What Is CAS 4 in PC Memory?

RAM latency can greatly affect performance.

Many factors influence the performance of RAM memory. Capacity and speed are the most often cited factors. However, latency of RAM, which can be measured by CAS, is an important variable as well.

Latency

RAM latency is the delay between the processor's request for information from memory and the availability of data to the processor's bus. Basically, it means how long it takes for the RAM to "find" the data and hand it to the processor. One measure of latency is CAS.

CAS

CAS stands for column access strobe (or select). Memory on RAM modules is organized in arrays of rows and columns. The CAS figure represents how many clock cycles it takes to find the data. CAS-4 would mean the RAM needs four clock cycles to find data.

Significance

Lower latency numbers is better. Lower CAS speed indicates data can be accessed faster. The faster the processor can access information, the more time it can spend on other processes and applications. This can affect the RAM module's performance almost as much as the RAM's frequency.

References

• DEW Associates: CAS Latency, What Is It?
• Memory Suppliers: CAS Latency--What is It, and How Does It Affect Performance?
• Extreme Tech: RAM Latency Explained
• Dev Hardware: Effects of Memory Latency

Resources

• Tom's Hardware: Memory Question Cas. Latency and Timings
• PC Stats: Memory Bandwidth vs. Latency Timings