You’ve wrapped your head around SDRAM, DIMMs, and DDR generations. But what about the other long strings of numbers in the RAM model? What do they mean? What is RAM measured in? And what about ECC and Swap? Here are the other RAM specification terms you need to know.
CLOCK SPEED, TRANSFERS, BANDWIDTH #
- You may have seen RAM referred to by two sets of numbers, like DDR3-1600 and PC3-12800. These both reference and allude to the generation of the RAM and its transfer speed. The number after DDR/PC and before the hyphen refers to the generation: DDR2 is PC2, DDR3 is PC3, and DDR4 is PC4.
- The number paired after DDR refers to the number of mega transfers per second (MT/s). For example, DDR3-1600 RAM operates at 1,600MT/s. The DDR5-6400 RAM mentioned above will operate at 6,400MT/s—much faster! The number paired after PC refers to the theoretical bandwidth in megabytes per second. For example, PC3-12800 operates at 12,800MB/s.
- It is possible to overclock RAM, just like you can overclock a CPU or graphics card. Overclocking increases the RAM’s bandwidth. Manufacturers sometimes sell pre-overclocked RAM, but you can overclock it yourself. Just make sure your motherboard supports the higher RAM clock speed!
- You might be wondering if you can mix RAM modules of different clock speeds. The answer is that yes, you can, but they’ll all run at the clock speed of the slowest module. If you want to use faster RAM, don’t mix it with your older, slower modules. You can, in theory, mix RAM brands, but it isn’t advisable. You run a greater chance of encountering a blue screen of death or other random crashes when you mix RAM brands or different RAM clock speeds.
TIMING AND LATENCY #
- You will sometimes see RAM modules with a series of numbers, like 9-10-9-27. These numbers are referred to as timings. RAM timing is a measurement of the performance of the RAM module in nanoseconds. The lower the numbers, the quicker the RAM reacts to requests.
- The first number (9, in the example) is the CAS latency. The CAS latency refers to the number of clock cycles it takes for data requested by the memory controller to become available to a data pin.
- You might notice that DDR3 RAM generally has higher timing numbers than DDR2, and DDR4 generally has higher timing numbers than DDR3. Yet, DDR4 is faster than DDR3, which is faster than DDR2. Weird, right?
- We can explain this using DDR3 and DDR4 as examples.
- The lowest speed DDR3 RAM runs is 533MHz, which means a clock cycle of 1/533000000, or 1.87 ns. With a CAS latency of 7 cycles, the total latency is 1.87 x 7 = 13.09 ns. (“ns” stands for nanoseconds.)
- Whereas the lowest speed DDR4 RAM runs at is 800MHz, which means a clock cycle of 1/800000000, or 1.25 ns. Even if it has a higher CAS of 9 cycles, the total latency is 1.25 x 9 = 11.25 ns. That’s why it’s faster!
- For most people, capacity trumps clock speed and latency every time. You will get much more benefit from 16GB of DDR4-1600 RAM than you get from 8GB of DDR4-2400 RAM. In most cases, timing and latency are the last points of consideration.
ECC #
- Error Correcting Code (ECC) RAM is a special kind of memory module that aims to detect and correct data corruption. ECC ram is used in servers where errors in mission-critical data could be disastrous. For example, personal or financial information stored in RAM while manipulating a linked database.
- Consumer motherboards and processors don’t usually support ECC-compatible RAM. Unless you are building a server that specifically requires ECC RAM, you should stay away from it.
