Win7 System desktop Configuration 8G memory shows available memory only 3.4G What's going on?

win7 System desktop configuration 8G memory display available memory only 3.4G What's going on?

The specific steps are as follows:

1, start-run (Input msconfig)-click: OK or enter;

2, in the pop-up "System Configuration" window, we click: Guide;

3, then we click: Advanced Options (V);

4, in the Pop-up Boot Advanced Options window, we see: Maximum memory is 256MB, and we configure the memory is 8GB, so the problem is here.

Now we change the maximum memory to 8000MB, then click: OK, back to "System Configuration Window" also click: OK;

5, in the pop-up "System Configuration" window, we point: Reboot (R), restart the computer;

6, restart the computer, we click: Computer-Properties, visible: Installed memory (RAM): 8.00GB (7.40G available), now the computer's memory is normal.