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Random access memory (usually known by its acronym, RAM) is a type of computer data storage. Today it takes the form of integrated circuits that allow the stored data to be accessed in any order, i.e. at random. The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.

This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.

The word RAM is mostly associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. However, many other types of memory are RAM as well (i.e. Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash.

DDR SDRAM (double data rate synchronous dynamic random access memory) is a class of memory integrated circuit used in computers. It achieves nearly twice the bandwidth of the preceding [single data rate] SDRAM by double pumping (transferring data on the rising and falling edges of the clock signal) without increasing the clock frequency.

With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.

JEDEC has set standards for speeds of DDR SDRAM, divided into two parts: The first specification is for memory chips and the second is for memory modules. As DDR-SDRAM is superseded by the newer DDR2 SDRAM, the older version is sometimes referred to as DDR1-SDRAM.

Memory Timing
* RAS - Row Address Strobe or Row Address Select
* CAS - Column Address Strobe or Column Address Select
* tRAS - Active to precharge delay; this is the delay between the precharge and activation of a row
* tRCD - RAS to CAS Delay; the time required between RAS and CAS access
* tCL - (or CL) CAS Latency
* tRP - RAS Precharge; the time required to switch from one row to the next row, for example, switch internal memory banks
* tCLK – ClocK; the length of a clock cycle
* Command Rate - the delay between Chip Select (CS), or when an IC is selected and the time commands can be issued to the IC
* Latency - The time from when a request is made to when it is answered; the total time required before data can be written to or read from the memory.

Some of the above terms are more important to system stability and performance than others. However, to understand the whole, it is important to understand the role of each of these settings/signals. Therefore, the numbers 2-3-2-6-T1 refer to CL-tRCD-tRP-tRAS-Command Rate and are measured in clock cycles.

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.

tRCD 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 in instability.

CAS Latency
Certainly, one of the most important timings is the CAS Latency, which is also the one most people understand. Since data is often accessed sequentially (same row), the CPU need only select the next column in the row to get the next piece of data. In other words, CAS Latency is the delay between the CAS signal and the availability of valid data on the data pins (DQ). The latency between column accesses (CAS) then plays an important role in the performance of the memory. The lower the latency, the better the performance. However, the memory modules must be able to support low-latency settings.

tRP is the time required to terminate one row access and begin the next row access. tRP might also be seen as the delay required between deactivating the current row and selecting the next row. So in conjunction with tRCD, the time required (or clock cycles required) to switch banks (or rows) and select the next cell for reading, writing, or refreshing is a combination of tRP and tRCD.

tRAS is the time required before (or delay needed) between the active and precharge commands. In other words, how long the memory must wait before the next memory access can begin.

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.

Command Rate
The Command Rate is the time needed between the chip select signal and when commands can be issued to the RAM module IC. Typically, these are either 1 clock or 2.

This covers much of the basic settings for memory and how they work. As mentioned earlier, it is important to understand what timings your memory will support. Refer to your memory vendor’s website or datasheets for more information.

Memory Overclocking
There are two modes of configuring the memory clock: synchronous, were the memory clock is tied to the CPU external clock, and asynchronous, where the memory clock can be configured independently from the CPU clock. The mode your computer has depends on its motherboard.

The synchronous mode is usually found in entry-level motherboards while asynchronous mode is usually found in high-end motherboards. Also, notice that many low-end motherboards don't have any overclocking configuration at all.

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post #2 of 2
I'm pretty sure all of this info is stickied in the AMD and Intel memory sections.
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