The Java Card virtual machine is defined in terms of an abstract storage unit called a word. This specification does not mandate the actual size in bits of a word on a specific platform. A word is large enough to hold a value of type byte, short, reference or returnAddress. Two words are large enough to hold a value of type int.

The actual storage used for values in an implementation is platform-specific. There is enough information present in the descriptor component of a CAP file to allow an implementation to optimize the storage used for values in variables and on the stack.