Protected mode intialization code example

Protected mode intialization code example
			page	66,132
			.386p
			stack	segment	stack use16
					dd	100 dup(0)
			stack	ends
			data segment use16
			gdt		dd	0
					dd	0
			code_desc	dd	0000ffffh
					dd	00cf9a00h
			data_desc	dd	0000ffffh
					dd	00cf9200h
			ret_desc	dd	0000ffffh
					dd	00cf9a00h
			gdt_label	label	fword
			gdt_limit	dw	4*8-1
			gdt_base	dd	?
			protected_code_address	dd	0
						dw	08h
			data ends
			code segment para public use16 'code'
			assume cs:code,ds:data,ss:stack
	start:
			mov	ax,data
			mov	ds,ax
					; setup gdt
			mov	eax,0
			mov	ax,data
			shl	eax,4
			add	eax,offset gdt
			mov	gdt_base,eax
					; runtime fixup of far jump into pmode
			mov	eax,0
			mov	ax,code32
			shl	eax,4
			add	eax,offset protected_code
			mov	protected_code_address,eax
					; runtime fixup for far jump back to rmode
			mov	eax,0
			mov	ax,code32
			mov	es,ax
			mov	word ptr es:[4],18h
			
			cli
			db	66h
			lgdt	gdt_label
			smsw	ax
			or	ax,1
			lmsw	ax
			jmp	next
	next:		jmp	fword ptr protected_code_address
	ret_real:	smsw	ax
			and	ax,0fffeh
			lmsw	ax
			jmp	far ptr s16
	s16:
			mov	ax,4c00h
			int	21h
	code		ends
	code32		segment	use32
			assume	cs:code32
	sto:
			jmp	far ptr ret_real
	protected_code:
			mov	ax,10h
			mov	ds,ax
			mov	ebx,0b8000h
			mov	ax,0741h
			mov	ecx,80*25
	st1:
			mov	[ebx],ax
			add	ebx,2
			loop	st1
			jmp	sto
			
	code32		ends
			end	start