REAL-PROTECTED

.386

descr struc
lim     dw 0
base_l  dw 0
base_m  db 0
attr_1  db 0
attr_2  db 0
base_h  db 0
descr   ends

; DATA segment
data segment use16

gdt_null    descr   <0,0,0,0,0,0>
gdt_data    descr   <data_size-1, 0,0,92h, 0,0>
gdt_code    descr   <code_size-1, 0,0, 98h, 0,0>
gdt_stack   descr   <255, 0,0,92h,0,0>
gdt_screen  descr   <3999,8000h, 0Bh, 92h, 0,0,>
gdr_size = $-gdt_null

pdescr  df 0
sym     db 1
attr    db 1Eh
msg     db 27,'Returned to real mode!',27,'$'
data_size=$-gdt_null
data ends

text segment use16
    assume CS:text, CS:data
main proc
    xor eax, eax
    mov ax, data
    mov ds, ax

    shl eax, 4
    mov ebp, eax
    mov bx, offset gdt_data
    mov [bx].base_l, ax
    shr eax, 16
    mov [bx].base_m, AL

    xor eax, eax
    mov ax, CS
    shl eax, 4
    mov BX, offset gdt_code
    mov [bx].base_l, ax
    shr eax, 16
    mov [bx].base_m, al

    xor eax, eax
    mov ax, ss
    shl eax, 4
    mov bx, offset gdt_stack
    mov [bx].base_l, ax
    shr eax, 16
    mov [bx].base_m, al

    mov dword ptr pdescr+2, ebp
    mov word ptr pdescr, gdt_size-1
    lgdt pdescr

    mov ax, 40h
    mov es, ax
    mov word ptr es:[67h], offset return
    mov es:[69h], cs
    mov al, 0Fh
    out 70h, al
    mov al, 0Ah
    out 71h, al
    cli

    mov eax, cr0
    or eax, 1
    mov cr0, eax

;; Now in protected mode ;;

    db 0EAh
    dw offset continue
    dw 16

continue:

    mov ax, 8
    mov ds, ax

    mov ax, 24
    mov ss, ax

    mov ax, 32
    mov es, ax

    mov di, 1920
    mov cx, 80
    mov ax, word ptr sym

scrn:
    swosw
    inc al
    loop scrn

    mov al, 0FEh
    out 64h, al
    hlt

;; Now in real mode ;;

return:
    mov ax, data
    mov ds, ax
    mov ax, stk
    mov ss, ax
    mov sp, 256
    sti

    mov ah, 09h
    mov dx, offset msg
    int 21h
    mov ax, 4c00h
    int 21h
main endp

code_size=$-main
text    ends

stk segment stack use16
    db 256 dup('^')
stck ends
    end main