【PM复习】保护模式下的一点迷你应用（下）

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上一节中的与显示有关的函数已经写好了，下面来继续写关于读和写的代码。我们打算在5M地址处读和写，那么必须在GDT中添加一个数据段：

Code:

LABEL_DESC_TEST:
Descriptor 50000h,0ffffh,DA_DRW

添加选择子：

Code:

Selector_Test equ LABEL_DESC_TEST - LABEL_DESC_DUMMY

接下来是读测试函数：

Code:

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

代码很简单，要注意的一点是调用此函数前要把fs赋为指向测试段的选择子。

下面是写测试函数：

Code:

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

这个函数也很简单，没什么可说的，下面是全部代码：

Code:

%include "pm.inc"
org 0100h
jmp LABEL_BEGIN
[section .gdt]
LABEL_DESC_DUMMY:
Descriptor 0,0,0
LABEL_DESC_CODE32:
Descriptor 0,0ffffh,DA_C | DA_32
LABEL_DESC_CODE16:
Descriptor 0,0ffffh,DA_C
LABEL_DESC_VIDEO:
Descriptor 0b8000h,0ffffh,DA_DRW
LABEL_DESC_DATA:
Descriptor 0,0ffffh,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
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
[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 - $$
[section .s16]
[bits 16]
LABEL_BEGIN:
mov ax,cs
mov ds,ax
mov es,ax
mov ax,ss
mov sp,0100h
mov [LABEL_GO_BACK_TO_REAL + 3],ax
Fill_Descriptor LABEL_DESC_CODE32,LABEL_CODE32
Fill_Descriptor LABEL_DESC_CODE16,LABEL_BEGIN
Fill_Descriptor LABEL_DESC_DATA,LABEL_DATA
Fill_Descriptor LABEL_DESC_STACK32,LABEL_STACK32
xor eax,eax
mov ax,ds
shl eax,4
add eax,LABEL_DESC_DUMMY
mov dword [GDT_Ptr + 2],eax
lgdt [GDT_Ptr]
mov [w_SP_Value_In_Real_Mode],sp
cli
in al,92h
or al,00000010b
out 92h,al
mov eax,cr0
or al,1
mov cr0,eax
jmp Selector_Code32:0
_LABEL_PREPARE_GO_BACK_TO_REAL:
LABEL_PREPARE_GO_BACK_TO_REAL equ _LABEL_PREPARE_GO_BACK_TO_REAL - $$
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
[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
jmp Selector_Code16:LABEL_PREPARE_GO_BACK_TO_REAL
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 ax,0ffh
inc ax
mov bl,160
mul bl
mov [d_Disp_Pos],eax
pop ebx
pop eax
pop ebp
ret

运行结果如下：