Why does the Fujitsu ESPRIMO E3500 only have 3 gigabytes of RAM available?

Question

linux

FUJITSU SIEMENS E3500 Mini Tower Computer.
Installed 3 strips of 2 gigs.
In Linux Mint and in the BIOS, 3 gigabytes of memory are available.
Team

sudo dmidecode -t memory | grep -iE "Size|Type:|Speed"
Error Correction Type: None
Size: No Module Installed
Type: DDR2
Speed: 667 MHz
Size: 2048 MB
Type: DDR2
Speed: 667 MHz
Size: 2048 MB
Type: DDR2
Speed: 667 MHz
Size: 2048 MB
Type: DDR2
Speed: 667 MHz

That is, the memory works and is determined correctly. But why does the top command show only 3 gigs in bios?
Tried to leave two strips of 2 gigabytes. The result is the same.

Answer 1

1. Check under another operating system. A LiveCD is enough, it is not necessary to install it.
2. Incompatible memory

Answer 2

64bit Mint?

Answer 3

At least I can say that this model has an Intel 945G chipset, for which the total amount of memory is 4GB maximum.
And yes, the bit depth of the OS is of interest. Although the BIOS should have seen 4GB.