Program/Approach to find max heap size and max perm gen size for any system configuration?

To Test the max heap size and max permgen space, As per reply suggested by Gilli at Max amount of memory per java process in windows?, i wrote a simple java class with main method just printing system out

when i ran the class from Console with below param it ran fine but with 2048 it gave the error could not reserve Could not reserve enough space for object heap. It means for my system max heap size is allowed somewhere between 1536 to 2048

 java -Xms1536m -Xmx1536m  TestJVMParam
 

Fine till here. Now i want to find max permgen size for my system. So i ran the same program with below paramters

 java -Xms1536m -Xmx1536m -XX:PermSize=128m -XX:MaxPermSize=128m TestJVMParam
 

but again it says Could not reserve enough space for object heap. How is that possible as i have
8GB RAM with windows7 64 bit os. And its not allowing even 128m for permgen? i am sure i am missing something here but not able to figure it out?

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