Stack Machines vs. Register Machines & more

http://www.codeproject.com/Articles/461052/Stack-based-vs-Register-based-Virtual-Machine-Arch

Java VMs are stack-machines, here is one link
and the Android Davlik VM is based on a register-based-architecture.

"The stack machine style is in contrast to register file machines which hold temporary values in a small fast visible array of similar registers, or accumulator machines which have only one visible general-purpose temp register, or memory-to-memory machines which have no visible temp registers."

Here is a comparison (USENIX) of stack vs register machines. To quote from the paper:

"For example, the local variable assignment a = b + c might be translated to stack JVM code as ILOAD c, ILOAD b, IADD, ISTORE a. In a virtual register machine, the same code would be a single instruction IADD a, b, c. Thus, virtual register machines have the potential to significantly reduce the number of instruction dispatches."

and also

"In the stack-based JVM, a local variable is accessed using an index, and the operand stack is accessed via the stack pointer. In the register-based JVM both the local variables and operand stack can be considered as virtual registers for the method. There is a simple mapping from stack locations to register numbers, because the height and contents of the JVM operand stack are known at any point in a program"

Here is a comparison (Wikipedia) of different Application Virtual Machines

This is of course not a big deal, i.e. stack machines are just a way of compact code, but under the hood it of course uses registers to execute the code.