What is CAS latency

What is CAS latency

What is CAS latency

Column Address Strobe (CL) is the acronym for CAS Latency. An instruction for a column is given, and the moment it is available is measured by the number of clock cycles between that moment and its execution. The term CAS is still used in the industry today, although the term strobe has become antiquated as a remnant from the days of synchronous DRAMs. When you think about the spreadsheet analogy I made above, CAS latency is how long it takes a column in RAM to respond once the memory controller sends it over for access. All of the timings below, except for tCL, are exact numbers and not maximums or minimums.

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Calculating RAM Latency or CAS Latency

This is a measure of the length of time it takes for a memory to respond to a request that has come from a memory controller.

Here’s the formula that you use:


So, what would the CAS latency be for a DDR4-3600 CL18 memory kit?


CL= (18*2000)/3600

= 10 ns

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Is a Higher or Lower CAS Latency Better?

Performance is definitely impacted by CAS Latency. As the number decreases, the latency decreases and, therefore, the performance increases. As CAS Latency decreases, RAM performance increases. All other parameters, including the clock frequency, must remain the same for this to work.

When you request data from your RAM, the CAS Latency number simply indicates how long it takes to serve that data. In order to get the delay in seconds, multiply this number by the duration of one clock cycle.

Naturally, lower CAS Latency (CL) ratios produce faster RAM. However, the clock cycle duration for the RAM also affects the speed.

Lower CAS Latency results in faster response times and better performance since each clock cycle lasts a certain time.In addition, since the cycle time decreases as a function of the frequency, it is logical that a higher frequency means shortest change in cycle time as well as a shorter response time.

In general, I would say that because of cycle time and frequency dependence, a CAS time of 8 cycles at 800 MHz is not as attractive as a CAS time of 16 at 1600 MHz. Therefore, higher frequency numbers are both associated with better overall memory performance as are lower latency numbers.

RAM speed vs. RAM latency

It is important to understand RAM performance by considering CAS latency, which is a measure of how many mega transfers (one million transfer operations) can be performed in a second by a RAM (e.g., a DDR4-3200 RAM can conduct 3,200 mega transfers in one second).

To get a more accurate view of RAM’s overall latency, you should consider not only the number of cycles, but the duration of each cycle.

While DDR3 RAM is older and offers better power efficiency and storage density, DDR4 RAM has a higher CAS latency since it is newer. The CAS latency of DDR3 RAM is usually 9 or 10 ms, while DDR4 RAM will typically be 15 ms. Nevertheless, the faster clock speeds contribute to better performance overall for the newer standard.


Why CAS Latency Matters for RAM?

You are required to  determine which RAM stick is superior. when looking for best DDR4 module for you laptop. You should take into account the CAS Latency when comparing two RAM stihttps://tech1resource.com/best-ddr4-ram-for-dell-laptop/cks having the same capacity and clock speed. The one with a lower CL outperforms the other one because it has a lower latency.

As if it were a giant table, memory addresses are laid out in rows and columns as follows:

tCL: CAS Latency is the amount of time it takes to access a specific column when there is already a row that is open.

The RAS to CAS Delay is measured in clock cycles, and, when none of the rows are open, the number of cycles needed to open the correct row.

tRP: Row Precharge is how many cycles of the clock are needed to close an incorrect row.

tRAS: How long does it take the system to close a row that has been incorrectly opened and open the correct row?

Timings accumulate in some cases. If a cell on a different row has the required data, it will take tRAS plus tCL to open that cell (to retrieve the data).

Which is more important: speed or latency?

Our engineers analyzed this classic question in depth and tested it exhaustively in our Performance Lab, so we believe that the answer is BOTH! When it comes to system performance, speed and latency are both critical aspects, so when you’re upgrading you should consider:

Step 1: Check to see what the highest speed you can run your memory at based on your processor and motherboard (or by turning on overclocking profiles). This step is especially important when looking for the top DDR4 RAM for Ryzen 3700X.

Step 2: Select the memory with the lowest latency that fits within your budget at the speed you desire, keeping in mind that a lower latency means a better system performance.

Is CAS latency the same as tCL?

A DRAM’s read and write latency is measured by its CAS Latency (tCL/CL/tCAS). As opposed to other numbers which represent minimums, CAS Latency is an exact number. It is crucial that the memory and the memory controller come to an agreement about this number.

As such, you should also keep in mind the possibility of different CAS timings between two different RAM kits which have the same data transfer rate, for example, DDR4-3200. Let’s say you have the Team Group Delta Tuf Gaming RGB DDR4-3200 and G.Skill Trident Z Royal DDR4-3200. Their transfer rates are the same DDR4-3200. Their CAS timings are, however, different: CL16-18-18-38 (CAS 16) and CL14-14-14-34 (CAS 14).

How to Check my RAM’s CAS Latency?

In just a few seconds, you can determine your current  gaming memory latency! Download CPU-Z from CPUID’s official website and be ready to measure your memory latency. The first time you run CPU-Z, you should see a screen similar to the following (the specs, however, will be your own based on the hardware you’re using).

