# 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

---

## Getting Started

### Requirements

- GCC cross-compiler (or native x86-64 GCC)
- GNU-EFI library and headers
- QEMU with OVMF firmware
- GNU Make
- `xorriso` and `mtools` (for ISO builds only)

### Installation

<details>
<summary><strong>Debian / Ubuntu</strong></summary>

```bash
make install-deps
```

Or manually:

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

</details>

<details>
<summary><strong>Arch Linux</strong></summary>

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

</details>

<details>
<summary><strong>Fedora / RHEL</strong></summary>

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

</details>

### Building

```bash
make            # build both UEFI loader and kernel
```

This produces two binaries:

| Output | Description |
|--------|-------------|
| `build/EFI/BOOT/BOOTX64.EFI` | UEFI loader (PE32+) |
| `build/kernel.elf` | Kernel binary (ELF64) |

### Running

```bash
make run        # build and run with QEMU (FAT directory, nographic)
make runiso     # build, create ISO, and run with QEMU
```

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

| Key / Command | Action |
|---------------|--------|
| `Ctrl+A`, then `X` | Exit QEMU |
| `shutdown` (in shell) | Power off the OS |

### Make Targets

| Target | Description |
|--------|-------------|
| `all` | Build the UEFI loader and kernel (default) |
| `run` | Build and run with QEMU |
| `iso` | Create a bootable ISO image |
| `runiso` | Build, create ISO, and run in QEMU |
| `clean` | Remove all build artifacts |
| `install-deps` | Install required packages (Debian/Ubuntu) |
| `help` | Print target summary |

---

## Usage

### Shell Commands

Once the OS boots, an interactive prompt (`->`) is displayed. The following commands are available:

| Command | Description |
|---------|-------------|
| `help` | List all available commands |
| `man <command>` | Show detailed help for a command |
| `shutdown` | Shut down 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 four phases:

1. **Heap allocation** — allocates 8 blocks of increasing size (16–4096 bytes) via `kmalloc()`
2. **Heap free** — frees all 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 test manually 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).

---

## Architecture

### Boot Flow

```
UEFI Firmware
  └─ efi_main()          [main.c]       PE32+ UEFI application
       ├─ InitializeLib()                GNU-EFI setup
       ├─ read_file_to_buffer()          read kernel.elf from ESP
       ├─ load_elf_kernel()              parse ELF64, map PT_LOAD segments
       ├─ populate BootInfo              wrap UEFI services as fn pointers
       └─ kmain(&Boot)    [kernel.c]     jump to kernel
            ├─ idt_init()                install exception handlers
            ├─ memory_init()             PMM → paging → heap
            ├─ task_init()               scheduler + task 0
            └─ shell loop                read-eval-print with cooperative yield
```

### Memory Management

| Layer | Description |
|-------|-------------|
| **PMM** | Bitmap-based page-frame allocator managing a 16 MB pool (4096 pages). Supports single and contiguous multi-page allocation. |
| **Paging** | Walks and creates 4-level x86-64 page tables (PML4 → PDPT → PD → PT). Supports map, unmap, and virtual-to-physical translation for 4 KB, 2 MB, and 1 GB page sizes. |
| **Heap** | First-fit free-list allocator with block splitting and bidirectional coalescing. 16-byte alignment. Magic-number corruption detection. |

### Task System

| Aspect | Detail |
|--------|--------|
| **PCB** | `Task` struct: PID, state, name, saved RSP, stack base, entry function, scheduling metadata |
| **States** | `FREE` → `READY` → `RUNNING` → `TERMINATED` → `FREE` |
| **Scheduler** | Round-robin among `READY` tasks via `task_yield()` |
| **Context Switch** | Assembly routine saves/restores callee-saved registers (`rbp`, `rbx`, `r12`–`r15`) and `RFLAGS`, then swaps RSP |
| **Stack** | 32 KB (8 pages) per task, 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 in `commands.c`:

```c
static Command commands[] = {
    { L"name", L"description", L"usage", handler_fn },
    ...
    { NULL, NULL, NULL, NULL }   /* sentinel */
};
```

`execute_command()` splits user input into command + arguments and dispatches to the matching handler. `man` provides detailed help by looking up command metadata.

---

## Project Structure

```
.
├── main.c              # UEFI loader — reads kernel.elf, sets up BootInfo
├── kernel.c            # Kernel entry point (kmain) and interactive shell
├── kernel.ld           # Linker script — kernel loaded at 0x100000
├── boot_info.h         # BootInfo struct shared between loader and kernel
│
├── commands.c          # Command registry and all handler implementations
├── commands.h          # Command type and dispatch API
├── string_utils.c      # CHAR16 string helpers (trim, compare)
├── string_utils.h      # String utility declarations
│
├── idt.c               # IDT setup, exception handlers, PIC EOI
├── idt.h               # ISRFrame layout and idt_init() declaration
├── isr.S               # ISR stub table (vectors 0–255) and common handler
│
├── memory.c            # PMM (bitmap), paging (4-level), heap (first-fit)
├── memory.h            # Memory subsystem constants, types, and API
│
├── task.c              # PCB pool, round-robin scheduler, yield/exit
├── task.h              # Task types (Task, TaskState) and task API
├── context_switch.S    # Cooperative context switch (x86-64 assembly)
│
├── Makefile            # Build system
├── README.md           # This file
└── build/              # Build output (generated)
    ├── EFI/BOOT/BOOTX64.EFI
    ├── kernel.elf
    └── image/          # ISO staging area
```

---

## Development

### Adding a New Command

1. Open `commands.c`
2. Add a forward declaration:
   ```c
   static void cmd_foo(BootInfo *Boot, CHAR16 *Args);
   ```
3. Append an entry to the `commands[]` array (before the `NULL` sentinel):
   ```c
   { L"foo", L"Short description", L"Usage: foo [args]\n\r  Details.", cmd_foo },
   ```
4. Implement `cmd_foo`
5. Rebuild with `make`

### Building for Real Hardware

1. Build the project: `make`
2. Format a USB drive with GPT and a FAT32 EFI System Partition
3. Mount the ESP
4. Copy `build/EFI/BOOT/BOOTX64.EFI` → `<mount>/EFI/BOOT/BOOTX64.EFI`
5. Copy `build/kernel.elf` → `<mount>/kernel.elf`
6. Boot from the USB in UEFI mode

> **Warning:** Always back up your data before creating bootable media.

---

## Troubleshooting

| Problem | Solution |
|---------|----------|
| **GNU-EFI headers not found** | Install GNU-EFI. Headers are typically at `/usr/include/efi`. Update `EFI_INC` in the Makefile if installed elsewhere. |
| **OVMF firmware not found** | Check `/usr/share/OVMF/`, `/usr/share/qemu/`, or `/usr/share/edk2-ovmf/`. The Makefile auto-detects common paths. |
| **QEMU crashes or won't start** | Ensure `qemu-system-x86_64` and OVMF are installed. Verify with `qemu-system-x86_64 --version`. |

---

## Technical Details

| Property | Value |
|----------|-------|
| Architecture | x86-64 (long mode) |
| Firmware Interface | UEFI 2.x |
| Development Library | GNU-EFI |
| Loader Format | PE32+ (EFI application) |
| Kernel Format | ELF64 (static, loaded at 0x100000) |
| Boot Protocol | UEFI Boot Services |
| Memory Model | Identity-mapped (UEFI), 4-level paging (kernel) |
| Scheduling | Cooperative round-robin multitasking |

## Resources

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

## License

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