# Boot Process and System Layout

## Disk Layout
```
FAT32 Partition
┌─────────────────────────┐
│ /                       │
├─────────────────────────┤
│ ├── EFI/               │
│ │   └── BOOT/          │
│ │       └── BOOTX64.EFI│ <- UEFI bootloader
├─────────────────────────┤
│ ├── boot/              │
│ │   └── bootloader.bin │ <- BIOS bootloader
├─────────────────────────┤
│ └── kernel             │ <- Main kernel binary
└─────────────────────────┘
```

## Kernel Binary Format
```
Kernel Header Structure
┌─────────────────────────┐
│ Magic Number (4 bytes)  │ 0x00
├─────────────────────────┤
│ Entry Point (8 bytes)   │ 0x04
├─────────────────────────┤
│ Stack Pointer (8 bytes) │ 0x0C
├─────────────────────────┤
│ Flags (4 bytes)         │ 0x14
├─────────────────────────┤
│ Text Offset (4 bytes)   │ 0x18
├─────────────────────────┤
│ Text Size (4 bytes)     │ 0x1C
├─────────────────────────┤
│ Data Offset (4 bytes)   │ 0x20
├─────────────────────────┤
│ Data Size (4 bytes)     │ 0x24
└─────────────────────────┘
```

## Memory Layout (After Boot)
```
Virtual Memory Layout
┌─────────────────────┐ 0xFFFFFFFF_FFFFFFFF
│    Higher Half      │
├─────────────────────┤ 0xFFFFFFFF_FF600000
│     Recursive       │
│    Page Mapping     │
├─────────────────────┤ 0xFFFFFFFF_C0000000
│   Kernel Stacks     │
├─────────────────────┤ 
│   Kernel Heap       │
├─────────────────────┤ 0xFFFFFFFF_80000000
│   Kernel Code       │
├─────────────────────┤ 0x00007FFF_FFFFFFFF
│       Guard         │
├─────────────────────┤ 0x00007FFF_00000000
│    User Space       │
├─────────────────────┤ 0x0000000000400000
│       Guard         │
└─────────────────────┘ 0x0000000000000000

Physical Memory Layout
┌─────────────────────┐
│    Available RAM    │
├─────────────────────┤ 
│   Kernel Binary     │ <- Loaded at 1MB (0x100000)
├─────────────────────┤
│   Reserved/BIOS     │
└─────────────────────┘ 0x00000000
```

## Boot Process

### BIOS Boot Flow
1. BIOS loads MBR (stage1.bin)
2. Stage 1 bootloader:
   - Loads Stage 2 bootloader (stage2.bin) starting at sector 2048
3. Stage 2 bootloader:
   - Switches to protected mode
   - Sets up initial page tables
   - Finds and loads kernel from FAT32
   - Enables long mode
   - Jumps to kernel entry point

### UEFI Boot Flow
1. UEFI firmware loads BOOTX64.EFI
2. UEFI bootloader:
   - Gets memory map
   - Finds and loads kernel
   - Exits boot services
   - Sets up page tables
   - Enables long mode
   - Jumps to kernel entry point

### Kernel Entry Point
```rust
extern "C" {
   fn kmain(magic: u64, boot_info: *const BootInfo) -> !;
}
```   

## Common Boot Environment
Both bootloaders must provide:

### CPU State
```
- Long mode enabled
- Paging enabled
- Interrupts disabled
- GDT set up for long mode
- IDT not required (kernel will set up)
```

### Register State
```
RAX = Boot magic value (e.g., 0xCAFEBABE)
RBX = Pointer to boot info structure
RCX = 0
RDX = 0
RSI = 0
RDI = 0
RBP = 0
RSP = Valid stack pointer (as specified in kernel header)
```

### Boot Info Structure
```c
struct BootInfo {
    uint64_t magic;          // Boot info magic number
    uint64_t mem_map_addr;   // Physical address of memory map
    uint64_t mem_map_size;   // Size of memory map
    uint64_t fb_addr;        // Framebuffer address (if available)
    uint32_t fb_width;       // Framebuffer width
    uint32_t fb_height;      // Framebuffer height
    uint32_t fb_pitch;       // Framebuffer pitch
    uint8_t  fb_bpp;         // Bits per pixel
    uint8_t  boot_type;      // 0 = BIOS, 1 = UEFI
    uint8_t  reserved[6];    // Padding to 64-bit align
};
```

## Required Kernel Features
1. Position-independent code (PIC)
2. No assumptions about physical memory layout beyond boot info
3. Own interrupt handling
4. Own memory management after boot

## Development Notes
1. Kernel must be compiled with:
   - No red zone
   - No MMX/SSE initially
   - Position-independent code
   - No standard library dependencies

2. Testing can be done with:
   ```bash
   # BIOS boot
   qemu-system-x86_64 disk.img

   # UEFI boot
   qemu-system-x86_64 -bios /usr/share/OVMF/OVMF_CODE.fd disk.img
   ```