But I still do not understand why Mandriva only find half of the amount
The mandriva kernel was not compiled to use more than 1GB of memory.
You can sort this out my recompiling your kernel. I think CONFIG_HIGHMEM is the parameter you need to change, from 1GB to something larger.
Never recompiled your kernel before? Now's the perfect excuse to try it. Lots of guides on the web, and it is a great feeling when you get it to work!