Can a “Dual Rank Memory Module” be used as a single module and not have it affect other slots

memory

Current configuration

My laptop is capable of supporting up to 32 GB.

I presently have 4 slots filled with the following memory:

Samsung 4GB PC3-12800 DDR3-1600MHz non-ECC Unbuffered CL11 204-Pin SODIMM 1.35V Low Voltage Single Rank Memory Module Mfr P/N M471B5173BH0-YK0

RAM
    16.0GB Dual-Channel DDR3 @ 799MHz (11-11-11-28)
        Memory slots
            Total memory slots  4
            Used memory slots   4
            Free memory slots   0
        Memory
            Type    DDR3
            Size    16384 MBytes
            Channels #  Dual
            DRAM Frequency  799.1 MHz
            CAS# Latency (CL)   11 clocks
            RAS# to CAS# Delay (tRCD)   11 clocks
            RAS# Precharge (tRP)    11 clocks
            Cycle Time (tRAS)   28 clocks
            Command Rate (CR)   2T
        Physical Memory
            Memory Usage    26 %
            Total Physical  16 GB
            Available Physical  12 GB
            Total Virtual   32 GB
            Available Virtual   27 GB
        SPD
            Number Of SPD Modules   4
                Slot #1
                Slot #2
                Slot #3
                Slot #4

Question

If I add a single module of 8 GB that is marked "dual rank", will the system work or not? In other words, if I make the configuration:

8 | 4 | 4 | 4

will it affect the functionality of the memory in the other slots?

Best Answer

Most CPUs support a limited number of memory ranks.
Most consumer DIMMs are dual-rank (this is the most economical solution).

However there are also single rank DIMMs and quad-rank DIMMS. The difference betyween these is rather technical (and I had to look it up myself about a year ago), but it seems to come down to this:

  • A CPU likes to have more memory channels to make things faster.
  • This can be done with more DIMMs (seel dual-, triple- or even quad-channel CPU's), or by using one physical modules with 'sort-of' the content of two DIMMs on one stick.
    This is the rank.
  • Most systems use dual channel dual rank memory. (two DIMMs, each with two ranks).
  • A CPU can only drive a limited number of ranks. (Think of it as each rank draining the address bus a bit. More ranks is more draining. This limits how many ranks can be used, though sometimes more ranks can be used at a lower speed.

This (https://serverfault.com/questions/69612/dimms-single-vs-double-vs-quad-rank) might be interesting reading to grok the background.


But to answer your question:

My laptop is capable of supporting up to 32 GB.

That simple is not enough information. To answer it with any certainty we need something like:

  • My laptop is capable of supporting up to 32 GB.
  • In four DIMM sockets.
  • With a maximum of X ranks
  • And at a voltage of a.bc per DIMM.

My best guess it that it will work.

Depending on he chipset you may and up with:

8GB DIMM 4+4) First part of the 8GB DIMM Only in dual channel mode 4GB DIMM (4)

4GB DIMM (4) In dual channel mode 4GB DIMM (4)

Or with no dual channel mode at all for 12GB of your memory.

Note that the ranks at this point are mostly about 'will it work', and the channels are about 'will I get the 5% average speed increase from going dual-channel`.