stackmaster-vm

A tiny stack-based virtual machine written in C, built to explore the core ideas behind computer organization: program counters, stack pointers, and the fetch-decode-execute cycle.

What it does

The VM lets you interactively build a small "program" made up of instructions (PUSH, POP, ADD, SUB), then executes them one at a time against an internal stack — similar in spirit to how a real CPU or a bytecode interpreter (like the JVM) operates.

On startup, it repeatedly prompts:

which operation would you like to do? 1 for PUSH, 2 for POP, 3 for ADD, 4 for SUB, else exit

Entering 1 (PUSH) also prompts for a value to push. Entering anything other than 1, 2, or 3 ends input and runs the program you built. Up to 10 instructions can be entered.

Example session

Entering PUSH 9, PUSH 2, POP, PUSH 1, ADD, then exiting produces this trace:

Step	Stack
PUSH 9	[9]
PUSH 2	[9, 2]
POP	[9]
PUSH 1	[9, 1]
ADD	[10]

Final output: 10

How it works

• struct Instruction — holds an opcode (enum OpCode) and an operand value.
• struct VM — holds the instruction array, a program counter (pc), a value stack, and a stack pointer (sp).
• push / pop — manipulate the VM's internal stack.
• run — the fetch-decode-execute loop: reads the instruction at pc, branches on its opcode via a switch, executes it, then advances pc.
• main — interactively collects up to 10 instructions from the user (with bounds-checking so input stops once the instruction array is full), then calls run.

Build & run

gcc stack.c -o stack
./stack

Follow the prompts to build your program, then see the final top-of-stack value printed at the end.

Possible next steps

• Additional opcodes (MUL, JMP, PRINT)
• A simple text-based assembler so programs can be written as strings instead of typed in interactively
• Bounds-checking on the value stack itself (currently only the instruction array is guarded)
• Step-by-step execution tracing/printing