The JDK  comes in both 32 bit and 64 bit flavors. 64 bit allows to address memory range > 3GB compared to 32 bit JVM’s. In order to run a JVM in 64 bit, add -d64 argument to the java command line in your shell script e.g.

java_cmd_line=”java -d64 –Xms512m -Xmx6g  -cp $classpath $env_options”

This will cause the JVM to run in 64bit mode.

The -d64 option means to use the 64-bit version of the JVM. The JVM has three basic configurations: -client, -server (both for 32-bit JVMs) and -d64. 

In order to use -d64, you have to separately install the 64-bit JVM for your platform.

*If you want to check if a JDK can support  64 bit version or not, just run the java -version command but with -d64 argument as well. If the JDK does not support it, it will give the below error:
~/server/bin$ java -d64 -version
Error: This Java instance does not support a 64-bit JVM.
Please install the desired version.