
平台论坛博客文库

› 论坛 › 程序设计 › 内核源码 › 向大家请教几个保护模式编程的问题！很初级，请大家费心 ...

向大家请教几个保护模式编程的问题！很初级，请大家费心了。 [复制链接]

myxd2000

白手起家

论坛徽章:: 0

电梯直达

1楼 [收藏(0)] [报告]

发表于 2005-03-20 23:20 |只看该作者 |倒序浏览

下面这段程序用nasm编译通过，可以运行，是一个从实模式到保护模式再到实模式的例子程序。

我这里有几个问题请加大家
1、在mov cr0,eax和mov cr0,eax之间加上loop1:jmp loop1这条指令，都会导致重启。这是为什么呢？
2、如果我不修改0x0D号中断，也就是屏蔽掉mov [es:0x0D * 4],edx ，mov edx,[es:0x0D * 4]，mov [es:0x0D * 4 + 2]，cs，lea ax,[trap]，mov [es:0x0D * 4],ax这5条指令，则屏幕上什么也不打印，这又是为什么呢？

说明一下，我是在Bochs 2.1.1上调试一下程序的。程序有点长，附在下面，请大家帮忙看看，我都快急死了。

[ORG 0x100]
[BITS 16]

start:  xor ebx,ebx
mov bx,ds                      ; BX=segment
shl ebx,4                      ; BX="linear" address of segment base
mov eax,ebx
mov [gdt2 + 2],ax             ; set base address of 32-bit segments
mov [gdt3 + 2],ax
mov [gdt4 + 2],ax             ; set base address of 16-bit segments
mov [gdt5 + 2],ax
shr eax,16
mov [gdt2 + 4],al
mov [gdt3 + 4],al
mov [gdt4 + 4],al
mov [gdt5 + 4],al

mov [gdt2 + 7],ah
mov [gdt3 + 7],ah
mov [gdt4 + 7],ah
mov [gdt5 + 7],ah

      lea eax,[gdt + ebx]          ; EAX=PHYSICAL address of gdt
      mov [gdtr + 2],eax

xor ax,ax
mov es,ax
mov edx,[es:0x0D * 4]    ; INT 0Dh vector ->; EDX
mov [es:0x0D * 4 + 2],cs
lea ax,[trap]
mov [es:0x0D * 4],ax

mov ebx,0xB809A       ; ES still 0
mov byte [es],'R'    ; 'R' in upper right corner of screen

mov ax,cs
mov [RealModeCS],ax
lea ax,[do_rm]
mov [RealModeIP],ax

mov ax,0xB800
mov es,ax

lgdt [gdtr]

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

      lea si,[msg0]                ; ->; "still in real mode!"
      mov di,(80 * 1 + 2) * 2       ; row 1, column 2
      mov cx,38
      cld
      rep movsb

jmp SYS_CODE_SEL:do_pm       ; jumps to do_pm

trap: mov ax,0xB800
mov fs,ax
mov byte [fs:0x9C],'='
pop ax             ; point stacked IP beyond...
add ax,5          ; ...the offending instruction
push ax
iret

[BITS 32]
do_pm:

      xor edi,edi
      xor esi,esi

lea si,[msg1]                ; ->; "ES, DS still real mode!"
      mov di,(80 * 2 + 3) * 2       ; row 2, column 3
      mov ecx,46
      cld
      rep movsb

mov ax,SYS_DATA_SEL
mov ds,ax
mov ss,ax

mov ax,LINEAR_SEL
mov es,ax

; questionable PM code here
mov byte [es:dword 0xB8000],'0'

; more questionable PM code here
mov byte [es:dword 0xB8002],'1'

; still more questionable PM code here
mov byte [es:dword 0xB8004],'2'

; (you get the picture)
mov byte [es:dword 0xB8006],'3'

lea esi,[msg2]                ; ->; "Finally in protected mode!"

mov edi,0xB8000 + (80 * 3 + 4) * 2    ; row 3, column 4
mov ecx,52
cld
rep movsb

jmp REAL_CODE_SEL:do_16
[BITS 16]
do_16:

mov ax,REAL_DATA_SEL
mov ss,ax
mov ds,ax          ; leave ES alone

mov eax,cr0
and al,0xFE
loop1:jmp loop1
mov cr0,eax

jmp far [RealModeIP]

[BITS 16]

do_rm:  mov byte [es:dword 0xB8008],'4'

xor ax,ax
mov es,ax
mov byte [es:dword 0xB800A],'5'

lea esi,[msg3]       ; ->; "ES, DS still protected mode!"
mov edi,0xB8000 + (80 * 4 + 5) * 2    ; row 4, column 5
mov ecx,56
cld
;rep movsb

a32          ; same as 'db 0x67'
rep movsb

mov ax,cs
mov ds,ax
mov ss,ax
mov ax,0xB800
mov es,ax

lea si,[msg4]                ; ->; "back to BORING old real mode"
mov di,(80 * 5 + 6) * 2       ; row 5, column 6
mov cx,56
cld
rep movsb

xor ax,ax
mov es,ax
mov [es:0x0D * 4],edx    ; EDX ->; INT 0x0D vector

sti
mov ax,0x4C00
int 0x21
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; data
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
RealModeIP:
      dw 0

RealModeCS:
dw 0
msg0: db "s t i l l i n r e a l m o d e ! "
msg1: db "E S , D S s t i l l r e a l m o d e ! "
msg2: db "F i n a l l y i n p r o t e c t e d m o d e ! "
msg3: db "E S , D S s t i l l p r o t e c t e d m o d e ! "
msg4: db "b a c k t o B O R I N G o l d r e a l m o d e "

gdtr: dw gdt_end - gdt - 1 ; GDT limit
dd gdt                ; (GDT base gets set above)
gdt: dw 0          ; limit 15:0
dw 0          ; base 15:0
db 0          ; base 23:16
db 0          ; type
db 0          ; limit 19:16, flags
db 0          ; base 31:24

; linear data segment descriptor
LINEAR_SEL  equ $-gdt
dw 0xFFFF    ; limit 0xFFFFF
dw 0          ; base 0
db 0
db 0x92       ; present, ring 0, data, expand-up, writable
      db 0xCF                ; page-granular, 32-bit
db 0

; code segment descriptor
SYS_CODE_SEL equ $-gdt
gdt2: dw 0xFFFF             ; limit 0xFFFFF
dw 0          ; (base gets set above)
db 0
db 0x9A       ; present, ring 0, code, non-conforming, readable
      db 0xCF                ; page-granular, 32-bit
db 0

; data segment descriptor
SYS_DATA_SEL equ $-gdt
gdt3: dw 0xFFFF             ; limit 0xFFFFF
dw 0          ; (base gets set above)
db 0
db 0x92       ; present, ring 0, data, expand-up, writable
      db 0xCF                ; page-granular, 32-bit
db 0

; a code segment descriptor that is 'appropriate' for real mode
; (16-bit, byte-granular, limit=0xFFFF)
REAL_CODE_SEL equ $-gdt
gdt4: dw 0xFFFF
dw 0          ; (base gets set above)
db 0
db 0x9A       ; present, ring 0, code, non-conforming, readable
db 0          ; byte-granular, 16-bit
db 0

; a data segment descriptor that is 'appropriate' for real mode
; (16-bit, byte-granular, limit=0xFFFF)
REAL_DATA_SEL equ $-gdt
gdt5: dw 0xFFFF
dw 0          ; (base gets set above)
db 0
db 0x92       ; present, ring 0, data, expand-up, writable
db 0          ; byte-granular, 16-bit
db 0

gdt_end: