# Simple UEFI Operating System

A minimal bootable UEFI operating system written in C that demonstrates UEFI boot loading, memory management, interrupt handling, and cooperative multitasking.

## Features

- **UEFI Boot**: Boots directly on UEFI firmware via a PE32+ loader
- **ELF64 Kernel Loader**: Reads and maps a standalone kernel from the EFI partition
- **Console I/O**: Interactive keyboard input and screen output
- **Interrupt Handling**: IDT setup with CPU exception handlers (vectors 0–31) and hardware IRQ dispatch
- **Memory Management**: Bitmap-based physical memory manager (PMM), 4-level x86-64 paging, and a first-fit heap allocator with block coalescing
- **Cooperative Multitasking**: Process control blocks, assembly context switching, and a round-robin task scheduler
- **Interactive Shell**: Command registry with `help`, `man`, built-in commands, and test utilities

## 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 xorriso mtools
```

### On Arch Linux:

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

### On Fedora/RHEL:

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

## Building

Build the UEFI loader and kernel:

```bash
make
```

This creates `build/EFI/BOOT/BOOTX64.EFI` (UEFI loader) and `build/kernel.elf` (kernel binary).

## Running

Test with QEMU (no graphics mode):

```bash
make run
```

To build and run from an ISO image:

```bash
make runiso
```

The loader expects `kernel.elf` at the root of the EFI partition (next to the `EFI/` directory).

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

## Commands

Once the OS boots, you can use these commands:

| Command | Description |
|---------|-------------|
| `help` | Display all available commands |
| `man <command>` | Display detailed help for a command |
| `shutdown` | Shutdown the system |
| `clear` | Clear the screen |
| `about` | Display system information |
| `mem` | Display memory statistics (PMM, heap, paging) |
| `ps` | List all active tasks with PID, state, and name |
| `spawn [name]` | Create a demo background task |
| `memtest` | Run memory allocation/deallocation tests |
| `tasktest` | Spawn multiple concurrent tasks to test the scheduler |

### Example Session

```
-> help
-> about
-> mem
-> memtest
-> spawn worker1
-> spawn worker2
-> ps
-> tasktest
-> shutdown
```

### Testing Memory Management

The `memtest` command runs through four test phases:

1. **Heap allocation** — allocates 8 blocks of increasing size (16 to 4096 bytes) via `kmalloc()`
2. **Heap free** — frees all allocated blocks via `kfree()` and verifies coalescing
3. **PMM single page** — allocates and frees a single 4 KB page
4. **PMM multi-page** — allocates and frees 4 contiguous pages

Each phase reports addresses, success/failure, and usage statistics.

### Testing Task Scheduling

The `tasktest` command:

1. Spawns 3 worker tasks (`worker-A`, `worker-B`, `worker-C`)
2. Each worker prints 3 progress steps, yielding between each
3. The shell yields to let workers run in round-robin order
4. Displays the task list after completion

You can also manually test with `spawn` and `ps`:

```
-> spawn myTask
Created task 'myTask' (PID 1)
-> ps
  PID  STATE       SWITCHES  NAME
  ---  ----------  --------  ----
    0  RUNNING            5  kernel
    1  READY              0  myTask

  Active tasks: 2 / 32
```

Background tasks run whenever the shell yields (which happens automatically while waiting for keyboard input).

## Project Structure

```
.
├── main.c              # UEFI loader (reads and loads kernel ELF)
├── kernel.c            # Kernel entry point and interactive shell loop
├── kernel.ld           # Kernel linker script (base address 0x100000)
├── boot_info.h         # Shared boot interface (BootInfo struct)
├── commands.c          # Command registry and handler implementations
├── commands.h          # Command interface definitions
├── string_utils.c      # String utility functions (trim, compare, etc.)
├── string_utils.h      # String utility declarations
├── idt.c               # IDT setup, exception handlers, PIC EOI
├── idt.h               # IDT types (ISRFrame) and init declaration
├── isr.S               # ISR stub table (vectors 0–255) with common handler
├── memory.c            # PMM (bitmap), paging (4-level), heap (first-fit)
├── memory.h            # Memory subsystem declarations and constants
├── task.c              # Task manager: PCB pool, scheduler, yield/exit
├── task.h              # Task types (Task, TaskState) and API
├── context_switch.S    # Assembly cooperative context switch (x86-64)
├── Makefile            # Build configuration
└── README.md           # This file
```

## Architecture

### Boot Flow

1. **UEFI Entry** — `efi_main()` in `main.c` is called by firmware
2. **GNU-EFI Init** — library initialized with the system table
3. **Kernel Load** — `kernel.elf` is read from the EFI partition, ELF64 PT_LOAD segments are mapped into memory
4. **BootInfo Setup** — function pointers for console I/O, shutdown, and page allocation are packaged into a `BootInfo` struct
5. **Kernel Entry** — `kmain()` is called with the `BootInfo` pointer

### Kernel Initialization

1. **IDT** — CPU exception vectors (0–31) are installed; firmware IRQ handlers are preserved
2. **Memory** — PMM allocates a 16 MB page pool via UEFI, bitmap tracks free/used pages; paging reads CR3; heap carves 256 KB from the PMM
3. **Tasks** — scheduler initializes; the running kernel becomes task 0
4. **Shell** — interactive loop with non-blocking input and cooperative yielding

### Memory Management

| Component | Description |
|-----------|-------------|
| **PMM** | Bitmap-based page-frame allocator managing a 16 MB pool (4096 pages); supports single and contiguous multi-page allocation |
| **Paging** | Walks/creates 4-level x86-64 page tables (PML4 → PDPT → PD → PT); supports map, unmap, and virtual-to-physical translation |
| **Heap** | First-fit free-list allocator with block splitting and bidirectional coalescing; 16-byte alignment; magic number corruption detection |

### Task System

| Component | Description |
|-----------|-------------|
| **PCB** | `Task` struct with PID, state, name, saved RSP, stack, entry function, and scheduling metadata |
| **States** | `FREE` → `READY` → `RUNNING` → `TERMINATED` → `FREE` |
| **Scheduler** | Round-robin selection among `READY` tasks via `task_yield()` |
| **Context Switch** | Assembly routine saves/restores callee-saved registers (`rbp`, `rbx`, `r12`–`r15`) and `RFLAGS`, swaps RSP |
| **Stack** | Each task gets 32 KB (8 pages) allocated from PMM; freed on `task_exit()` |
| **Limits** | Up to 32 concurrent tasks; cooperative (voluntary yield) only |

### Command System

- Commands are registered in a static array with metadata (name, description, usage, handler function)
- `execute_command()` parses input, splits command and arguments, and dispatches to the matching handler
- `man` provides detailed help by looking up command metadata
- New commands are added by writing a handler and appending to the `commands[]` array

## 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. Copy `build/kernel.elf` to the root of the EFI partition
6. 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
- **Loader Format**: PE32+ (Portable Executable for EFI)
- **Kernel Format**: ELF64
- **Boot Protocol**: UEFI Boot Services
- **Memory Model**: Identity-mapped (UEFI), 4-level paging (kernel)
- **Scheduling**: Cooperative round-robin multitasking

## License

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

## Adding New Commands

1. Open `commands.c`
2. Add a forward declaration: `static void cmd_yourcommand(BootInfo *Boot, CHAR16 *Args);`
3. Add an entry to the `commands[]` array (before the sentinel)
4. Implement the handler function
5. Rebuild with `make`

## Resources

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