mirror of
https://github.com/gbowne1/ClassicOS.git
synced 2025-11-17 06:45:25 -08:00
28 lines
798 B
Markdown
28 lines
798 B
Markdown
# ClassicOS 2-stage bootloader
|
|
|
|
Bootloader documentation for ClassicOS
|
|
|
|
## Disk image organization:
|
|
|
|
```
|
|
[ 512 B ] [ 2048 B ] [ Unspecified ]
|
|
Stage 1 Stage 2 Kernel
|
|
```
|
|
|
|
## Stage 1 (`stage1.asm`)
|
|
|
|
Responsible for loading the second stage using BIOS routines, and switching to protected mode.
|
|
|
|
- Queries CHS parameters from BIOS
|
|
- Loads the second stage bootloader (2048 B) to `0x7c00`
|
|
- Sets up a GDT with descriptor entries for code and data both covering the whole 32-bit address space
|
|
- Enables A20
|
|
- Set CR0.PE (enable protected mode) and jump to stage 2
|
|
|
|
## Stage 2 (`stage2.asm, stage2_load.c`)
|
|
|
|
- Set up segment registers
|
|
- Load the kernel ELF header
|
|
- Parse the program headers, and load all `PT_LOAD` segments from disk
|
|
- Jump to the kernel entry
|