x86-bootloader
The project is an experimental x86 bootloader (i386) for the learning purpose. Bootloader is a piece of program that runs before the operating system when a computer is turned on. Its primary goal is to ready the system for control by the kernel. The project is emulated with QEMU, which is a powerful machine emulator and virtualizer.
- Print a string and hex bytes (not just ASCII characters) in 16-bit real mode with BIOS interrupt call (int 0x10)
- Understand the concepts of stack and function calls
- Understand the memory addressing of 8086/80286/80386: segmentation and flat model
- Load data from disk with BIOS interrupt call (int 0x13)
- Implement the global descriptor table (GDT) structure
- Switch to 32-bit protected mode: disable interrupt, load GDT, enable PE flag in CR0, flush pipeline
- Print a string in 32-bit protected mode with video memory (VGA)
- Load the kernel
Usage
Install QEMU emulator in Kali Linux
$ sudo apt-get install qemu-system-x86
Type the following commands or simply run make command
-
Compile the assembly code into binary
$ nasm boot.asm -f bin -o boot.bin -
Compile the files about kernel and link object files into binary
$ nasm kernel_entry.asm -f elf -o kernel_entry.o $ gcc -m32 -fno-pie -ffreestanding -c kernel.c -o kernel.o $ ld -m elf_i386 -s -o kernel.bin -Ttext 0x1000 kernel_entry.o kernel.o --oformat binary -
Concatenate two binary files into one image
$ cat boot.bin kernel.bin > bootloader -
Boot QEMU off a floppy disk
$ qemu-system-i386 -fda bootloader
If you want to debug the code, you can connect to gdb remotely with the port 1234
(gdb) target remote localhost:1234
Result
The program first prints a string in 16-bit real mode with BIOS interrupt call. Then prepare to load the kernel from the disk and switch to 32-bit protected mode. Finally, it lands on 32-bit protected mode and loads the kernel. As you can see, the program executes the function that is located in the kernel. That is, print 'X' with video memory successfully.
