# BIOS Boot Process and Disk Layout

## Disk Layout

```
Sector 0 (MBR):
+-----------------------------------+ 0x000
|                                   |
|        Bootstrap Code             |
|        (Stage 1 Bootloader)       |
|                                   |
|                                   |
+-----------------------------------+ 0x1BE (446)
|     Partition Entry 1 (16 bytes)  |
+-----------------------------------+ 0x1CE (462)
|     Partition Entry 2 (16 bytes)  |
+-----------------------------------+ 0x1DE (478)
|     Partition Entry 3 (16 bytes)  |
+-----------------------------------+ 0x1EE (494)
|     Partition Entry 4 (16 bytes)  |
+-----------------------------------+ 0x1FE (510)
|     Boot Signature (0x55AA)       |
+-----------------------------------+ 0x200 (512)

Sectors 1-2047:
+-----------------------------------+ 0x200
|                                   |
|        Stage 2 Bootloader         |
|        (Up to 1023.5 KB)         |
|                                   |
+-----------------------------------+ 0x100000 (Sector 2048)

Sector 2048 onwards:
+-----------------------------------+ 0x100000
|                                   |
|        FAT32 Partition           |
|        (Rest of disk)            |
|                                   |
+-----------------------------------+
```

## Partition Entry Format (16 bytes)
```
Offset  Size    Description
0x00    1       Boot flag (0x80 = bootable, 0x00 = non-bootable)
0x01    3       Starting CHS address
0x04    1       Partition type
0x05    3       Ending CHS address
0x08    4       Starting sector (LBA)
0x0C    4       Number of sectors
```

## Boot Process

1. **BIOS Power-On**
   - BIOS performs POST (Power-On Self Test)
   - Looks for bootable devices

2. **MBR Load**
   - BIOS loads Sector 0 (MBR) into memory at 0x7C00
   - Verifies 0x55AA signature
   - Executes Stage 1 Bootloader

3. **Stage 1 Bootloader**
   - Runs in 16-bit real mode
   - Limited to 446 bytes
   - Main task: Load Stage 2 Bootloader
   - Typically contains minimal disk I/O code

4. **Stage 2 Bootloader**
   - Located in sectors 1-2047
   - Has more space for complex operations
   - Tasks:
     - Switch to 32-bit protected mode
     - Set up basic memory management
     - Parse FAT32 filesystem
     - Switch to long mode
     - Load and execute kernel

5. **Kernel Load**
   - Stage 2 loads kernel from FAT32 partition
   - Sets up necessary environment
   - Transfers control to kernel

## Memory Layout During Boot
```
0x00000000 - 0x000003FF: Interrupt Vector Table
0x00000400 - 0x000004FF: BIOS Data Area
0x00000500 - 0x00007BFF: Free Memory
0x00007C00 - 0x00007DFF: Stage 1 Bootloader (MBR)
0x00007E00 - 0x0007FFFF: Free Memory (Stage 2 can be loaded here)
0x00080000 - 0x0009FFFF: Extended BIOS Data Area
0x000A0000 - 0x000FFFFF: BIOS ROM, Video Memory, etc.
0x00100000 onwards:      Free Memory (Kernel typically loaded here)
```