wrote a basic bootloader

2025-02-25 01:14:52 +00:00
commit 012b0454ed
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+# 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
+```