Hello Rio Chang,
As correctly pointed by Sandra, the global variables value is saved neither in ABAP memory nor SAP Memory.
I suppose you have a top include with all the global data declaration, used by all Function modules in the function group. The values of these variables are saved in temporary memory at run time, and are available for the particular session.
Also check that global variables in one function module are not cleared, if you want to use the same values is another function module as well.
Please elaborate the issue further so we can understand and respond better.
You may visit the below link for another chain on global variables:
http://scn.sap.com/thread/1066150
Thanks and Regards,
Sheily Babel.