ClassicOS/bootloader

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