What does RAM speed mean?
The next issue to consider is speed. RAM speeds can be quite confusing, as they can be expressed in several ways. Starting with the oldest DDR modules, the basic models run at an internal frequency of 100MHz, while more advanced modules increase the internal clock speed to 133MHz, 166MHz and up to 200MHz.
It might seem logical to refer to these different modules by their internal speeds but, thanks to the double data rate that gives DDR its name, a 100MHz module can carry out a theoretical maximum of 200 million transfers per second, while the 200MHz module can carry out 400 million transfers per second. For this reason, 100MHz DDR is known as DDR-200, 133MHz modules are labelled DDR-266 and so forth.
This is a fairly obvious system, but RAM transfers aren’t very convenient units to work in. It’s much more common to talk about data in terms of bytes. So to make DIMM speeds more easily understandable, they’re also given a “PC-rating”, which expresses their bandwidth in megabytes per second.
PC ratings can be calculated very simply. Each RAM transfer consists of a 64-bit word, or eight bytes. So to convert transfers-per-second into bytes-per-second, you simply multiply by eight. DDR-200 is thus equivalent to PC-1600.
DDR2 uses almost the same naming conventions, but the chips communicate with the CPU at twice the speed of DDR. The slowest DDR2 is therefore capable of 400 million transfers per second, and is designated DDR2-400, or PC2-3200. As you’d expect, DDR2 goes up to DDR2-800, also known as PC2-6400, and above this there’s a high-end part, based on 266MHz chips, to give DDR2-1066. Its PC-rating is rounded down to PC2-8500 for convenience – its peak bandwidth is more like 8533MB/sec.
DDR3 extends this process, running the I/O bus at four times the speeds of DDR – so the basic part can handle 800 million transfers per second, earning the labels DDR3-800 and PC3-6400, with faster chips being named accordingly.
The maximum standard RAM speeds approved by JEDEC – the body behind the three DDR standards – are DDR-400, DDR2-1066 and DDR3-1600. You may also hear of modules with higher speed ratings, such as DDR2-1250 and DDR3-2000, designed to run at overclocked speeds in enthusiast motherboards.
What are the benefits of buying extra speed?
Not every motherboard will support every RAM speed. Older boards may be unable to run fast DIMMs at their full rated speed, while more recent models can refuse to boot if you use memory that’s too slow.
But if you do splash out on super-fast RAM, what benefit can you expect to see? In theory, the fastest DIMMs can communicate with the CPU at more than twice the speed of slower modules. In practice, however, very few systems spend half their time waiting for RAM transfers, so expensive DIMMs won’t magically double your performance.
To find out what difference RAM speed makes, we ran our standard benchmarks on a Vista system equipped with 4GB of DDR3-800, then repeated the test with 4GB of DDR3-1600 RAM.
For many applications, the performance advantage gained through faster RAM is nominal – dBpoweramp, Photoshop and 3ds Max gained no benefit at all, while the multi-applications test ran a mere 0.5% faster. But the benefit wasn’t negligible: our Microsoft Office tests received a 1.5% boost, and the Canopus ProCoder video suite completed its tasks an impressive 5.5% more quickly.
Games can receive a 2-4% boost. So if you use your PC for heavy gaming or entertainment, where every second counts, investing in the fastest RAM can pay off – just. You may even want to overclock your RAM.
But for everyday usage, RAM speed is almost a non-issue, and investing in faster DIMMs won’t give a noticeable performance boost. You’ll see far greater benefits from adding more RAM, or investing in a faster hard
disk or CPU.
Does faster RAM really make a difference?
By Darien Graham-Smith
Nov 17, 2008 2:35PM
Nov 17, 2008 2:35PM