User:Imate900/32-bit assembler bare bones
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This tutorial will teach you how to make a 32-bit assembler kernel.
GRUB
All we need is something like this (GNU as):
.global loader # making entry point visible to linker # setting up the Multiboot header - see GRUB docs for details .set ALIGN, 1<<0 # align loaded modules on page boundaries .set MEMINFO, 1<<1 # provide memory map .set FLAGS, ALIGN | MEMINFO # this is the Multiboot 'flag' field .set MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header .set CHECKSUM, -(MAGIC + FLAGS) # checksum required .align 4 .long MAGIC .long FLAGS .long CHECKSUM _start: # Write the kernel here jmp _start
Link with:
as -o kernel.o kernel.s ld --oformat binary -o kernel.bin kernel.o
For NASM:
global loader ; making entry point visible to linker ; setting up the Multiboot header - see GRUB docs for details ALIGN equ 1<<0 ; align loaded modules on page boundaries MEMINFO equ 1<<1 ; provide memory map FLAGS equ ALIGN | MEMINFO ; this is the Multiboot 'flag' field MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header CHECKSUM equ -(MAGIC + FLAGS) ; checksum required mboot: dd MAGIC dd FLAGS dd CHECKSUM _start: ; Write the kernel here jmp _start
Link with:
nasm -f bin kernel.asm
Make sure you have a GRUB entry...
title Assembler Barebones kernel (hd0,0)/boot/kernel.bin
Now, reboot, and enjoy!
Without GRUB
Starting In Protected Mode
If your bootloader loads flat binary images and gives you protected mode for you, use this code as a basis:
_start:
; Write kernel here. It might be good to load a new GDT. I include the GDT business here nevertheless.
lgdt [kern_gdtr]
jmp 0x08:normalize_regs
normalize_regs:
mov ax, 0x10 ; 0x10 points at the new data selector
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
jmp kernel
kernel:
; After all that tricky business! Write your kernel here, anyway.
jmp kernel
kern_gdt:
db 0, 0, 0 ; null entry
db 0, 0xffffffff, 0x9A
db 0, 0xffffffff, 0x92
kern_gdtr DW 0 ; For limit storage
DD 0 ; For base storage
Starting In Real Mode
Usually (in most FAT bootloaders), you must first switch to Protected Mode in order to write a 32-bit kernel if your kernel starts in Real Mode. For reference, this code is enough (GDT is composed of null entry, a code entry and a data entry:)
BITS 16
cli
lgdt [kern_gdtr]
mov eax, cr0
or eax, 1
mov cr0, eax
jmp 0x08:normalize_regs
BITS 32
normalize_regs:
mov ax, 0x10 ; 0x10 points at the new data selector
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
jmp kernel
kernel:
; After all that tricky business! Write your kernel here, anyway.
jmp kernel
kern_gdt:
db 0, 0, 0 ; null entry
db 0, 0xffffffff, 0x9A
db 0, 0xffffffff, 0x92
kern_gdtr DW 0 ; For limit storage
DD 0 ; For base storage