wrote a basic bootloader

2025-02-25 01:14:52 +00:00
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+# 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)
+```