What is a Bootloader?

A bootloader is a special program that is executed each time a bootable device is initialized by the computer during its power on or reset that will load the kernel image into the memory. This application is very close to hardware and to the architecture of the CPU. All x86 PCs boot in Real Mode. In this mode you have only 16-bit instructions. Our bootloader runs in Real Mode and our bootloader is a 16-bit program.

https://manybutfinite.com/img/boot/bootProcess.png

How this works?

When you switch on the PC the BIOS want to boot up an OS which must be found somewhere in hard disks, floppy disk, CDs, etc. The order in which BIOS searches an OS is user configurable. Next the BIOS reads the first 512 byte sector of the bootable disk. Usually a sector is 512 bytes in size. This is known as the Master Boot Record (MBR). BIOS simply loads the contents of the MBR into memory location “0x7c00” and jumps to that location to start executing whatever code is in the MBR. Our bootloader should be 512 bytes in size as well.

https://manybutfinite.com/img/boot/masterBootRecord.png



Code 440 bytes Disk signature 4 bytes Null 2 bytes Partition tables 64 bytes MBR signature 2 bytes Total 512 bytes

The boot process is:

Turn on your PC and BIOS executes

The BIOS seeks the MBR in boot order which is user configurable.

The BIOS loads a 512 byte boot sector into memory location “0x7c00” from the specified media and begins executing it.

Those 512 bytes then go on to load the OS itself or a more complex bootloader.

BIOS Interrupts

These interrupts help OS and application invoke the facilities of the BIOS. This is loaded before the bootloader and it is very helpful in communicating with the I/O. Since we don’t have OS level interrupts this is the only option that would be helpful.

For example to print a character to the screen using BIOS interrupt calls.

mov ah , 0x0e ; function number = 0Eh : Display Character mov al , 'O' ; AL = code of character to display int 0x10 ; call INT 10h, BIOS video service

This is a simple boot loader written in AT&T syntax.

/* * Title: A Si mple Bootloader in AT & T Assembly * Author: Osanda Malith Jayathissa ( @ OsandaMalith ) * Website: http: // OsandaMalith.wordpress.com */ .code16 .section .text .global main main: /* Disk description table , to make it a valid floppy FAT12 file system format */ jmp _start .byte 144 # NOP .ascii "OsandaOS" # OEMLabel .word 512 # Byte sP erSector .byte 1 # SectorsPerCluster .word 1 # ReservedForBoot .byte 2 # NumberOfFats .word 224 # RootDirEntries ( 224 * 32 = 7168 = 14 sectors to read ) .word 2880 # LogicalSectors .byte 0xf0 # MediumByte .word 9 # SectorsPerFat .word 18 # SectorsPerTrack .word 2 # Si des .long 0 # HiddenSectors .byte 0 # LargeSectors .byte 0 # DriveNo .byte 0x29 # Si gnature ( 41 for Floppy ) .long 0x12345678 # VolumeID .ascii "My First OS" # VolumeLabel .ascii "FAT12 " # FileSystem _start: movw $ 0 , % ax movw % ax , % ss movw % ax , % ds movw % ax , % es movw $ string , % si loop: movb $ 0xe , % ah movb ( % si ), % al cmpb $ 0 , % al je done int $ 0x10 addw $ 1 , % si jmp loop done: jmp done # infinite loop string: .ascii "Welcome to @OsandaMalith's First OS :)" .byte 0 .fill 0x1fe - ( . - main ) , 1 , 0 # Pad remainder of boot sector with 0 s .word 0xaa55 # The standard PC boot si gnature

https://github.com/OsandaMalith/bootloader/blob/master/loader.S

Assemble and link the code. I have explicitly specified to load the text section to load at 0x7c00 and it will calculate the absolute addressing.

as --32 -o loader.o loader.S

ld -m elf_i386 loader.o --oformat=binary -o loader.bin -Ttext 0x7c00

The same in NASM using Intel syntax.

