simple legacy bootloader

procedure

• enable A20
• get memory info
  • memory map
  • continuous memory (for BIOSes not supporting memory map)
• check vbe support
• get best vbe mode
• enter protected mode
• load kernel from FAT16 drive (file named "KERNEL")
• enter long mode
• pass important information (in rdi)
• give control to the kernel (elf64 file)

build

• $ make / $ make release
  • creates a bootable image without any kernel inside

test it with qemu

• $ make test
  • creates a bootable image with a test kernel inside (test.asm)
  • test.asm just prints "BLOADER" to the screen

debug with qemu

• $ make monitor
  • $(qemu) info tlb
    • all valid pages and there frames
  • $(qemu) info mem
    • compact paging tables
  • $(qemu) info registers
    • show all regs and there values
  • $(qemu) x/<num>x <addr>
    • show num words at addr
  • $(qemu) x/<num>i <addr>
    • show num instructions at addr

create log with qemu

• $ make log
  • log all interrupts (state of all regs)

try it on a real device

• $ make / $ make release
• $ sudo dd if=build/bloader.img of=/dev/<YourDevice> (or any other way)
• put your elf64 kernel into the root directory and call it "KERNEL" (or change name in OPTIONS)
• reboot and open BIOS
• make sure legacy boot(CSM) is enabled when using uefi device
• boot into USB device