What does the 1Rx8 mean vs. 2Rx8 for RAM and are they compatible?

Are a

• 4GB 1Rx8 PC3L 12800S

and a

• 4GB 2Rx8 PC3 12800S

compatible?

What do 1Rx8 and 2Rx8 mean? Can I put them together?

1Rx8 means it is a single-rank module and 2Rx8 means it is a dual-rank module module. Rank is a data block which is 64 bits wide without Error Correction Code (ECC) created using some, or all of the memory chips on a module. The x8 in them specifies the number of banks in the memory module. Higher the number of banks, the fewer the chips in the memory module, the better the reliability and power consumption.

Compatibility is generally not an issue in most cases. But you should go through the motherboard manufacturer's guide of supported RAM modules just to make sure if they are compatible or not.

Coming to the PC3 and PC3L part, PC3 RAMs requires an operating voltage of 1.5V whereas PC3L requires an operating voltage of 1.35V. The 'L' in PC3L signifies Low Voltage. So using PC3 RAM in a PC3L RAM slot will not provide it sufficient voltage and it will fail to operate. On the other hand using a PC3L RAM in a PC3 RAM slot may damage it due to overvoltage. So PC3 and PC3L RAMs are not compatible with each other.

So in short 1Rx8 PC3L 12800S and 2Rx8 PC3 12800S RAMs are not compatible with each other. But if it is only the 1Rx8 and 2Rx8 in concern, they may be compatible with each other but it's best to check the motherboard manufacturer's list of compatible RAMs.

These modules aren't compatible with each other.

• PC3 is standard voltage (1.5 V) DDR3 memory, while PC3L is DDR3L low-voltage (1.35 V) memory. Newer Intel processors (since fourth-generation Haswell) require DDR3L memory; regular DDR3 modules are out of specification for these processors and can cause the system to fail:

Notebooks Cannot Complete Power On Self Test (POST) When Used With DDR3 Memory Module

Notebooks utilizing 4th Generation Intel (Haswell) processors require a new type of memory known as DDR3-Low Voltage or “DDR3L”.

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Upgraders: 1.35V memory is required!

Please note we fit 1.35V memory into our G750s. New Haswell CPUs require this and using 1.5V memory can cause problems along the lines of:

• Overheating
• Damage to the CPU IMC (due to the increased potential difference)
• Greater battery use than our stated values.

• 1R and 2R refer to the number of memory ranks on the module. Dual-rank (2R) modules appear to the system as if two memory modules were inserted in that memory channel, which may cause compatibility issues if you have multiple such modules on the same channel.

• The highest density memory modules (32 GB or more) are intended for server use and generally will not work on consumer systems that use Intel Core processors. Consumer CPUs are typically limited to two or four ranks per channel while server processors can often handle eight or more ranks; this explains why many Intel Xeon processors can accept extremely large amount of memory (768 GB for Xeon E5 V3 parts).

PC3L can generally replace PC3, but not vice-versa. PC3L is DDR3 RAM spec'd to run at 1.35v, but is generally also fine at 1.5v. PC3 is spec'd to run at 1.5v; it might not work at 1.35v.

So, if your system came with PC3L then it probably requires it, so don't try to replace it with PC3. But if your system came with PC3 then you can probably replace it with either PC3 or PC3L.

Generally you can mix 1Rx8 and 2Rx8, as well, though mixing different kinds of RAM sometimes prevents interleaving (i.e., it might be a tad slower).

I do recommend running a memory test after RAM upgrades, though. Windows Memory Diagnostic is probably sufficient, though MemTest86 or MemTest86+ is better.