%if 0 ; * Title: A Si mple Bootloader in NASM * Author: Osanda Malith Jayathissa ( @ OsandaMalith ) * Website: http: // OsandaMalith.wordpress.com %endif ; BITS 16 jmp short _start ; Jump past disk description section nop ; Disk description table, to make it a valid floppy OEMLabel db "OsandaOS" ; Disk label BytesPerSector dw 512 ; Bytes per sector SectorsPerCluster db 1 ; Sectors per cluster ReservedForBoot dw 1 ; Reserved sectors for boot record NumberOfFats db 2 ; Number of copies of the FAT RootDirEntries dw 224 LogicalSectors dw 2880 ; Number of logical sectors MediumByte db 0F0h ; Medium descriptor byte SectorsPerFat dw 9 ; Sectors per FAT SectorsPerTrack dw 18 ; Sectors per track (36/cylinder) Sides dw 2 ; Number of sides/heads HiddenSectors dd 0 ; Number of hidden sectors LargeSectors dd 0 ; Number of LBA sectors DriveNo dw 0 ; Drive No: 0 Signature db 41 ; Drive signature: 41 for floppy VolumeID dd 12345678h ; Volume ID: any number VolumeLabel db "My First OS" ; Volume Label: any 11 chars FileSystem db "FAT12 " ; File system type: don't change! _start: mov ax , 07C0h ; move 0x7c00 into ax mov ds , ax ; set data segment to where we're loaded mov si , string ; Put string position into SI call print_string ; Call our string-printing routine jmp $ ; infinite loop! string db "Welcome to @OsandaMalith's First OS :)" , 0 print_string: mov ah , 0Eh ; int 10h 'print char' function .loop: lodsb ; load string byte to al cmp al , 0 ; cmp al with 0 je .done ; if char is zero, ret int 10h ; else, print jmp .loop .done: ret times 510- ( $ - $$ ) db 0 ; Pad remainder of boot sector with 0s dw 0xAA55 ; The standard PC boot signature

https://github.com/OsandaMalith/bootloader/blob/master/loader.nasm

Assemble the file using binary as the format.

nasm -f bin -o loader.bin loader.nasm

If you use the file utility you will see that it’s a legit 1.4MB floppy Disk and a 32-bit boot sector.

$ file loader.bin loader.bin: DOS floppy 1440k, x86 hard disk boot sector

If you open our bootloader in a hex editor you will see our 512 size program and at the end there should be our boot signature 0xaa55.

After that convert the binary file to floppy image.

dd status=noxfer conv=notrunc if= loader.bin of=floppy.flp

You can also convert the binary file to an ISO using tools such as UltraISO, etc. You may burn and try in your PC instead of emulating.

Use Qemu to test our newly created bootloader 🙂

qemu-system-i386 -fda floppy.flp



You can develop something like this 🙂

Bootloader In C

We can use inline assembly using C to write a simple bootloader. I’ll be showing a simple example to print “Hello”.

__asm__(".code16

"); __asm__("jmpl $0x0000, $main

"); void main() { __asm__ __volatile__("movb $'H' , %al

"); __asm__ __volatile__("movb $0x0e, %ah

"); __asm__ __volatile__("int $0x10

"); __asm__ __volatile__("movb $'e' , %al

"); __asm__ __volatile__("movb $0x0e, %ah

"); __asm__ __volatile__("int $0x10

"); __asm__ __volatile__("movb $'l' , %al

"); __asm__ __volatile__("movb $0x0e, %ah

"); __asm__ __volatile__("int $0x10

"); __asm__ __volatile__("movb $'l' , %al

"); __asm__ __volatile__("movb $0x0e, %ah

"); __asm__ __volatile__("int $0x10

"); __asm__ __volatile__("movb $'o' , %al

"); __asm__ __volatile__("movb $0x0e, %ah

"); __asm__ __volatile__("int $0x10

"); }

https://github.com/OsandaMalith/bootloader/blob/master/loader.c

This will be the linker script specified in the linker as “test.ld”

.ENTRY(main); SECTIONS { . = 0x7C00; .text : AT(0x7C00){ *(.text); } .sig : AT(0x7DFE) { SHORT(0xaa55); } }

https://github.com/OsandaMalith/bootloader/blob/master/test.ld

After that compile using GCC and we generate only a object file and manually link using the linker specifying our linker script.

gcc -m32 -c -Os -march=i686 -ffreestanding -Wall -Werror -o loader.o loader.c

ld -m elf_i386 -static -Ttest.ld -nostdlib --nmagic -o test.elf test.o

objcopy -O binary loader.elf loader.bin

dd status=noxfer conv=notrunc if=loader.bin of=floppy.flp

After that boot our image using Qemu 🙂

qemu-system-i386 -fda floppy.flp

Using BIOS interrupts you can write nice programs to the boot sector of your PC 🙂

References

http://duartes.org/gustavo/blog/post/how-computers-boot-up/

http://wiki.osdev.org

http://mikeos.sourceforge.net/

https://en.wikipedia.org/wiki/BIOS_interrupt_call

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