👻 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.
• ANSI Terminal Control: Clear screen and cursor positioning.
• File System Commands: format, ls, save, load for Flash memory file operations.
• Concurrency Handling: Atomic I/O operations with Critical Section Protection using disable_irq / enable_irq.
• Built-in Tetris Game: Launchable Colorful Tetris game within the shell.

4. Built-in Tetris Game (tetris.c)

• Graphical Interface: ANSI terminal control for rendering the game board.
• Game Logic: Piece rotation, line clearing, scoring system.
• Persistent High Score: Saves and loads the best score from Flash memory.

5. Flash Memory File System (flash.c/h)

• Simple File Storage: Save and load files to/from Flash memory.
• File Management Commands:
  • format: Format the Flash storage.
  • ls: List files stored in Flash.
  • save <name> <text>: Save text data to a file.
  • load <name>: Load and display file content.
• File Structure: Basic file header with name, size, and offset management.

6. 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
├── flash.c/h    # Simple Flash memory disk file system
├── console.c    # ANSI terminal control and screen handling
├── tetris.c     # Tetris game implementation
├── 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
• dd if=/dev/zero of=flash.bin bs=1M count=64 to create a 64MB flash image.

Build & run

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

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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
whoami	Display current user information
tetris	Start the TETRIS game
format	Format the flash storage
ls	List files in flash storage
save <name> <text>	Save text to a file in flash storage
load <name>	Load and display a file's content from flash storage

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📝 Future Roadmap

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