bits 32 section start alloc exec write progbits align=16 mov ecx, 0x400000 ; Set up stack to top of multiboot's mov esp, ecx ; reserved bss area, we can use the stack ; freely without worry of overwriting ; the multiboot structure cmp eax, 0x2BADB002 ; Check magic signature in eax jne bad_magic mov eax, 0 ; Replace multiboot header, temporarily mov edi, 0x100400 ; stored in the gdt, with zero's, so that mov ecx, 15 ; there will not be random segments defined cld ; eax = replacement value, edi = location rep stosd ; of header, ecx = length to replace / 4 mov eax, [ebx] bt eax, 1 jc boot_device_ok ; If neither boot_device nor cmdline is bt eax, 2 ; specified, we dont know where to boot from. jnc no_boot_device ; It's still possible there is a cmdline but boot_device_ok: ; with no boot device specified bt eax, 3 ; The root filesystem driver has to be loaded jnc no_mods ; as a module, along with supporting drivers bt eax, 0 jc mem_map_ok bt eax, 6 jnc no_mem_map mem_map_ok: mov esi, test_string jmp print_msg no_boot_device: mov esi, no_boot_device_string jmp print_msg no_mods: mov esi, no_mods_string jmp print_msg no_mem_map: mov esi, no_mem_map_string jmp print_msg bad_magic: mov esi, bad_magic_string jmp print_msg print_msg: mov edi, 0xb8000 print_msg_loop: lodsb cmp al, 0 je halt stosb mov al, 0xF ; White on black, non-blinking stosb jmp print_msg_loop halt: jmp halt bad_magic_string db 'The boot loader is incompatible with this OS (Bad signature)', 0 no_mem_map_string db 'The boot loader is incompatible with this OS (No memory map)', 0 no_mods_string db 'Unable to boot; no filesystem driver loaded', 0 no_boot_device_string db 'Unable to boot; no boot device specified', 0 test_string db 'Testing...', 0 section initsys alloc noexec write progbits align=4096