D Bare Bones

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Difficulty level
Beginner

Kernel Designs
Models
Monolithic Kernel Microkernel Hybrid Kernel Exokernel Nano/Picokernel Cache Kernel Virtualizing Kernel Megalithic Kernel
Other Concepts
Modular Kernel Higher Half Kernel 64-bit Kernel

In this Tutorial we will write a kernel in the D language and boot it.

Preface

The following tutorial assumes basic knowledge of a compiler, linker and assembler toolchain. It also of course assumes prior knowledge of the D programming language.

Overview

In this tutorial we will create a simple D kernel that prints 'D' on to the screen. The basic setup will consist of three files:

• start.asm
• kernel.main.d
• linker.ld

start.asm

global start
extern kmain        ; Allow kmain() to be called from the assembly code
extern start_ctors, end_ctors, start_dtors, end_dtors

MODULEALIGN        equ        1<<0
MEMINFO            equ        1<<1
FLAGS              equ        MODULEALIGN | MEMINFO
MAGIC              equ        0x1BADB002
CHECKSUM           equ        -(MAGIC + FLAGS)

section .text      ; Next is the Grub Multiboot Header

align 4
MultiBootHeader:
       dd MAGIC
       dd FLAGS
       dd CHECKSUM

STACKSIZE equ 0x4000  ; 16 KiB if you're wondering

static_ctors_loop:
   mov ebx, start_ctors
   jmp .test
.body:
   call [ebx]
   add ebx,4
.test:
   cmp ebx, end_ctors
   jb .body

start:
       mov esp, STACKSIZE+stack

       push eax
       push ebx

       call kmain

static_dtors_loop:
   mov ebx, start_dtors
   jmp .test
.body:
   call [ebx]
   add ebx,4
.test:
   cmp ebx, end_dtors
   jb .body

    
cpuhalt:
       hlt
       jmp cpuhalt

section .bss
align 32

stack:
      resb      STACKSIZE

Assemble that with:

nasm -f elf -o start.o start.asm

kernel.main.d

module kernel.main;
import core.volatile;
 
extern(C) void kmain(uint magic, uint addr) {
 
	ubyte* vidmem = cast(ubyte*)0xFFFF_8000_000B_8000; //Video memory address
 
	for (int i = 0; i < 80*25*2; i++) { //Loops through the screen and clears it
			volatileStore(vidmem + i, 0);
	}
 
	volatileStore(vidmem, 'D' & 0xFF); //Prints the letter D
	volatileStore(vidmem + 1, 0x07); //Sets the colour for D to be light grey (0x07)
 
	for (;;) { //Loop forever. You can add your kernel logic here

	}
}

You then compile that with:

gdc -fno-druntime -m32 -c kernel.main.d -o kernel.main.o -g

linker.ld

OUTPUT_FORMAT(elf32-i386)
ENTRY (start)

SECTIONS{
    . = 0x00100000;

    .text :{
        code = .; _code = .; __code = .;
        *(.text)
        *(.rodata)
    }

    .rodata ALIGN (0x1000) : {
        *(.rodata)
    }

    .data ALIGN (0x1000) : {
        data = .; _data = .; __data = .;
        *(.data)
        start_ctors = .; *(.ctors)   end_ctors = .;
        start_dtors = .; *(.dtors)   end_dtors = .;
    }

    .bss : {
        sbss = .;
        bss = .; _bss = .; __bss = .;
        *(COMMON)
        *(.bss)
        ebss = .;
    }
    end = .; _end = .; __end = .;
}

Now finally you can link all of that with:

ld -melf_i386 -T linker.ld -o kernel.bin start.o kernel.main.o

Your kernel is now kernel.bin, and can now be booted by grub, or run in qemu:

qemu-system-i386 -kernel kernel.bin

Note the "-fno-druntime" argument above. This is how gdc spells the -betterC flag from other D compilers, and the "Better C" page of the D language reference explains how this limits the language. If you want to drop that you'll have to add the D runtime to your kernel.

Hopefully this has gotten you started on writing your operating system in the D programming language.

Further reading

Creating a minimal output to the console

D BareBone with 64 bit and ldc2