Foundry-Boot/doc/uefi_boot.md

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