04/10/23 Protected Mode Assignment 1

Switching from Real to Protected Mode

• Programme Assignments

• Define Global Descriptor Table

• Main Programme Body

0000h

4Gb

x00h

04/10/23 Protected Mode Assignment 2

Programme Assignments

.386p

Code segment use16

Assume cs: code , ss: stackrealadd dd 0taskadd dd 0

msg0 db ‘ in Real Mode ‘,0msg1 db ‘ in Protected Model Mode ‘,0msg2 db ‘ Back in Real Mode ‘,0msg3 db ‘ < Your own declarations> ‘,0

x00h

04/10/23 Protected Mode Assignment 3

Define Global Descriptor Table

Code Segment cs_32

Data Segment ds_32

Extra Segment es_32

04/10/23 Protected Mode Assignment 4

Define Global Descriptor Table

<Code Segment cs_32>------------------------------------------------------------------------------------------cs_32: ; 08h -----------------------------------------------------------------------------

dw 0ffffh ; segment Limitc1: dw 00h ; Base Address 15:00c2: db 00h ; Base Address 23:16

db 09Ah : b ’1001 1010’db 00h ; b ‘ 0000 0000’

C3 db 00h ; Base Address 24:31---------------------------------------------------------------------------------------------

04/10/23 Protected Mode Assignment 5

Define Global Descriptor Table

<Extra Segment es_32>------------------------------------------------------------------------------------------es_32: ; 10h------------------------------------------------------------------------------------------

dw 0ffffh ; segment Limite1: dw 8000h ; Base Address 15:00e2: db 0bh ; Base Address 23:16

db 092h : b ’1001 1010’db 00h ; b ‘ 0000 0000’

e3 db 00h ; Base Address 24:31---------------------------------------------------------------------------------------------

04/10/23 Protected Mode Assignment 6

Define Global Descriptor Table

<Data Segment ds_32>------------------------------------------------------------------------------------------ds_32: ; 18h------------------------------------------------------------------------------------------

dw 0ffffh ; segment Limitd1: dw 0000h ; Base Address 15:00d2: db 00h ; Base Address 23:16

db 092h : b ’1001 1010’db 00h ; b ‘ 0000 0000’

d3 db 00h ; Base Address 24:31---------------------------------------------------------------------------------------------

04/10/23 Protected Mode Assignment 7

Global Descriptor Table

x00h

cs_32

es_32

ds_32

gdt_Start

cs_32(08h)

es_32(10h)

ds_32(18h)

gdt_End

Null Desc(0h)

04/10/23 Protected Mode Assignment 8

Global Descriptor Table

x00h

gdt_start:cs_32:es_32:ds_32:

gdt_end:

gdt_reg label fwordgdt_limit dw gdt_end - gdt_startgdt_base dd 0

cs_32(08h)

es_32(10h)

ds_32(18h)

gdt_End

Null Desc(0h)

gdt_Start

04/10/23 Protected Mode Assignment 9

Main Programme Body

Begin:

Display Some Message (Graphics Mode)

Set up Global Descriptor Table

Load Global Descriptor Table Register

Switch to Protected Mode

Jump to Protected Mode

Display Message

Switch to Real Mode

Exit:

pm:

rmd:

04/10/23 Protected Mode Assignment 10

Display Some Message (Graphics Mode)

Graphics Display B800h

es = B800

ds = cs

clear screen

si = msg0

di = (80 * Lines(hor) ) * 2

cx = length of Msg

Loop es: [di] = al = [si]

inc si ; inc di

loop cx

04/10/23 Protected Mode Assignment 11

Display Some Message (Graphics Mode)

ax = cslea si, realaddmov word ptr [si+2],axlea ax,[rmd]mov word ptr [si],ax

pm:

rmd:

04/10/23 Protected Mode Assignment 12

Set up Global Descriptor Tableax = cseax << axebx << eaxebx << 4 ; Address is 20 bitseax = ebx

c1 = d1 = axeax >> 16 ; Shift Right by 16

d2 = c2 = ald3 = c3 = ah----------------------------------------------------------------------

c1 , d1

c2 d2c3 , d3

04/10/23 Protected Mode Assignment 13

Load Global Descriptor Table Register

----------------------------------------------------------------------------------eax = ax = cseax << 4ebx << bx = gdt_Startebx = eax + ebxcs:gdt_Base = ebxlgdt cs:gdtreg

04/10/23 Protected Mode Assignment 14

Switch To Protected Mode

cr0<PE> = 1{

eax = cr0or al,01cr0 = eax

}

jump to Protected Mode

pm:es = 10ds = 18

display as before using si di

pm:

rmd:

04/10/23 Protected Mode Assignment 15

Switch To Protected Mode

cr0<PE> = 1{

eax = cr0or al,01

` cr0 = eax}

db oeahdw pmdw 08h

pm:es = 10ds = 18

display as before using si di

pm:

rmd:

04/10/23 Protected Mode Assignment 16

Main Programme Body

CR0<PE> = 0

jmp dword ptr realadd

rmd:

Display Data in Real Mode

Exit using int 21(4ch)

pm:

rmd: