👻 Ghost-ARM

A lightweight, bare-metal operating system for ARM926EJ-S architecture, built from scratch using C and Assembly. This project implements preemptive multi-tasking, a priority-based scheduler, dynamic memory management, and an interactive shell interface.

🚀 Features

1. Kernel Core

• Preemptive Multi-tasking: Context switching using IRQ (Timer Interrupt).
• Priority-Based Scheduler: Round-Robin scheduling with weighted time slices (Quantum).
• Process Lifecycle Management: Supports READY, WAIT, and DEAD states.
• Context Switching: Full register context saving/restoring (R0-R12, SP, LR, PC, CPSR).

2. Memory Management (mm.c)

• Dynamic Allocation: Custom malloc and free implementation.
• Heap Management: Linked-list based memory block management with First-Fit algorithm.
• Split & Coalesce: Efficient memory block splitting logic.

3. Interactive Shell (shell.c)

• CLI Interface: UART-based command line interface.
• Process Control Commands:
  • jobs: List running tasks with PID, Priority, and State.
  • kill <pid>: Terminate a specific process.
  • stop <pid>: Suspend (pause) a process.
  • resume <pid>: Resume a suspended process.
  • sleep <sec>: Put the shell to sleep for N seconds (non-blocking for background tasks).
• System Info: uptime, memtest, whoami.
• Concurrency Handling: Atomic I/O operations with Critical Section Protection using disable_irq / enable_irq.

4. Hardware Abstraction

• Target Architecture: ARMv5TE (ARM926EJ-S) via QEMU VersatilePB.
• Drivers:
  • UART0 (PL011): Serial communication.
  • Timer0 (SP804): System tick generation.
  • VIC (PL190): Vector Interrupt Controller management.

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📂 Project Structure

.
├── ghost.h      # Global headers, structures (TCB), and constants
├── main.c       # Entry point, hardware initialization, and task creation
├── kernel.c     # Scheduler, IRQ handlers, and context switching logic
├── shell.c      # Shell interface and command parsing
├── mm.c         # Memory Manager (malloc/free)
├── lib.c        # String utilities and hardware I/O helpers
├── startup.s    # Startup, IVT, and Assembly interrupt handlers
├── linker.ld    # Linker script for memory layout
└── Makefile     # Build script

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🛠 How to Build & Run

Prerequisites

• arm-none-eabi-gcc toolchain
• qemu-system-arm

Build

make clean && make
# Cleans previous build and compiles ghost-arm.axf

Run

qemu-system-arm -M versatilepb -m 128M -nographic -kernel ./ghost-arm.axf

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💻 Shell Commands Manual

Command	Description
help	Show available commands
jobs	Show list of running processes (PID, Prio, State)
kill <pid>	Terminate process with ID <pid>
stop <pid>	Pause (Suspend) process <pid>
resume <pid>	Resume paused process <pid>
sleep <sec>	Sleep shell for <sec> seconds
memtest	Test dynamic memory allocation (malloc/free)
uptime	Show system uptime in seconds
clear	Clear the terminal screen

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📝 Future Roadmap

• Implement IPC (Inter-Process Communication) via Message Queues.
• Dynamic Task Spawning (spawn command).
• File System support (Simple RAM Disk).
• User Mode separation (currently runs in SVC mode).