The idea is the following: I plane to upgrade the RAM of my two laptops, and I want to play some games with the laptop I use to work. So increasing dedicated RAM of intel HD graphics is supposed to work, no?
This depends on your CPU / graphics model, for the latest CPU it is limited by the operating system to a maximun of a half of the RAM installed and it is dinamically allocated by the OS to the apps, you do not have control of this...
For more info check this link
This is correct; any allocation specified in the BIOS is only applicable while the BIOS is operation (i.e. during BIOS Setup and other UEFI Shell environments).
I have been told that there is a way to specify an increased default allocation via the Registry, but I do not have the details for this. If someone else has the information, please chime in.
Nice. So, in theory, if I use a laptop equipped with an I5 5th gen, and 16go ram, I can dedicate 4go of memory to Intel HD.
Does it means that Intel HD graphics with these 4go ram will reach the performance of a separate graphic card? (Can we make a comparison with the Nvidia 950M gpu installed in the laptop used for the experience?)
Or Intel HD graphics is limited, regardless the amount of ram allocated?
Ps. Excuse me for the potential language mistakes, I'm french ^^
You could do this, but it isn't worth it. The driver will allow up to (in your case) 8GB of memory to be used if conditions warrant.
Let's face it; Intel HD Graphics, especially as far back as your 5th gen. processors, is not going to keep up with any of these discrete graphics solutions. It is a basic capability for budget-minded people. If you want to understand how it compares to other solutions, just look at the charts published all over the interweb. Check out those at Tom's Hardware, for example.
If you are not comfortable communicating in English, just do so in French; we can translate easy enough.