Pointer access from qsys custom component

Hello

My nios code is running on a DDR

I have also a custom component connected to the DDR

The purpose is to define a pointer register in DDR and to overwrite its value from the custom component through its avalon mm bus

So, if i define a pointer on the c code . lets assume its address will be 0x800.

Which address should be used on the avalon mm bus of the custom component in order to do that ?

Thanks in advance