Only 8 of 16 GB recognized at BIOS - XPS 8500, i7-3770, Win10 Home...

These rams are old - originally came with system. There were removed and plug-in again ?

These are the original rams came with system - NOT new rams? it was just removed and plug-in again.

These are old ram, came with the system.

Did you reset BIOS before or after you reinstalled all 16 GB of RAM?

If you reset BIOS before reinstalling all 16 GB, try resetting BIOS, but this time with all 16 GB installed.

I've reset BIOS with installed 16 GB through the following method I've found from forum.

Open BIOS setup and copy down all the current settings

Power off and unplug

Press/hold power botton for ~15 sec

Open case and remove motherboard battery

Press/hold power button for ~30 sec

Reinstall battery (right-side-up!)

See if BIOS recognizes all RAM now

But SAME result only 8 GB showing at BIOS, is there any other method to reset BIOS ?

thanks for helping....

That's the best way to reset BIOS. Sure the modules are all seated correctly? Is the motherboard battery old and weak/dead? CR2032 3-volt coin cell battery, ~US$2.00.

I guess it's possible you have a motherboard failure.  If you run the free CPU-Z, does it report a 4 GB module in all 4 slots?

www.cpuid.com/.../cpu-z.html

I'm now going to purchase new coin cell and replace it, let's see it would make any difference.

BTW: Did you -at some point- increase the amount of RAM installed from 12 GB to 16GB by installing the module that's in slot 4? Or maybe you replaced an original module that was in slot 4?

The modules in slots 1-3 are all from Hyuandi and the one in slot 4 is from A-Data. So either that A-Data module is defective or it's just not compatible with the other three so BIOS is ignoring that entire bank of modules (eg slots 3 and  4).

Looks like CPU-Z is seeing all four 4-GB RAM modules.  BUT -- the module in slot 4 is only being reported as "single rank" while the other three are all reported as "dual rank".  That might be why BIOS is only reporting 8 GB instead of 16.

How much RAM does Windows (what version?) report, and how much is "available" to the OS?

You may have a failed module in slot 4 or a failed motherboard slot 4.  Remove the modules from slots 3 and 4 and use some canned air to blow out the motherboard slots. Then swap the module from slot 4 into slot 3 and the one from slot 3 into slot 4. Rerun CPU-Z.

If "single rank" goes with the position of the modules, then you probably have a failed RAM module. If "single rank" is still reported for slot 4 after the swap, you have a motherboard problem.

If it's a RAM problem, you'd have to replace both modules in that pair so you have matched modules. Just buying one module with the same specs doesn't mean they will be matched. And if not matched, they won't run as DDR RAM.

EDITED

I've replaced slot 4 ram (A-Data) year ago, when one of the ram stopped working. But it was working fine until I had to reset CMOS because my system wasn't starting and unplug/plug all ram modules last week.

I've replaced new coin battery but same result. I've Windows 10 home and it is showing 8 GB.

I will replace (slot 3/4) positions and let you know.

Thanks for helping.

BTW: You probably should try the module swap I previously suggested. If the "single rank" goes with the A-Data module as shown by CPU-Z, then replace 2 RAM modules.

But, if "single rank" stays with slot 4, there's probably no point spending money on new RAM, since it may be a motherboard failure.

Possible the A-Data module recently failed which caused the booting problems, and you had to clear BIOS so it would see the "change".

Keep in mind you will need to swap the A-Data module and the one that's in the other slot with the same color retention clips. That may NOT be slot 3, and physical slots on the motherboard may not be in same order the PC recognizes them.  So swap the A-Data module and the Hyuandi module from the other slot with matching color clips and you'll be ok.

You can keep the extra (old) Hyuandai module as a spare in case one of the other Hyuandi modules ever fails, since those 3 are all matched.

Not point paying extra for any RAM that's faster than what you have now because faster RAM will run at the speed of the slowest. Many around here buy RAM for Dell PCs from crucial.com because they guarantee it works or they take it back.

www.crucial.com/.../xps-8500

I will do the swapping on weekend and let you know the result of CPU-Z. Btw I tried only Hyundai rams in different slots previously and it was showing 8 GB instead of 12 GB but not sure about which slots I've used. I will try again as you said.

Thanks for helping.