assembly - X86 切换到 32 位保护模式

Question

另一个切换到 32 位保护模式的问题。我目前正在编写一个安全操作系统(仅文本)并研究了有关切换模式的教程，但要么我遗漏了某些内容，要么列出的代码没有所有详细信息。我有一个简单的引导加载程序，它加载这段代码然后运行它。然后这段代码将加载内核并可能切换到 64 位长模式。 mov cr0, eax 行的代码错误。我现在真的很困惑，所以真的很感激有人看一看。如果有帮助，我在 VirtualBox 中将其作为 VM 运行，在调试器中它会出现“u32: error: DBGCCmdHlpVarToDbgfAddr failed on '0000:00009362 L a': VERR_INVALID_SELECTOR”错误，当我尝试拆开。该代码的某些部分是从其他人那里借来的，因此我已尽力承认这一点。

;;
;; enableA20.s (adapted from Visopsys OS-loader)
;;
;; Copyright (c) 2000, J. Andrew McLaughlin
;; You're free to use this code in any manner you like, as long as this
;; notice is included (and you give credit where it is due), and as long
;; as you understand and accept that it comes with NO WARRANTY OF ANY KIND.
;; Contact me at jamesamc@yahoo.com about any bugs or problems.
;;
org 0x9210

enableA20:
;; This subroutine will enable the A20 address line in the keyboard
;; controller.  Takes no arguments.  Returns 0 in EAX on success, 
;; -1 on failure.  Written for use in 16-bit code, see lines marked
;; with 32-BIT for use in 32-bit code.u 9210

[BITS 16]

mov bx,jumper
call prstr

pusha

;; Make sure interrupts are disabled
cli

;; Keep a counter so that we can make up to 5 attempts to turn
;; on A20 if necessary
mov CX, 5

.startAttempt1:     
;; Wait for the controller to be ready for a command
.commandWait1:
xor AX, AX
in AL, 64h
bt AX, 1
jc .commandWait1

;; Tell the controller we want to read the current status.
;; Send the command D0h: read output port.
mov AL, 0D0h
out 64h, AL

;; Wait for the controller to be ready with a byte of data
.dataWait1:
xor AX, AX
in AL, 64h
bt AX, 0
jnc .dataWait1

;; Read the current port status from port 60h
xor AX, AX
in AL, 60h

;; Save the current value of (E)AX
push AX         ; 16-BIT
;; push EAX     ; 32-BIT

;; Wait for the controller to be ready for a command
.commandWait2:
in AL, 64h
bt AX, 1
jc .commandWait2

;; Tell the controller we want to write the status byte again
mov AL, 0D1h
out 64h, AL 

;; Wait for the controller to be ready for the data
.commandWait3:
xor AX, AX
in AL, 64h
bt AX, 1
jc .commandWait3

;; Write the new value to port 60h.  Remember we saved the old
;; value on the stack
pop AX          ; 16-BIT
;; pop EAX      ; 32-BIT

;; Turn on the A20 enable bit
or AL, 00000010b
out 60h, AL

;; Finally, we will attempt to read back the A20 status
;; to ensure it was enabled.

;; Wait for the controller to be ready for a command
.commandWait4:
xor AX, AX
in AL, 64h
bt AX, 1
jc .commandWait4

;; Send the command D0h: read output port.
mov AL, 0D0h
out 64h, AL 

;; Wait for the controller to be ready with a byte of data
.dataWait2:
xor AX, AX
in AL, 64h
bt AX, 0
jnc .dataWait2

;; Read the current port status from port 60h
xor AX, AX
in AL, 60h

;; Is A20 enabled?
bt AX, 1

;; Check the result.  If carry is on, A20 is on.
jc .success

;; Should we retry the operation?  If the counter value in ECX
;; has not reached zero, we will retry
loop .startAttempt1


;; Well, our initial attempt to set A20 has failed.  Now we will
;; try a backup method (which is supposedly not supported on many
;; chipsets, but which seems to be the only method that works on
;; other chipsets).


