# Simple UEFI Operating System

A minimal bootable UEFI operating system written in C that demonstrates basic UEFI functionality.

## Features

- **UEFI Boot**: Boots directly on UEFI firmware
- **Console I/O**: Interactive keyboard input and screen output
- **System Information**: Displays firmware details
- **Simple Commands**: Interactive command interface

## Requirements

- GCC compiler
- GNU-EFI library
- QEMU with OVMF firmware
- Make

## Installation

### On Debian/Ubuntu:

```bash
make install-deps
```

Or manually:

```bash
sudo apt-get install gnu-efi qemu-system-x86 ovmf gcc binutils make
```

### On Arch Linux:

```bash
sudo pacman -S gnu-efi qemu-system-x86 edk2-ovmf gcc binutils make
```

### On Fedora/RHEL:

```bash
sudo dnf install gnu-efi qemu-system-x86 edk2-ovmf gcc binutils make
```

## Building

Build the UEFI application:

```bash
make
```

This will create `build/EFI/BOOT/BOOTX64.EFI`

## Running

Test with QEMU (no graphics mode):

```bash
make run
```

### QEMU Controls:
- **Exit QEMU**: Press `Ctrl+A`, then `X`
- **Shutdown OS**: Press `q` in the OS

## Commands in the OS

Once the OS boots, you can use these commands:

- `h` - Display help
- `i` - Display system information
- `c` - Clear screen
- `q` - Shutdown the system

## Project Structure

```
.
├── main.c          # Main UEFI application source code
├── Makefile        # Build configuration
└── README.md       # This file
```

## How It Works

1. **UEFI Entry Point**: The `efi_main` function is the entry point called by UEFI firmware
2. **Initialization**: GNU-EFI library is initialized with the system table
3. **Console Setup**: Output is configured and screen is cleared
4. **Main Loop**: The OS enters a loop waiting for keyboard input
5. **Command Processing**: User commands are processed and executed
6. **Shutdown**: UEFI runtime services are used to shutdown the system

## Building for Real Hardware

To create a bootable USB drive:

1. Build the project: `make`
2. Format a USB drive with GPT and create an EFI partition (FAT32)
3. Mount the EFI partition
4. Copy `build/EFI/BOOT/BOOTX64.EFI` to `/EFI/BOOT/` on the USB drive
5. Boot from the USB drive in UEFI mode

**Warning**: Always backup your data before creating bootable media!

## Cleaning

Remove build artifacts:

```bash
make clean
```

## Troubleshooting

### GNU-EFI headers not found

Make sure GNU-EFI is installed. The headers are typically in `/usr/include/efi`.

If installed in a different location, update the `EFI_INC` variable in the Makefile.

### OVMF firmware not found

OVMF files might be in different locations depending on your distribution:
- `/usr/share/OVMF/`
- `/usr/share/qemu/`
- `/usr/share/edk2-ovmf/`

Update the paths in the Makefile if needed.

### QEMU crashes or fails to start

Ensure you have QEMU with x86_64 support and OVMF firmware installed.

## Technical Details

- **Architecture**: x86_64
- **Firmware Interface**: UEFI 2.x
- **Development Library**: GNU-EFI
- **Executable Format**: PE32+ (Portable Executable for EFI)
- **Boot Protocol**: UEFI Boot Services

## License

This is a minimal educational example. Feel free to use and modify as needed.

## Further Development

Ideas for expansion:
- File system support
- Memory management
- Graphics output
- Network stack
- Device drivers
- Multi-tasking

## Resources

- [UEFI Specification](https://uefi.org/specifications)
- [GNU-EFI Documentation](https://sourceforge.net/projects/gnu-efi/)
- [OSDev Wiki](https://wiki.osdev.org/)