Enter/Leave Protected Mode

stub.asm

org 0x100
start: use16
        cli					; disable interrupts

        in al, 0x92				; IO port method to enable A20
        or al, 2
        and al,0xFE
        out 0x92, al

	lgdt [descriptor] 			; load GDT header into GDT register, so the CPU can find the GDT entries

        mov eax,cr0				; enable protected mode
        or eax,1
        mov cr0,eax

	dw _start32				; jump to start of 32bit protected mode segment
	db 0xEA
	db 8

LeaveProtectedMode:	use16
			pop     ds
			popf
			
			mov     ah, 4ch		; exit
			mov     al, cl		; status = the byte
			int     21h
			
descriptor:					; This header describes the GDT itself
        dw 31					; GDT limit is 31, which means its size is 32 bytes
        dd gdt_entries+0x11C70			; GDT flat memory address is the flat memory address of the start
																		; program (0x11C70, as determined in Bochs debugger) + the offset
																		; into the program at which which the actual GDT entries are stored
gdt_entries:					; memory blocks are 4kB in size, and the first block in memory is block 0
        dq 0x0000000000000000			; blank entry
        dq 0x00C09A0010000008			; code segment, start of segment is block 1, and is 8 blocks in size
        dq 0x00C0920090000008			; data segment, start of segment is block 9, and is 8 blocks in size
        dq 0x00C0960110000008			; stack segment, start of segment is block 17, and is 8 blocks in size

_start32: use32					; initialize the segment registers with indecies into the GDT
        mov ax,2
        mov ds,ax
        mov ax,3
        mov ss,ax

_star: 	mov     cr0, eax			; 
	db      0eah   				; far jump to restore CS & clear prefetch queue
	dd      LeaveProtectedMode 		; it MUST be a far jump - return crashes!