Memory Matters
FEATURE
THE OBSESSION WITH SPEED
Memory speed is one of the most
convoluted specifications around, and is
often misrepresented. Way back when, in
the early days of SDRAM development, the
megahertz measurement was the correct
way of advertising the associated speeds of
memory. In short, every single solid-state
component in your machine operates at a
specific frequency, or Hz – whether it’s your
processor, GPU, memory, or even SSD, each
one operates on a cycle. Like the ticking of a
clock, each tick represents a single hertz or
cycle (the opening and closing of atransistor gate, in this case). A speed of 1Hz,
for example, is one cycle per second; 2Hz is
two per second; a MHz is 1,000,000 cycles
per second; you get the picture. Once we got
our heads round it, it made perfect sense,
and continued to do so for years.
The problem is, when DDR (or double
data rate) RAM came on the scene, it
changed how data transfers were registered.
Instead of only actuating once on the rising
of each clock cycle, it could now also
process an additional operation on the fall
of that same clock cycle, effectively doublingthe rate at which the DIMM could process
data. The figure for accurate measurement
of data transfer requests then shifted from
MHz to MT/s to adjust for this change,
despite the fact that memory still operated
at the same frequency. However, marketing
apparently didn’t get that memo, because
many companies, in a bid to tout it as the
next big thing, ignored the MT/s figure,
instead referring to it as MHz, while
modern-day memory quoted at 2,400MHz,
for instance, actually only operates at half
that frequency.TIMINGS AND LATENCY
The next part of the holy trinity of memory
specifications revolves around timings and
latency. There’s a ton of them, but the most
important one you need to keep in mind is
the CAS latency. Referring to the Column
Address Strobe, this figure indicates just
how many clock cycles it’s going to take for
the memory module to access a particular
memory location, either to store or retrieve
a bit of data held there, ready for processing
by the CPU.
That said, this figure on its own doesn’t
give you all the information you need. It’s
only when you combine it with the memorytransfer rate we mentioned above that you
get a better picture of just how fast your
memory modules are. So, how do we get a
figure that makes any sort of logical sense to
us consumers? Well, there’s a handy
formula that converts CAS latency and
MT/s into a real-world latency: Latency =
(2,000/Y) x Z, where Y is your RAM’s
speed in MT/s, and Z is your CAS latency.
So, as an example, if we take a 2,666MT/s
memory kit, operating with a CAS latency
of 15, we get a real-world result of 11.25ns.
This tells us exactly the total time it takes
for that memory module to access, store, orrequest a bit of data from its particular
location on the module.
This is where the concept of overclocking
your memory typically comes unstuck. As a
general rule of thumb, the higher the
frequency, the higher the CAS latency, and
as such, real-world performance gains are
often slim, unless the rise of that CAS
latency is slowed as well. When it comes to
the “best” performing memory, what you’re
looking for is a kit that has a high frequency,
a low CAS latency, and the necessary
capacity (see opposite) to do what you want
to do.Technology Memory Chip Capacity Transfer Rate (MT/s) CAS Latency (ns) Real Latency (ns)DDR2 500MB 400 5 25.00DDR2 500MB 800 5 12.50DDR3 1GB 800 9 22.50DDR3 1GB 2,400 11 9.17DDR4 2GB 2,133 15 14.06DDR4 2GB 4,500 19 8.44DDR5* 4GB 4,266 23 10.78DDR5* 4GB 6,400 27 8.44RAM LATENCIES