What is the system resource in the Intel SE7221BK1-E motherboard and how to disable them?

Question

Iron

I decided to put 4GB of memory into the Intel SE7221BK1-E motherboard and... it wasn't there... almost 800MB just disappeared, got "fucked up" by some mythical System Resources!? I found only a mention of them, but what is it, what is it for, why do they eat it and how can I turn it off!?
in the dock www.mavin.com/pdf/intel_se7221bk1.pdf they
write 4GB is supported. BUT! 750MB disappear somewhere???
<< The maximum memory capacity is 4 GB
Note* Although the Intel® Server Board SE7221BK1-E supports a maximum memory
capacity of 4 GB, system resources consume roughly 750 MB of physical memory in the
maximum memory configuration. As a result, when 4 GB of memory is used, the amount of
memory made available to the operating system is significantly lower than 4 GB; approximately
3200 MB. THIS IS ONLY AN ISSUE WHEN 4 GB OF MEMORY IS USED. A memory
configuration of less than 4 GB will not be susceptible to this issue. Please refer to Intel®
Technical Advisory TA 719-01>>
Technical Advisory TA 719-01 did not find, but found TA 719-02
download.intel.com/support/motherboards/server/sb/...
and somehow did not very understood, the reasons...??
addressing under PCI???
And how to decide that the entire amount of memory is available?

Answer 1

Most likely, these 750 meters are gobbled up for address space or caches of all kinds of equipment by the motherboard itself. There's nothing to be done here.

Answer 2

Does the processor support x64 mode? Most probably not. And if so, then the system can only address 4gb of the address space along with the address space of the rest of the hardware (pci, video, etc). Accordingly, in order for the system to work stably, the available RAM is cut.