【PM复习】修正段界限

     在前几节中的GDT和LDT中，大部分描述符的段界限都是0xffff，这其实是不严谨的，段界限是多少，要从实际出发，在这节中各个段的最后，都要定义一个宏表示此段的长度，在描述符中用此宏来表示段界限。

     完整代码如下：

%include "pm.inc"

org 0100h
jmp LABEL_BEGIN

[section .gdt]
LABEL_DESC_DUMMY:
    Descriptor  0,0,0
LABEL_DESC_CODE32:
    Descriptor  0,GDT_Code32_Len - 1,DA_C | DA_32
LABEL_DESC_CODE16:
    Descriptor  0,GDT_Code16_Len - 1,DA_C
LABEL_DESC_VIDEO:
    Descriptor  0b8000h,0ffffh,DA_DRW
LABEL_DESC_DATA:
    Descriptor  0,Data_Len - 1,DA_DRW
LABEL_DESC_TEST:
    Descriptor  50000h,0ffffh,DA_DRW
LABEL_DESC_STACK32:
    Descriptor  0,Stack_Len - 1,DA_DRW | DA_32
LABEL_DESC_NORMAL:
    Descriptor  0,0ffffh,DA_DRW
LABEL_DESC_LDT:
    Descriptor  0,LDT_LEN - 1,DA_LDT

GDT_Len equ $ - LABEL_DESC_DUMMY
GDT_Ptr:
    dw  GDT_Len - 1
    dd  0

Selector_Code32 equ LABEL_DESC_CODE32 - LABEL_DESC_DUMMY
Selector_Code16 equ LABEL_DESC_CODE16 - LABEL_DESC_DUMMY
Selector_Video  equ LABEL_DESC_VIDEO - LABEL_DESC_DUMMY
Selector_Data       equ LABEL_DESC_DATA - LABEL_DESC_DUMMY
Selector_Test       equ LABEL_DESC_TEST - LABEL_DESC_DUMMY
Selector_Stack32 equ    LABEL_DESC_STACK32 - LABEL_DESC_DUMMY
Selector_Normal equ LABEL_DESC_NORMAL - LABEL_DESC_DUMMY
Selector_LDT        equ LABEL_DESC_LDT - LABEL_DESC_DUMMY

[section .ldt]
LABEL_LDT:
    Descriptor  0,0,0
LABEL_LDT_DESC_CODE32:
    Descriptor  0,LDT_Code32_Len - 1,DA_C | DA_32

Selector_LDT_Code32 equ LABEL_LDT_DESC_CODE32 - LABEL_LDT + SA_TIL
LDT_LEN equ $ - $$


[section .stack32]
[bits 32]
LABEL_STACK32:
    times 512 db 0
Stack_Len   equ $ - $$

[section .data]
LABEL_DATA:
_d_Disp_Pos dd  160 * 5
d_Disp_Pos  equ _d_Disp_Pos - $$
_w_SP_Value_In_Real_Mode    dw  0
w_SP_Value_In_Real_Mode equ _w_SP_Value_In_Real_Mode - $$

Data_Len    equ $ - $$

[section .s16]
[bits 16]
LABEL_BEGIN:
    mov ax,cs
  mov ds,ax
  mov es,ax
  mov   ax,ss
  mov   sp,0100h

  mov   [LABEL_GO_BACK_TO_REAL + 3],ax

  Fill_Descriptor   LABEL_DESC_CODE32,LABEL_CODE32
  Fill_Descriptor   LABEL_DESC_CODE16,LABEL_BEGIN
  Fill_Descriptor   LABEL_DESC_DATA,LABEL_DATA
  Fill_Descriptor   LABEL_DESC_STACK32,LABEL_STACK32
  Fill_Descriptor   LABEL_DESC_LDT,LABEL_LDT
  Fill_Descriptor   LABEL_LDT_DESC_CODE32,LABEL_LDT_CODE32

