; 3. Program to switch to protected mode
dosseg
.MODEL SMALL
.586p
codeseg_offset equ 08h
dataseg_offset equ 10h
extraseg_offset equ 18h
stackseg_offset equ 20h
;-----------------------------------------------------------------------------
; Segment Descriptor Structure
;-----------------------------------------------------------------------------
segment_descriptor struc
seg_length0_15 dw 0
base_addr0_15 dw 0
base_addr16_23 db 0
flags db 0
access db 0
base_addr24_31 db 0
segment_descriptor ends
;-----------------------------------------------------------------------------
; Macro used for 32-bit address calculation
;-----------------------------------------------------------------------------
ADDR32 macro base,offsetadd
mov eax,0
mov ebx,eax
mov ax,base
shl eax,4
mov bx,offsetadd
add ebx,eax
endm
;-----------------------------------------------------------------------------
; Macro used for Displaying PE bit
;-----------------------------------------------------------------------------
DISP macro
mov ah,06H
mov bl,6
add al,30H
mov dl,al
int 21H
endm
dsetup macro dest
movzx eax, ax
shl eax, 4
mov dest.base_addr0_15, ax
shr eax, 8
mov dest.base_addr16_23, ah
endm
;-----------------------------------------------------------------------------
; Stack
;-----------------------------------------------------------------------------
stack16 segment para public use16
assume cs:code16, ds:data16
db 100 dup (?)
stack16_end label word
db 100 dup (?)
stack16 ends
;-----------------------------------------------------------------------------
; Data
;-----------------------------------------------------------------------------
data16 segment para public use16
assume cs:code16, ds:data16
MSG1 db "Switching to Protected Mode...Wait for sometime to switch back $"
MSG2 db "PE BIT OF CRO after switching back is $"
global_descriptor_table label fword
gdt_start dw gdt_size,0,0
dummy_des segment_descriptor <0,0,0,0,0,0>
code16_des segment_descriptor <0ffffh,0,0,9ah,0,0>
data16_des segment_descriptor <0ffffh,0,0,92h,0,0>
extraseg_des segment_descriptor <0ffffh,0,0,92h,0,0>
stak16_des segment_descriptor <00000h,0,0,96h,0,0>
gdt_size = $-(offset dummy_des)
temp dw ?
data16 ends
;-----------------------------------------------------------------------------
; Code starts
;-----------------------------------------------------------------------------
code16 segment para public use16
assume cs:code16, ds:data16
;-----------------------------------------------------------------------------
; Procedure used to implement delay
;-----------------------------------------------------------------------------
delay PROC NEAR
mov ecx,02ffffffH
lop: dec ecx
jnz lop
ret
delay endp
start16:
cli ; turn off interrupts
; Set up real mode registers
mov ax, data16
mov ds, ax
mov ax, stack16
mov ss, ax
mov sp, offset stack16_end
; Set up up the various pmode descriptiors
mov ax, code16
dsetup code16_des
mov ax, data16
dsetup data16_des
mov ax, stack16
dsetup stak16_des
; Set up the global descriptor table
mov temp, offset dummy_des
ADDR32 data16, temp
mov dword ptr ds:gdt_start[2],ebx
; Load Global Descriptor Table Register
lgdt fword ptr ds:global_descriptor_table
; Display a message before switching to Protected mode
mov dx,offset MSG1
mov ah,09H
int 21H
; Switch to Protected mode by setting PE bit of CR0
mov eax,cr0
or al,1
mov cr0,eax
db 0eah
dw $+4, codeseg_offset
; Set up the pmode descriptors
mov ax, dataseg_offset
mov ds, ax
mov ax, stackseg_offset
mov ss, ax
mov ax, extraseg_offset
mov es, ax
mov fs, ax
mov gs, ax
; Give some delay in Protected mode
call delay
; Switch back to real mode by resetting PE bit of CR0
mov eax,cr0
and al,not 1
mov cr0,eax
db 0eah
dw $+4,code16
mov ax, data16
mov ds, ax
mov es, ax
mov ax, stack16
mov ss,ax
mov sp, offset stack16_end
; Turn interrupts back on and get back to DOS
sti
; Print the PE bit of CR0
mov dl,0AH
mov ah,02H
int 21H
mov dl,0DH
mov ah,02H
int 21H
mov dx,offset MSG2
mov ah,09H
int 21H
mov eax,cr0
and al,0001H
;Use the macro DISP to display PE bit of CR0
disp
mov ax,4c00h
int 21h
code16 ends
end start16
end