Memory Timing

As we mentioned, timing plays a crucial role in memory modules. This determines how quickly the modules can transfer data. Many high-end computers come with name brand parts on the inside. Memory manufacturers like Corsair, Crucial, or PDP produce high quality products, but timing does make a difference when it comes to quality memory modules.

Memory timing can be changed on the motherboard, but before a person takes the leap into that level of customization they should first have more than a toe in the water – they should be submerged. Let’s get wet.

The timing parameters appear as 2-3-2-6-T1 or some variant of that string. The numbers refer to CL-tRCD-tRP-tRAS-Command Rate and are measured in clock rate. It’s also important to visualize how memory works inside the modules. The data is accessed via columns and rows, like a large table. As we mentioned earlier in the article, those cells are very small, but each houses information that can be accessed quickly. So let’s take a moment to look at the number references to better understand how memory timing works.

CL – CAS Latency is the number of clock cycles it will take the memory module to access and return data to the CPU. Very simply, this is the access time of the memory module. Most in the industry refer to this number first, since it reflects the most important aspect of the RAM – how fast can it do its job. For example, a memory module with a CL of 4 would take 4 clock cycles to return data to the CPU, while a CL of 3 would only take 3 clock cycles.

tRCD – RAS to CAS delay refers to the time it would take the memory module to access the column of data and the line (row) where it’s stored in the table.

tRP – RAS Precharge refers to the time it takes to disable access to a row of data and begin access to another.

tRAS – Active to Precharge Delay refers to how long the memory has to wait before it can request another memory address.

CMD – The command rate refers to the how long it takes the memory module to receive a request after being activated. This is usually T1 or one clock cycle, but sometimes may be T2 or two clock cycles.

The performance of two modules with the same size and bus speed can be very different depending on timing settings. The stickers on the memory stick should give an idea of the module’s performance. Sometimes the sticker may only indicate the CL value of the module, but many times the numbers referenced after the specific make of the module should indicate the performance. For instance, the following picture displays a label from a RAM module. What can you decipher from the numbers listed there? What’s the CL value? How quickly can the memory access a specific column? How long will it have to wait to field a second request?

The 512MB stick of RAM is DDR and runs at 400MHz. The CAS Latency of the memory module is 3, which means it would take 3 clock cycles to access the module and return data to the CPU. The number referenced after the model PC3200U indicated all you would need to know about the module. 3-0-3-3-1 describes the CL-tRCD-tRP-tRAS-Command Rate. As you can see, the CL3 reference next to DDR400MHz is just another place a person could find the CAS latency of the module.

The Big RAM Picture

RAM is that piece of the computer that many feel is not quite as glamorous as the rest. It doesn’t have the flare of the CPU or the dazzling on-screen presence of the GPU. However, it does carry information to the CPU in order for the system to work. Without RAM, modern computers would still be without the means to carry out many of their tasks. As an overview, we hope this helped demystify how RAM works and gave you the knowledge to buy what you want in a memory module.