Assembly Programming Guide (Part 2): Entering Protected Mode

Welcome back! In this part, we will cover how to switch the CPU from real mode to protected mode and load a simple 32-bit kernel.

What is Protected Mode?

Protected mode is a CPU mode that allows access to more memory (up to 4GB), hardware protection, paging, and 32-bit instructions. Modern operating systems run in this mode.

Warning

This code won't use a BIOS — once in protected mode, you can't call BIOS interrupts like `int 0x10`. We'll write everything low-level.

Files We'll Create

Directory Layout

/project

|-- boot.asm

|-- kernel.asm

|-- linker.ld

|-- Makefile

|-- iso/

boot.asm


[BITS 16]
[ORG 0x7C00]

start:
cli
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7C00
; Load kernel (assume it's at sector 2, LBA)
mov ah, 0x02        ; BIOS read sectors
mov al, 2           ; number of sectors
mov ch, 0
mov cl, 2           ; LBA = 1
mov dh, 0
mov dl, 0x80        ; First HDD
mov bx, 0x1000
int 0x13
jc disk_error

; Setup GDT
lgdt [gdt_descriptor]

; Enable protected mode
mov eax, cr0
or eax, 1
mov cr0, eax
jmp CODE_SEG:init_pm

[BITS 32]
init_pm:
mov ax, DATA_SEG
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov esp, 0x90000
call kernel_main
hang:
jmp hang

disk_error:
mov ah, 0x0E
mov al, 'E'
int 0x10
jmp $

; Global Descriptor Table
gdt_start:
dw 0 ; null
dw 0, 0, 0, 0

gdt_code:
dw 0xFFFF ; limit
dw 0x0000 ; base
db 0x00
db 10011010b
db 11001111b
db 0x00

gdt_data:
dw 0xFFFF
dw 0x0000
db 0x00
db 10010010b
db 11001111b
db 0x00

gdt_end:

gdt_descriptor:
dw gdt_end - gdt_start - 1
dd gdt_start

CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

kernel_main:
call 0x1000
ret

TIMES 510 - ($ - $$) db 0
DW 0xAA55

kernel.asm


[BITS 32]
[GLOBAL _start]
_start:
mov eax, 0xC0FFEE
mov ebx, 0xDEADBEEF
.loop:
hlt
jmp .loop

linker.ld


ENTRY(_start)
SECTIONS {
. = 0x1000;
.text : {
*(.text)
}
}

Makefile


all: os-image

boot.o: boot.asm
nasm -f bin boot.asm -o boot.o

kernel.o: kernel.asm
nasm -f elf kernel.asm -o kernel.o

kernel.bin: kernel.o linker.ld
ld -T linker.ld -o kernel.bin kernel.o --oformat binary

os-image: boot.o kernel.bin
cat boot.o kernel.bin > os-image.bin

run: os-image
qemu-system-i386 -fda os-image.bin

clean:
rm -f *.o *.bin

Running It

make

make run

If all goes well, it will boot into protected mode and hang the CPU (`hlt`), ready for more code.

What's Next?

In future parts, you can explore:

Stay tuned!