blog_os/_posts/2015-08-18-multiboot-kernel.md

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post	A minimal x86 kernel in small steps

This post explains how to create a minimal x86 operating system kernel. In fact, it will just boot and print OK to the screen. The following blog posts we will extend it using the Rust programming language.

I tried to explain everything in detail and to keep the code as simple as possible. If you have any questions, suggestions or other issues, please leave a comment or create an issue on Github. The source code is available in a repository, too.

Overview

When you turn on a computer, it loads the BIOS. It first runs self test and initialization routines of the hardware. Then it looks for bootable devices. If it finds one, the control is transferred to its bootloader, which is a small portion of executable code stored at the device's beginning. The bootloader has to determine the location of the kernel image on the device and load it into memory. It also needs to switch the CPU to the so-called Protected Mode because x86 CPUs start in the very limited Real Mode by default (to be compatible to programs from 1978).

We won't write a bootloader because that would be a complex project on its own (if you really want to do it, check out Rolling Your Own Bootloader). Instead we will use one of the many well-tested bootloaders out there. But which one?

Multiboot

Fortunately there is a bootloader standard: the Multiboot Specification. So our kernel just needs to indicate that it supports Multiboot and every Multiboot-compliant bootloader can boot it. We will use the Multiboot 2 specification (PDF) together with the well-known GRUB 2 bootloader.

To indicate our Multiboot 2 support to the bootloader, our kernel must contain a Multiboot Header, which has the following format:

Field	Type	Value
magic number	u32	0xE85250D6
architecture	u32	0 for i386, 4 for MIPS
header length	u32	total header size, including tags
checksum	u32	-(magic + architecture + header length)
tags	variable
end tag	(u16, u16, u32)	(0, 0, 8)

Converted to a x86 assembly file it looks like this (Intel syntax):

section .multiboot_header
header_start:
    dd 0xe85250d6                ; magic number (multiboot 2)
    dd 0                         ; architecture 0 (protected mode i386)
    dd header_end - header_start ; header length
    ; checksum
    dd 0x100000000 - (0xe85250d6 + 0 + (header_end - header_start))

    ; insert optional multiboot tags here

    ; required end tag
    dw 0    ; type
    dw 0    ; flags
    dd 8    ; size
header_end:

If you don't know x86 assembly, here is some quick guide:

• the header will be written to a section named .multiboot_header (we need this later)
• header_start and header_end are labels that mark a memory location, we use them to calculate the header length easily
• dd stands for define double (32bit) and dw stands for define word (16bit). They just output the specified 32bit/16bit constant.
• the additional 0x100000000 in the checksum calculation is a small hack¹ to avoid a compiler warning

We can already assemble this file (which I called multiboot_header.asm) using nasm. It produces a flat binary by default, so the resulting file just contains our 24 bytes (in little endian if you work on a x86 machine):

> nasm multiboot_header.asm
> hexdump -x multiboot_header
0000000    50d6    e852    0000    0000    0018    0000    af12    17ad
0000010    0000    0000    0008    0000
0000018

The Boot Code

To boot our kernel, we must add some code that the bootloader can call. Let's create a file named boot.asm:

global start

section .text
bits 32
start:
    ; print `OK` to screen
    mov dword [0xb8000], 0x2f4b2f4f
    hlt

There are some new commands:

• global exports a label (makes it public). As start will be the entry point of our kernel, it needs to be public.
• the .text section is the default section for executable code
• bits 32 specifies that the following lines are 32-bit instructions. It's needed because the CPU is still in Protected mode when GRUB starts our kernel. When we switch to Long mode in the [next post] we can use bits 64 (64-bit instructions).
• the mov dword instruction moves the 32bit constant 0x2f4f2f4b to the memory at address b8000 (it prints OK to the screen, an explanation follows in the next posts)
• hlt is the halt instruction and causes the CPU to stop

Through assembling, viewing and disassembling we can see the CPU Opcodes in action:

> nasm boot.asm
> hexdump -x boot
0000000    05c7    8000    000b    2f4b    2f4f    00f4
000000b
> ndisasm -b 32 boot
00000000  C70500800B004B2F  mov dword [dword 0xb8000],0x2f4b2f4f
         -4F2F
0000000A  F4                hlt

Building the Executable

Now we create an ELF executable from these two files. We therefore need the object files of the two assembly files and a custom linker script, that we call linker.ld:

ENTRY(start)

SECTIONS {
    . = 1M;

    .boot :
    {
        /* ensure that the multiboot header is at the beginning */
        *(.multiboot_header)
    }

    .text :
    {
        *(.text)
    }
}

Let's translate it:

• start is the entry point, the bootloader will jump to it after loading the kernel
• . = 1M; sets the load address of the first section to 1 MiB, which is a conventional place to load a kernel²
• the executable will have two sections: .boot at the beginning and .text afterwards
• the .text output section contains all input sections named .text
• Sections named .multiboot_header are added to the first output section (.boot) to ensure they are at the beginning of the executable. This is necessary because GRUB expects to find the Multiboot header very early in the file.

