pk001,

The OptiPlex 390 system supports up to 8 GB of memory in the mini-tower (MT), desktop (DT), and small form factor (SFF) chassis with 64-bit operating system. The memory speed supported is 1333MHz DDR3 memory. The Optiplex 390 supports up to 2 DIMM slots and is capable of taking non-ECC dual-channel memory.

 However,32-bit operating systems can only use maximum 4GB memory address space. Certain components within the computer require address space in the 4GB range, any address space reserved for these components cannot be used by computer memory the amount of memory available to the operating system is less than 4GB.

This is a limitation of a 32-bit architecture: the system can only address 4 GB of allocated memory. Allocated memory is made up of physical RAM and any I/O space needed by devices. Normally this is not a problem, but when a system has 4 GB of physical memory, the memory addresses needed to map RAM overlap the space needed for the I/O devices. In this case the need for I/O space takes precedence, and the amount of RAM visible to the operating system is limited to 4 GB minus the I/O allocation.

Integrated

1. Intel HD Graphics (with Celeron/Pentium class CPU-GPU combo)
2. Intel HD Graphics 2000 (with iCore Dual/Quad core class CPU-GPU combo)
   
   
   • up to 512 MB shared video memory (with 1 GB system memory) - Windows XP
   • up to 272 MB shared video memory (with 1 GB system memory) - Windows Vista & 7
   • up to 3856 MB shared video memory (with 8 GB system memory) - Windows Vista & 7

Hi Jesse, thanks so much for the prompt response.  I guess it's confusing, as I called Dell to see if I could add memory and they said that for the desktop associated with my service tag I could add 4gb, as there were two memory slots.  From what you are saying, I can't add memory, and I am restricted to 4gb?

Sorry, I should have added "So it's a Windows 10 operating system restriction instead of a hardware issue?"

FYI  Optiplex 390SFF with Palm Beach MB and bios A14 supports 2x8gb for 16gb.  Verified.