/mb/showdoc.aspx?i=3208&p=2
tWCL Write CAS number ClocK; the length of a clock cycle. (Unknown source)

Write CAS number. Write to whatever bank is open to be written too. Operates at a rate of 1T, but can be set to others. It does not seem to work with other settings than 1T on DDR. DDR2 is different though.

From: From: http://www.techpowerup.com/articles/overclocking/AMD/memory/131/2

Here's another table for acronyms not to be confused with RAM timings:
Acronym Company Description
MCH Intel Memory Controller Hub (not to be confused with the northbridge)
SPP nVidia System Platform Processor
MCP nVidia Media and Communications Processor
ICH N/A I/O Controller Hub (Southbridge or southbridge support chip)
IOH Intel Input/Output Hub (Northbridge - e.g. Intel x58)
VTT N/A Voltage termination
PLL N/A Phase Locked Loop
GTL N/A Gunning Transceiver Logic