This trace follows the actual state transitions behind the companion A Bytecode VM Executes a Portable Instruction Set. It describes a common execution path; implementation details can vary, so keep the contract separate from the mechanism.
Step 1: Fetch the next instruction
Bytecode is a compact instruction representation designed for a virtual machine rather than one physical processor. A VM fetches instructions, decodes operands, and updates execution state such as a stack, registers, frames, and heap. This layer can provide portability and runtime checks.
Step 2: Decode opcode and operands
A stack-based instruction sequence might push two constants, add the top values, and return the result. The VM’s instruction definitions specify operand widths, stack effects, and control-flow behavior. A verifier can reject malformed bytecode before execution if it proves stack and type constraints.
Step 3: Update VM execution state
For a stack-based ADD, pop the right operand and then the left, push their sum, and preserve the stack-effect invariant expected by the next instruction.
At this point, record the state that changed and check the invariant before advancing. If the operation repeats, make clear which values persist and which are recomputed.
Step 4: Follow a branch or call
Interpretation has dispatch overhead, while compiled native code may execute more directly but requires architecture-specific generation. Bytecode is only portable when runtimes agree on format and semantics. Sandboxing a VM also requires controlling native calls, memory, and resource consumption.
Step 5: Return a value or exception
For a stack VM, state the stack before and after an ADD instruction that consumes two numeric values. What invariant should a bytecode verifier enforce before this instruction executes?
The trace is complete when the result satisfies the stated contract. Compare this model with the concrete runtime or system you are studying before making a performance claim.