The Novice Computer Upgrade Series: Memory


More than a decade ago memory was single access. Meaning writing to memory required one cycle, one clock tick. In the next tick it could read memory. But, the memory could not be read and written to in the same clock tick.

The newer DDR (Double Data Rate) memory transfers data twice per clock tick. The older style wrote at the peak voltage and the newer style on the change, the increasing and decreasing voltage. Thus the DOUBLE. This allows moving twice as much data per clock tick.

So, the letters DDR in memory stick designates how data can transfer. The number following ‘DDRx’ is the frequency the memory can run at, so about chip speed .

Memory cannot go faster than the CPU or the motherboards data bus (the circuits that carry the data). But, the memory has to keep up with the CPU, otherwise the CPU waits, meaning things get slow. Everything has to stay synchronized, otherwise it is chaos, doesn’t work.

The memory in use today is:

  • DDR 266:1 GB/s – 2000 – 400 MTps*
  • DDR 333:6 GB/s
  • DDR 400:2 GB/s
  • DDR2 533:2 GB/s – 2003 – 1,066MTps
  • DDR2 667:3 GB/s – 240 pins
  • DDR2 800:4 GB/s
  • DDR3 1066:5 GB/s – 2007 – 2,133MTps
  • DDR3 1333:6 GB/s – 203 pins
  • DDR3 1600:8 GB/s
  • DDR3 1866:9 GB/s
  • DDR4 2133:17 GB/s – 2014 – 4,266MTps
  • DDR4 2400:2 GB/s – 288 pins
  • DDR4 2666:3 GB/s
  • DDR4 3200:6 GB/s

*From the PC Encyclopedia: (MegaTransfers per SECond) A measurement of bus and channel speed in millions of “effective” cycles per second. Also written as “MT/s,” it is a rating of the actual, delivered speed rather than the frequency of the clock. For example, if timing is derived from both the rising and falling edges of the cycle rather than one complete cycle, a 400 MHz clock yields 800 MT/sec.

Motherboards will handle one of the four types. A few will handle 2 types. But, not at the same time and they are rare. The motherboards handle one or more of the various speeds within a DDR type. Fast memory in slow boards only goes as fast as the board. A fast board with slow memory only goes as fast as the slow memory. Use the fastest memory the motherboard will support. Or if cost is more important than speed, use the slowest the board supports as slow is cheap.

You will see people using faster memory than the CPU and motherboard support. Those are the over-clockers. They change settings in the motherboard to push the envelope. They are unlikely to be reading this and my readers are unlikely to be over-clocking their memory. So, I’ll just say there is no point in spending for speed you will not use.

Generally the motherboard detects the memory speed and adjusts to the correct speed. You need only install the memory, the computer figures it out. This part of the computer is not too smart. So mixing memory of different speeds can confuse the computer and keep it from booting.

Speed is designated by a frequency number following DDR# or a PC# followed by a module name.  These two designations refer to the same memory module: DDR4 2133mhz or PC4-17000. Another example: DDR4 2400mhz or PC4-19200.  Your motherboard’s spec will specify memory using one or both of these names.

Why the two names? DDR# is about the memory chip speed. PC# module names are about the speed of the module, meaning chips, controller, and circuit board, the DIMM. You can multiple the DDR frequency by 8 to get the module transfer rate. If you are an engineer working with memory, the difference in names mean a lot. To us, not so much. We just have to pick the right module.

Each DDR type has a specific pin count and a specially located slot in the memory stick to prevent accidentally placing the wrong type in the computer.

More pages… links below.

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