Java Stack

Yes, Java is Stack based Architecture Virtual Machine,

A virtual machine (VM) is a high level abstraction on top of the native operating system, that emulates a physical machine. Here, we are talking about process virtual machines and not system virtual machines. A virtual machine enables the same platform to run on multiple operating systems and hardware architectures. The Interpreters for Java and Python can be taken as examples, where the code is compiled into their VM specific bytecode. The same can be seen in the Microsoft .Net architecture, where code is compiled into intermediate language for the CLR (Common Language Runtime).
What should a virtual machine generally implement? It should emulate the operations carried out by a physical CPU and thus should ideally encompass the following concepts:

• Compilation of source language into VM specific bytecode
• Data structures to contains instructions and operands (the data the instructions process)
• A call stack for function call operations
• An ‘Instruction Pointer’ (IP) pointing to the next instruction to execute
• A virtual ‘CPU’ – the instruction dispatcher that
  • Fetches the next instruction (addressed by the instruction pointer)
  • Decodes the operands
  • Executes the instruction

There are basically two main ways to implement a virtual machine: Stack based, and Register based. Examples of stack based VM’s are the Java Virtual Machine, the .Net CLR, and is the widely used method for implementing virtual machines. Examples of register based virtual machines are the Lua VM, and the Dalvik VM (which we will discuss shortly). The difference between the two approaches is in the mechanism used for storing and retrieving operands and their results.
Stack Based Virtual Machines
A stack based virtual machine implements the general features described as needed by a virtual machine in the points above, but the memory structure where the operands are stored is a stack data structure. Operations are carried out by popping data from the stack, processing them and pushing in back the results in LIFO (Last in First Out) fashion. In a stack based virtual machine, the operation of adding two numbers would usually be carried out in the following manner (where 20, 7, and ‘result’ are the operands):

1. POP 20
2. POP 7
3. ADD 20, 7, result
4. PUSH result

Because of the PUSH and POP operations, four lines of instructions is needed to carry out an addition operation. An advantage of the stack based model is that the operands are addressed implicitly by the stack pointer (SP in above image). This means that the Virtual machine does not need to know the operand addresses explicitly, as calling the stack pointer will give (Pop) the next operand. In stack based VM’s, all the arithmetic and logic operations are carried out via Pushing and Popping the operands and results in the stack.
Register Based Virtual Machines
In the register based implementation of a virtual machine, the data structure where the operands are stored is based on the registers of the CPU. There is no PUSH or POP operations here, but the instructions need to contain the addresses (the registers) of the operands. That is, the operands for the instructions are explicitly addressed in the instruction, unlike the stack based model where we had a stack pointer to point to the operand. For example, if an addition operation is to be carried out in a register based virtual machine, the instruction would more or less be as follows:

1. ADD R1, R2, R3 ;        # Add contents of R1 and R2, store result in R3

As I mentioned earlier, there is no POP or PUSH operations, so the instruction for adding is just one line. But unlike the stack, we need to explicitly mention the addresses of the operands as R1, R2, and R3. The advantage here is that the overhead of pushing to and popping from a stack is non-existent, and instructions in a register based VM execute faster within the instruction dispatch loop.
Another advantage of the register based model is that it allows for some optimizations that cannot be done in the stack based approach. One such instance is when there are common sub expressions in the code, the register model can calculate it once and store the result in a register for future use when the sub expression comes up again, which reduces the cost of recalculating the expression.
The problem with a register based model is that the average register instruction is larger than an average stack instruction, as we need to specify the operand addresses explicitly. Whereas the instructions for a stack machine is short due to the stack pointer, the respective register machine instructions need to contain operand locations, and results in larger register code compared to stack code

 Now the question can be Why JVM is stack based VM

There are a few attributes of a stack-based VM that fit in well with Java's design goals:

1. A stack-based design makes very few assumptions about the target hardware (registers, CPU features), so it's easy to implement a VM on a wide variety of hardware.
2. Since the operands for instructions are largely implicit, the object code will tend to be smaller. This is important if you're going to be downloading the code over a slow network link.

Going with a register-based scheme probably means that Dalvik's code generator doesn't have to work as hard to produce performant code. Running on an extremely register-rich or register-poor architecture would probably handicap Dalvik, but that's not the usual target - ARM is a very middle-of-the-road architecture.

Now Read out full Detail about Java Stack:

Java Stack

And Read out what is Native Stack

Native Stack