Go ahead and click on the ‘Memory’ tab you can see here, and you will see a list with your memory frequency (NOT data rate), as well as your memory timings, like the CAS Latency. Those of you who are curious enough to open your CPU-Z window will have noticed a tab called ‘SPD’ in the list of tabs.

This standardized EEPROM (Electrically Erasable Programmable Read Only Memory) provides access to memory specifications as a result of serial presence.

As a result, when your computer performs a POST (Power-On Self-Test), it uses this information to access memory specifications. You will also learn what specifications JEDEC has set for your memory in this step. As you can see in the screenshot above, it will also look for XMP profiles in your memory. BIOSes can then offer a ‘one-click’ way to read off an XMP profile and apply specifications to it.

Those aren’t the memory settings you currently have. Instead, you’ll see information from the modules it reads.

RAM operation background

Arrays of dynamic RAM are rectangular in shape. Their rows are separated by horizontal wordlines. The MOSFETs in each of those rows are enabled by a logical high signal, connecting each storage capacitor to its corresponding bit line. Bit lines are connected to amplifying sense amplifiers, which amplify the voltage change caused by storage capacitors. From the backside of the DRAM chip, the amplified signal is output and fed back up the bit line so that a new row is created each time the row is refreshed.

It is on this precharged state that the array always stays when there is no word line active. This way, a voltage halfway between high and low is held on all the bit lines. During the activation of a row, the storage capacitor deflects this indeterminate signal in a direction that is high or low.

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Making Memory row active

The sense amplifiers need to first be loaded with a row of memory in order to access it. A row becomes active when this occurs, and columns can be viewed or modified.

CAS latency is the time it takes the memory module to present the column address and column address strobe signal to the memory module, followed by the time the memory module provides the corresponding data. Having an active row is a prerequisite. If it isn’t, additional time must be put into the process.

In a typical memory module with 1 GiB of SDRAM, for example, you might find eight individual DRAM chips offering 128 MiB of capacity each. Essentially there are eight banks of 227=128 Mibits over eight channels of one chip. Each of these banks incorporates a separate array of DRAM chips. This means that there will be 214=16384 rows of 213=8192 bits in each bank. It is possible to access one byte of memory from each chip on the DIMM, i.e. 64 bits total (off of each chip), by providing a 3-bit bank number, a 14-bit row address, and a 13-bit column address.

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What Effect does CAS latency has on memory access speed

Rather than using a clock, asynchronous DRAM utilized a set timing on the memory bus to access the memory and was separate from the system bus. While synchronous DRAMs do not have a fixed CAS latency, they do have a clock rate-dependent CAS. SDRAM memory modules are therefore designated by their clock tick latency rather than by their absolute time.

Due to the multi-bank structure, a module’s output pins can be kept 100% busy regardless of CAS latency since data can be output from one bank to another during access latency for another.  Based solely on the clock speed, you can achieve the maximum bandwidth. Unfortunately, this maximum bandwidth can only be achieved using a long enough period of prior planning in order to determine the address of the data that needs to be read. There can be a loss of bandwidth if the address of the data being accessed is unpredictably different from the data address, which can result in pipeline stalls.

computing CAS

CAS latency is computed as the time to close any open rows – before the time to open the desired row and then the CAS latency to read data from the row – followed by the CAS latency to read data from the row for a completely unknown memory access. Despite this, it happens frequently for several words to be accessed simultaneously due to spatial locality. This case, the elapsed time is entirely determined by the CAS latency.

Clock ticks are used in place of time for CAS latencies on modern DRAM modules. It is fair to compare latencies at different clock speeds by converting them into absolute times. Even a numerical CAS latency of a higher number can result in a shorter response time if the clock is faster than usual. In a similar manner, a memory module which is underclocked may be offered the option to reduce its CAS latency time cycle count in order to maintain the same duration.

Two transfer per click

Usually, DDR RAM is described by the rate at which it performs two transfers per clock cycle. In contrast to transfer times, CAS latency is specified in terms of clock cycles, not transfer time (which occurs simultaneously on both the rising and falling edges of the clock).  If CAS latency times are being computed using the clock rate (half of the transfer rate), then it is critical to ensure that the clock rate is being used.

Additionally, burst transfers complicate matters. Typical microprocessors may have cache lines as large as 64 bytes, requiring eight transfers from memory that is 64 bits wide (eight bytes). In addition to the CAS latency, the time taken to transfer all eight words depends on data transfer rate. The CAS latency can only accurately measure the time taken to transfer the first word of memory. Since the burst is typically sent in critical word order, the processor does not have to wait for all eight words. Instead, the processor can use the first critical word immediately.

MT/s, or Megatransfers per Second, is substituted for MT/s in the table below, and Mhz, or million cycles per second, is substituted for Mhz in the table.

Final thoughts on what is CAS latency

Only a partial picture of CAS frequency is available. Since it should be considered alongside clock frequency, it should be taken into consideration. The reason is that it is based on the length of a single clock cycle. Secondly, you should also take the CL ratio into consideration when mixing two or more RAM modules with other factors. Hence, if you install a RAM with a high CL value, your system is likely to slow down.

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