Virtual Machine from Scratch

This project was made around December, 2024 and is a simple virtual machine (VM) implemented in C. It provides a basic understanding of how virtual machines work by simulating a simplified computer architecture.

Overview

The VM has its own memory, registers, and a limited instruction set. It can load and execute a program written in its assembly-like language(yup had to re-invent assembly).

Key Components

• tankvm.h: Header file containing definitions for data structures and function prototypes.
  • VM struct: Represents the virtual machine, including:
    • CPU struct: Represents the CPU, containing registers.
    • Memory : A byte array representing the VM's memory.
    • b: Break address, indicating the end of the program in memory.
  • Registers: 16-bit registers (ax, bx, cx, dx, sp, ip) within the s_register struct.
  • Opcode enum: Defines the instruction set (e.g., mov, nop, hlt).
  • Instruction struct: Represents a single instruction with an opcode and arguments.
  • instrmap: A lookup table mapping opcodes to instruction sizes.
• tankvm.c: Source file containing the implementation of the VM's functionality.
  • virtualmachine(): Allocates and initializes a new VM struct.
  • exampleprogram(): Creates a sample program in VM's memory (hardcoded).
  • execute(): The main execution loop that fetches, decodes, and executes instructions.
  • execinstr(): Executes a single instruction based on its opcode.
  • __mov(): Implements the mov instruction (move value into a register).
  • map(): Returns the size of an instruction based on its opcode.
  • error(): Handles errors like segmentation faults or halt instructions.
  • Utility functions: copy(), zero(), printhex().
  • main(): Entry point that creates a VM, loads the example program, executes it, and prints memory contents.
• Makefile: Defines the build process.

Instruction Set

The VM supports the following instructions:

• mov: Move a value into a register.
• nop: No operation.
• hlt: Halt the VM.

How it Works

1. Initialization: The main function creates a VM instance using virtualmachine().
2. Program Loading: The exampleprogram() function creates a simple, hardcoded program in the VM's memory.
3. Execution: The execute() function fetches instructions, decodes them, and executes them in a loop.
4. Termination: The program halts when a hlt instruction is encountered.

Building

To build the project, run:

make

Then go ahead and execute:

./tankvm

Limitations:

• Limited instruction set.
• Hardcoded program.
• No input/output.
• Limited memory (65KB).

Future Enhancements:

• Implement more instructions.
• Allow loading programs from a file.
• Add input/output capabilities.
• Improve error handling.