TestOS - Simple OS/Bootloader Script demonstrating jumping to 32 Bit Protected Mode from 16 Bit Real Mode

t1.asm

; TestOS - Simple OS/Bootloader Script 
; To demostrate jumping from 16-Bit Real Mode to 32 Bit Protected mode
; Tested in QEMU
; Simply by setting up only GDT. 
; A20 Gate is not part of of this sccript.
; No keyboard inputs required. I think, That would have made this interactive and better.
; 
; Single grouped Command to Test the script in Linux (Qemu is Required)
; nasm t1.asm -o t1.bin && dd conv=notrunc if=t1.bin of=t1.flp && qemu-system-i386 -fda t1.flp
; No license and Warranty - Use at your own risk after studying the code below.
;
; Have a great day,
; Krishna

BITS 16
org 0x7c00 ; Our code will be loaded here

jmp start

;*******************************************
; Global Descriptor Table (GDT)
;*******************************************

GDT: 
	; null descriptor
	dd 0                
	dd 0 
	
	; code descriptor
	dw 0xFFFF           ; limit low
	dw 0                ; base low
	db 0                ; base middle
	db 10011010b        ; access
	db 11001111b        ; granularity
	db 0                ; base high
	
	; data descriptor
	dw 0xFFFF           ; limit low (Same as code)10:56 AM 7/8/2007
	dw 0                ; base low
	db 0                ; base middle
	db 10010010b        ; access
	db 11001111b        ; granularity
	db 0                ; base high
END_GDT:

; GDT Pointer
GDT_PTR: 
	dw END_GDT - GDT - 1 	; limit (Size of GDT)
	dd GDT 			; base of GDT
	
; Location of entries in GDT
CODE_SEG equ 0x08
DATA_SEG equ 0x10

; Calling this enables Protected mode
; Shall be called only after setting up and loading GDT
; Otherwise, Processor will throw exception
EnablePM:					; clear interrupts
	mov	eax, cr0			; set bit 0 in cr0--enter pmode
	or	eax, 1
	mov	cr0, eax
	ret

Print16:
	lodsb
	or al, al
	jz .done
	mov ah, 0xe
	int 0x10
	jmp Print16
.done:
	ret

; Entry point in 16 bit Real Mode
; See the jmp instruction at the top of this file
start:
	;-------------------------------;
	;   Setup segments and stack	;
	;-------------------------------;
	
	xor	ax, ax				; null segments
	mov	ds, ax
	mov	es, ax
	mov	ax, 0x9000			; stack begins at 0x9000-0xffff
	mov	ss, ax
	mov	sp, 0xFFFF
	
	; Welcome Message
	mov si, wel
	call Print16
	
	lgdt [GDT_PTR] ; Install GDT
	
	; Jumping to Protected Mode
	; Notifying the user here
	mov si, pms
	call Print16
	
	; Enabling PM Mode
	call EnablePM
	
	; Jump to protected mode routine
	; Disabling the interupts because we didn't setup any interuppt handlers
	; Interuppts shall be enabled only after setting up and loading IDT table properly
	cli
	jmp CODE_SEG:PmodeEntry ; CS will be Auto-Updated to CODE_SEG
	
	jmp $
	wel db "Welcome! Warming Up ...", 0x0D, 0x0A, 0
	pms db "Now, Enabling and Jumping to protected mode...", 0x0D, 0x0A, 0

[BITS 32]
wel32Msg db "Welcome to the protected world!", 0

; ENTRY Point/subroutine for Protected mode
PmodeEntry:
	; Reload stack and data segment registers with GDT entry
	mov ax, DATA_SEG
	mov ds, ax
	mov ss, ax
	mov sp, 0xffff
	
	mov si, wel32Msg 	; Setting PM welcome message to be printed
	call Print32		; Printing PM Welcome message
	
	
	; Halting the system
	cli
	hlt
	
; Sub routine to print message in 32 bit protected mode
Print32:
	;Print to screen
	mov edi, 0xb8000
.repeat:
	lodsb
	or al, al
	jz .done
	mov byte [edi], al
	inc edi
	mov byte [edi], 0x1B
	inc edi
	call .repeat
.done:
	ret
	
	; Fill remaining bytes with zeros
	; and add boot signature
	times 510-($-$$) db 0
	dw 0xaa55