So let's create the ELF object files and link them using our new linker script. It's important to pass the -n flag to the linker because otherwise it may page align the sections in the executable. If that happens, GRUB isn't able to find the Multiboot header because the .boot section isn't at the beginning anymore. We can use objdump to print the sections of the generated executable and verify that the .boot section has a low file offset.

> nasm -f elf64 multiboot_header.asm
> nasm -f elf64 boot.asm
> ld -n -o kernel.bin -T linker.ld multiboot_header.o boot.o
> objdump -h kernel.bin
kernel.bin:     file format elf64-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .boot         00000018  0000000000100000  0000000000100000  00000080  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  1 .text         0000000b  0000000000100020  0000000000100020  000000a0  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE

Creating the ISO

The last step is to create a bootable ISO image with GRUB. We need to create the following directory structure and copy the kernel.bin to the right place:

isofiles
└── boot
    ├── grub
    │   └── grub.cfg
    └── kernel.bin

The grub.cfg specifies the file name of our kernel and it's Multiboot 2 compliance. It looks like this:

set timeout=0
set default=0

menuentry "my os" {
    multiboot2 /boot/kernel.bin
    boot
}

Now we can create a bootable image using the command:

grub-mkrescue -o os.iso isofiles

Booting

Now it's time to boot our OS. We will use QEMU:

qemu-system-x86_64 -hda os.iso


Notice the green OK in the upper left corner. Let's summarize what happens:

1. the BIOS loads the bootloader (GRUB) from the virtual hard drive (the ISO)
2. the bootloader reads the kernel executable and finds the Multiboot header
3. it copies the .boot and .text sections to memory (to addresses 0x100000 and 0x100020)
4. it jumps to the entry point (0x100020, you can obtain it through objdump -f)
5. our kernel prints the green OK and stops the CPU

You can test it on real hardware, too. Just burn the ISO to a disk or USB stick and boot from it.

Build Automation

Right now we need to execute 4 commands in the right order everytime we change a file. That's bad. So let's automate the build using a Makefile. But first we should create some clean directory structure for our source files to separate the architecture specific files:

…
├── Makefile
└── src
    └── arch
        └── x86_64
            ├── multiboot_header.asm
            ├── boot.asm
            ├── linker.ld
            └── grub.cfg

The Makefile looks like this (but indented with tabs instead of spaces):

arch ?= x86_64
kernel := build/kernel-$(arch).bin
iso := build/os-$(arch).iso

linker_script := src/arch/$(arch)/linker.ld
grub_cfg := src/arch/$(arch)/grub.cfg
assembly_source_files := $(wildcard src/arch/$(arch)/*.asm)
assembly_object_files := $(patsubst src/arch/$(arch)/%.asm, \
    build/arch/$(arch)/%.o, $(assembly_source_files))

.PHONY: all clean run iso

all: $(kernel)

clean:
    @rm -r build

run: $(iso)
    @qemu-system-x86_64 -hda $(iso)

iso: $(iso)

$(iso): $(kernel)
    @mkdir -p build/isofiles/boot/grub
    @cp $(kernel) build/isofiles/boot/
    @cp $(grub_cfg) build/isofiles/boot/grub
    @grub-mkrescue -o $(iso) build/isofiles 2> /dev/null
    @rm -r build/isofiles

$(kernel): $(assembly_object_files) $(linker_script)
    @ld -n -T $(linker_script) -o $(kernel) $(assembly_object_files)

# compile assembly files
build/arch/$(arch)/%.o: src/arch/$(arch)/%.asm
    @mkdir -p $(shell dirname $@)
    @nasm -felf64 $< -o $@

Some comments (see the Makefile tutorial if you don't know make):

• the $(wildcard src/arch/$(arch)/*.asm) chooses all assembly files in the src/arch/$(arch)` directory, so you don't have to update the Makefile when you add a file
• the patsubst operation for assembly_object_files just translates src/arch/$(arch)/XYZ.asm to build/arch/$(arch)/XYZ.o
• the $< and $@ in the assembly target are automatic variables
• the Makefile has rudimentary multi-architecture support, e.g. make arch=mips iso tries to create an ISO for MIPS (it will fail of course as we don't support MIPS yet).

Now we can invoke make and all updated assembly files are compiled and linked. The make iso command also creates the ISO image and make run will additionally start QEMU.

What's next?

In the [next post] we will create a page table and do some CPU configuration to switch to Long Mode.

[next post]: {% post_url 2015-08-25-entering-longmode %}

---

1. The formula from the table, -(magic + architecture + header length), creates a negative value that doesn't fit into 32bit. By subtracting from 0x100000000 (= 2^(32)) instead, we keep the value positive without changing its truncated value. Without the additional sign bit(s) the result fits into 32bit and the compiler is happy :). ↩︎

2. We don't want to load the kernel to e.g. 0x0 because there are many special memory areas below the 1MB mark (for example the so-called VGA buffer at 0xb8000, that we use to print OK to the screen). ↩︎