Virtual Machine Architecture Strategy

Comparative Study

Feature	Stack VM	Register VM
Examples	Python VM, JVM	Lua VM, Erlang BEAM
Execution Model	Operates on an implicit stack	Operates on explicit registers
Instruction Simplicity	High. Very small, simple instructions	Lower. Instructions encode more information
Instruction Count	High. More instructions for simple tasks	Low. Fewer instructions needed
Performance	Moderate. Extra stack manipulation overhead	High. Fewer memory accesses
Compiler Complexity	Low. Easy to generate code	Higher. Requires register allocation
VM Complexity	Low. Simple interpreter loop	Moderate. Instructions are richer
Maintainability	High. Easy to reason about	Moderate. More moving parts

1. Stack-Based Virtual Machine

A stack VM uses an implicit operand stack. Instructions do not name operands explicitly; instead, they consume values from the top of the stack and push results back.

Typical code:

PUSH a
PUSH b
ADD
POPRET

Pros:
- Simplicity. Instructions are small and uniform.
- Easy to implement and debug.
- Well-suited for simple compilers and educational languages.
Cons:
- Instruction-heavy. Even simple expressions require multiple instructions.
- Frequent stack pushes and pops introduce runtime overhead.
- Harder to optimize aggressively due to implicit data flow.

Stack VMs are ideal when implementation speed and conceptual clarity are more important than raw performance.

2. Register-Based Virtual Machine

A register VM operates on a fixed set of registers. Instructions explicitly specify their input and output operands.

Typical code:

ADD out, a, b

Pros:
- Performance. Fewer instructions and reduced memory traffic.
- More explicit data flow enables better optimizations.
- Instructions are more expressive and versatile.
Cons:
- Instructions are more complex.
- The compiler must manage register allocation.
- Slightly more complex VM implementation.