This directory has two assembly files, disk.asm and gdt.asm, which are two bootloaders that implement the disk and the GDT, respectively. It also contains a very simple kernel written in C.

Compiling and running these examples requires NASM and QEMU to be installed. Compiling the kernel requires an i386 cross-compiler.

• To compile: $ nasm disk.asm -f bin -o disk.bin
• To run: $ qemu-system-i386 -hda disk.bin

You should also try the commands $ xxd disk.bin and $ objdump -d disk.bin with these compiled files.

It is necessary to have a cross-compiler installed, prefixed with “i386-elf-“, and placed in the user’s $PATH. Once installed, simply type make to compile and run the kernel. The Makefile runs these commands:

nasm bootsect.asm -f bin -o bootsect.bin
nasm kernel_entry.asm -f elf -o kernel_entry.o
i386-elf-gcc -O2 -nostdlib -nostartfiles -ffreestanding -c kernel.c -o kernel.o
i386-elf-ld -o kernel.bin -Ttext 0x1000 kernel_entry.o kernel.o --oformat binary
cat bootsect.bin kernel.bin > kimage.bin
qemu-system-i386 -fda kimage.bin

void dummy(void) {
}

void main(void) {
    char *video_memory = (char *)0xb80a0;
    char *str = "Welcome to the kernel!";
    int x;

    /* prints the string in video memory, in alternating
     * colors, directly below "Landed in 32-bit Protected Mode" */
    for (x = 0; x < 22; x++) {
        (*video_memory++) = str[x];
        if (x % 2 == 0)
            (*video_memory++) = 0x0a;
        else if (x % 3 == 0)
            (*video_memory++) = 0x0d;
        else
            (*video_memory++) = 0x0c;
    }
}

[bits 16]
load_kernel:
    mov bx, MSG_LOAD_KERNEL
    call print
    call print_nl

    mov bx, KERNEL_OFFSET
    mov dh, 2
    mov dl, [BOOT_DRIVE]
    call disk_load
    ret