  xor eax,eax
  mov ax,ds
  shl eax,4
  add eax,LABEL_DESC_DUMMY
  mov dword [GDT_Ptr + 2],eax

  lgdt  [GDT_Ptr]

  mov   [w_SP_Value_In_Real_Mode],sp

  cli

  in  al,92h
  or  al,00000010b
  out 92h,al

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

  jmp dword Selector_Code32:0

_LABEL_PREPARE_GO_BACK_TO_REAL:
LABEL_PREPARE_GO_BACK_TO_REAL equ   _LABEL_PREPARE_GO_BACK_TO_REAL - $$
    mov ax,Selector_Normal
    mov ds,ax
    mov es,ax
    mov ss,ax
    mov sp,[w_SP_Value_In_Real_Mode]
    mov gs,ax
    mov fs,ax

    mov eax,cr0
    and al,11111110b
    mov cr0,eax

LABEL_GO_BACK_TO_REAL:
    jmp 0:LABEL_ALREADY_REAL

LABEL_ALREADY_REAL:
    in  al,92h
    and al,11111101b
    out 92h,al

    sti

    mov ax,4c00h
    int 21h

GDT_Code16_Len  equ $ - $$

[section .s32]
[bits 32]
LABEL_LDT_CODE32:
    mov ax,Selector_Video
  mov gs,ax
  mov ah,0ch
  mov al,'x'
  mov [gs:160 * 10],ax

  jmp   Selector_Code16:LABEL_PREPARE_GO_BACK_TO_REAL

LDT_Code32_Len  equ $ - $$

[section .s32]
[bits 32]
LABEL_CODE32:
  ;mov ax,Selector_Video
  ;mov gs,ax
  ;mov ah,0ch
  ;mov al,'x'
  ;mov [gs:80 * 10],ax

  mov   ax,Selector_Stack32
  mov   ss,ax
  mov   esp,Stack_Len

  mov   ax,Selector_Data
  mov   ds,ax
  mov   ax,Selector_Video
  mov   gs,ax
  mov   ax,Selector_Test
  mov   fs,ax

  call  Read_Test
  call  Disp_Return
  call  Write_Test
  call  Read_Test

  mov   ax,Selector_LDT
  lldt  ax

  jmp   Selector_LDT_Code32:0

Read_Test:
    push    ebp
    mov ebp,esp
    push    ecx
    push    esi

    mov ecx,8
    xor esi,esi

.loop:
    mov al,[fs:esi]
    call    Disp_Al
    inc esi
    loop    .loop

    pop esi
    pop ecx
    pop ebp
    ret

Write_Test:
    push    ebp
    mov ebp,esp
    push    eax
    push    ecx
    push    esi

    xor eax,eax
    mov eax,'A'
    xor esi,esi
    mov ecx,8

.loop:
    mov [fs:esi],al
    inc esi
    inc eax
    loop .loop

    pop esi
    pop ecx
    pop eax
    pop ebp
    ret

Disp_Al:
    push    ebp
    mov ebp,esp

    push    esi
    push    ecx
    push    eax
    mov ecx,2
    shr al,4

.loop:
    and al,0fh
    cmp al,9
    jb  .1
    add al,7

.1:
    add al,30h
    mov esi,[d_Disp_Pos]
    mov byte [gs:esi],al
    mov byte [gs:esi + 1],0ch
    add dword [d_Disp_Pos],2

    dec ecx
    cmp ecx,0
    je  .2
    pop eax
    jmp .loop

.2:
    pop ecx
    pop esi
    pop ebp
    ret

Disp_Return:
    push    ebp
    mov ebp,esp
    push    eax
    push    ebx

    mov eax,[d_Disp_Pos]
    mov bl,160
    div bl
    and eax,0ffh
    inc ax
    mov bl,160
    mul bl
    mov [d_Disp_Pos],eax

    pop ebx
    pop eax
    pop ebp
    ret

GDT_Code32_Len  equ $ - $$

     运行结果跟前一节一样。