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 in 16-bit real mode with BIOS interrupt call (int 0x10)
- Understand the concepts of stack and function calls
- 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
Install QEMU emulator in Linux
$ sudo apt-get install qemu-system-x86
Run the following commands or 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 would like to debug the code, you can connect to GDB remotely with the following command:
(gdb) target remote localhost:1234
The program initially prints a string in 16-bit real mode using a BIOS interrupt call. Subsequently, it prepares to load the kernel from the disk and transitions to 32-bit protected mode. Finally, it successfully enters 32-bit protected mode and proceeds to load the kernel. As evident, the program executes the function within the kernel, resulting in the successful display of 'X' on the video memory.