# UEFI Boot Process and Disk Layout

## Disk Layout with ESP (EFI System Partition)

```
Sector 0 (MBR/GPT):
+-----------------------------------+ 0x000
|        Protective MBR             |
|        (For BIOS compatibility)   |
+-----------------------------------+ 0x200

EFI System Partition (FAT32):
+-----------------------------------+ Sector 2048
|        FAT32 Filesystem           |
|        /EFI/                      |
|           /BOOT/                  |
|              BOOTX64.EFI          |
+-----------------------------------+ 

Main Partition (FAT32):
+-----------------------------------+
|        FAT32 Filesystem           |
|        (Kernel and other files)   |
+-----------------------------------+
```

## UEFI Boot Process

1. **System Startup**
   - Firmware initializes hardware
   - Loads UEFI runtime services
   - Sets up initial page tables and long mode
   - CPU already in 64-bit mode

2. **Boot Manager**
   - Scans for bootable devices
   - Looks for EFI System Partition (ESP)
   - Searches for bootloader at `/EFI/BOOT/BOOTX64.EFI`

3. **UEFI Bootloader**
   - Already in long mode (64-bit)
   - Has access to UEFI services:
     - File operations
     - Memory management
     - Video output
     - ACPI tables
   - Tasks:
     - Load kernel from main partition
     - Set up memory map
     - Exit boot services
     - Jump to kernel

4. **Kernel Load**
   - Bootloader passes:
     - Memory map
     - Framebuffer info
     - ACPI tables
   - Kernel takes control

## Key Differences from BIOS

1. **Initial State**
   - BIOS: 16-bit real mode
   - UEFI: 64-bit long mode

2. **Services**
   - BIOS: Limited interrupt-based services
   - UEFI: Rich API for system services

3. **Memory Management**
   - BIOS: Manual memory detection needed
   - UEFI: Provides memory map and allocation services

4. **File Access**
   - BIOS: Manual disk I/O and filesystem parsing
   - UEFI: Built-in filesystem support

5. **Graphics**
   - BIOS: VGA/VBE modes
   - UEFI: GOP (Graphics Output Protocol)

## UEFI Boot Services
```
Available until ExitBootServices() is called:
- Memory allocation
- File system access
- Graphics output
- Timer services
- Environment variables

Runtime Services (available after boot):
- Time services
- Variable services
- Reset system
- Virtual memory services
```

## Memory Map Example
```
Type                    Physical Address Range
EfiLoaderCode          0x0000000000000000-0x0000000000100000
EfiLoaderData          0x0000000000100000-0x0000000000200000
EfiBootServicesCode    0x0000000000200000-0x0000000000300000
EfiBootServicesData    0x0000000000300000-0x0000000000400000
EfiConventionalMemory  0x0000000000400000-...
EfiACPIReclaimMemory   ...
EfiACPIMemoryNVS       ...
EfiReservedMemoryType  ...
```

## Required UEFI Protocols
```
- EFI_LOADED_IMAGE_PROTOCOL: Access to bootloader image info
- EFI_SIMPLE_FILE_SYSTEM_PROTOCOL: File operations
- EFI_GRAPHICS_OUTPUT_PROTOCOL: Display output
- EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL: Text output
```

## Boot Flow Control
```
UEFI Firmware
    │
    ▼
Boot Manager
    │
    ▼
BOOTX64.EFI
    │
    ├── Get System Info
    │   ├── Memory Map
    │   ├── ACPI Tables
    │   └── GOP Info
    │
    ├── Load Kernel
    │   └── Parse FAT32
    │
    ├── ExitBootServices()
    │
    └── Jump to Kernel
        └── Pass Boot Info
```