Memory specifications are unusually easy to misread, because the two numbers manufacturers print largest — capacity and speed — interact with a third they print smallest. Understanding the mechanism makes the spec sheets readable and explains why the biggest number is often not the best buy.
Cells, refresh and why RAM is volatile
Each bit of DRAM is a tiny capacitor paired with a transistor. A charged capacitor is a 1, a drained one is a 0. Those capacitors leak, so the controller has to read and rewrite every row thousands of times a second — the “refresh” in Dynamic Random Access Memory. Cut the power and the charge is gone within moments, which is why RAM forgets everything on shutdown and an SSD does not.
MT/s, not MHz — and what bandwidth buys you
DDR stands for Double Data Rate: the module transfers on both the rising and falling edge of the clock. So DDR4-3200 runs on a 1600MHz clock but performs 3200 million transfers per second, and MT/s is the correct unit even though almost every listing writes “3200MHz”. Multiply MT/s by 8 bytes to get bandwidth per channel: DDR4-3200 gives 25.6 GB/s, DDR5-6000 gives 48 GB/s. Bandwidth matters most to integrated graphics, video encoding and simulation-heavy games; for a GPU-bound title at 1440p it barely registers.
CAS latency and the nanosecond that matters
CAS latency (the CL number) counts clock cycles between the controller asking for data and the module delivering it. Because a cycle gets shorter as speed rises, CL values are not comparable across speeds — you have to convert to time: CL ÷ MT/s × 2000 = nanoseconds. DDR4-3200 CL16 is 10.0ns. DDR5-5600 CL46 is 16.4ns. DDR5 is genuinely slower to first byte and only wins once you are moving enough data for its bandwidth advantage to dominate, which is the single most useful fact in this entire guide.
XMP, EXPO, and why new RAM runs slow
Every module boots at a conservative JEDEC default — often 2133 MT/s for DDR4 or 4800 for DDR5 — because that speed is guaranteed to work everywhere. The advertised speed lives in a stored overclocking profile: XMP on Intel, EXPO on AMD. Enabling it is one toggle in the BIOS, and skipping it is the most common reason a new stick feels no faster than the old one. Some modules, like the Crucial DDR5 here, carry both profiles; the Crucial DDR4 needs neither because 3200 is already its JEDEC default.
Channels and ranks
Desktop CPUs have a dual-channel memory controller, so two modules are read in parallel and deliver double the bandwidth of one. Within a module, rank describes how the chips are grouped: a dual-rank (2R) stick like the Acer UD100 lets the controller interleave between two sets of chips and typically edges out a single-rank module of the same capacity by a few percent. Four dual-rank sticks stress the controller hardest, which is why high-speed kits are usually sold as two modules rather than four.