Track the Top of an Operand Stack

The Operand Stack

Stack-based VMs evaluate expressions on an operand stack. A `PUSH` puts a value on top; arithmetic pops operands and pushes results. The single most useful mental model in this whole track is tracing what the stack looks like after each instruction.

Tracing By Hand, Then By Code

Given the sequence `PUSH 3`, `PUSH 4`, the stack holds `[3, 4]` and the top is 4. After an `ADD` it holds `[7]`. Bytecode compilers reason about exact stack depth at every instruction. The JVM verifier rejects classes where depths do not line up.

Implementation sketch:

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Stack operations you will implement:

• push X: increment top, store X
• pop: read stack[top], decrement top
• empty check: report when top == -1 after all commands

For example, three pushes followed by one pop leaves the top equal to the second value pushed.

Design Notes

Overflow and underflow checks come later, but the invariant is simple: top must never exceed the array bound and must never go below -1 on pop. Tracing stack depth by hand before writing code prevents an entire class of off-by-one bugs in the interpreter tasks ahead.

For This Exercise

This exercise asks you to simulate only pushes and pops before you add arithmetic. You will implement an array plus a `top` index and report the final top value, making the core data structure second nature.