Question 12GB ram but only 8GB usable?

[billy_b]

Had a look at a few posts but most seem to mention mixing completely different sticks. A friend decided to upgrade and give me his 2x4gb 'corsair vengeance LP' ram as one of mine broke. The ram sticks are the same brand and speeds (1600mhz) but slightly different models.

https://imgur.com/eH7epTE

View: http://imgur.com/eH7epTE


My question is why is Windows displaying 12gb but only 8 as usable and am I damaging performance but having my old 4gb stick in my pc?

 

[McKeu]

If you run uneven amounts of RAM, placement matters more than everything. I would probably never recommend a 3 DIMM setup, though. Rarely works as expected.
It can mess up the timings.

 

[Calvin7]

Memory is guaranteed in the form sold. Use of exact match, same thing, identical have no guarantee to be compatible together. Such combinations may raise the odds of memory working together over the use of random modules.

Workarounds to get extra memory working include:
Lower memory clock speed
Relax DRAM timings
Raise DRAM voltage