;; Keep a counter so that we can make up to 5 attempts to turn
;; on A20 if necessary
mov CX, 5

.startAttempt2:
;; Wait for the keyboard to be ready for another command
.commandWait6:
xor AX, AX
in AL, 64h
bt AX, 1
jc .commandWait6

;; Tell the controller we want to turn on A20
mov AL, 0DFh
out 64h, AL

;; Again, we will attempt to read back the A20 status
;; to ensure it was enabled.

;; Wait for the controller to be ready for a command
.commandWait7:
xor AX, AX
in AL, 64h
bt AX, 1
jc .commandWait7

;; Send the command D0h: read output port.
mov AL, 0D0h
out 64h, AL 

;; Wait for the controller to be ready with a byte of data
.dataWait3:
xor AX, AX
in AL, 64h
bt AX, 0
jnc .dataWait3

;; Read the current port status from port 60h
xor AX, AX
in AL, 60h

;; Is A20 enabled?
bt AX, 1

;; Check the result.  If carry is on, A20 is on, but we might warn
;; that we had to use this alternate method
jc .warn

;; Should we retry the operation?  If the counter value in ECX
;; has not reached zero, we will retry
loop .startAttempt2


;; OK, we weren't able to set the A20 address line.  Do you want
;; to put an error message here?
jmp .fail


.warn:
;; Here you may or may not want to print a warning message about
;; the fact that we had to use the nonstandard alternate enabling
;; method
mov bx,a20alt
call prstr
.success:
sti
popa
xor AX, AX
mov bx,a20norm
call prstr
jmp loader

.fail:
sti
popa
mov AX, -1
mov bx,a20err
call prstr
hlt
ret


;
;   Prints string to screen
;
prstr:
push bx
mov ah,0x0E
ploop:
mov al,[bx]             ;load byte of string
cmp al,0                ;if al=0 then string has ended so return
jz fin
mov bh,0
mov bl,7
int 0x10                ;print chr in string
pop bx
inc bx
push bx
jmp ploop               ;back to print next chr
fin:
pop bx                  ;get bx off stack
ret

section .data

a20err:             db 13,10,13,10,"A20 initialisation failed, halting",0
a20alt:             db 13,10,13,10,"Alternate A20 switch",0
a20norm:            db 13,10,13,10,"Normal A20 switch",0
jumper:             db 13,10,13,10,"Jumped to code",0



loader:

;
;jump to 32bit mode next
;
;32 bit mode code and gdt with help from Broken Thorn Website
;OS tutorial by Mike
;

cli             ; clear interrupts
xor ax,ax
mov ds,ax
[BITS 32]

lgdt [toc]          ; load GDT into GDTR
mov eax, cr0        ; set bit 0 in cr0--enter pmode
or  eax, 1
mov cr0, eax
jmp $   
mov ax, DATA_SEG
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax    
mov    esp, 0x090000  ; Set stack to grown downwards from 0x30000
jmp 0x08:clear_prefetch
nop
nop
clear_prefetch:

;;mov ax,cs     ;
;;push ax           ;
;;mov eax,Really32  ;
;;push eax          ;
;;retf          ;

jmp 0x08:Really32


Really32:

;
;test 32 bit mode
;

mov eax,0xb8000
mov word [Eax],0x41
jmp $








;
;GDT table  
; Offset 0 in GDT: Descriptor code=0
;
;First 1gb for code and then the rest for Data
;


gdt_data: 
dd 0                ; null descriptor
dd 0 

; Offset 0x8 bytes from start of GDT: Descriptor code therfore is 8

; gdt code:             ; code descriptor
gdt_code:
dw 0FFFFh           ; limit low
dw 0                ; base low
db 0                    ; base middle
db 10011010b            ; access
db 11001111b            ; granularity = Limit = 2GB-1
db 00               ; base high = Base =1GB

; Offset 16 bytes (0x10) from start of GDT. Descriptor code therfore is 0x10.

; gdt data:             ; data descriptor
gdt_dat:
dw 0FFFFh           ; limit low 
dw 0                ; base low
db 0                ; base middle
db 10010010b            ; access
db 11001111b            ; granularity = limit=rest of memry for data
db 00               ; base high

;...Other descriptors begin at offset 0x18. Remember that each descriptor is 8 bytes in size?
; Add other descriptors for Ring 3 applications, stack, whatever here...

end_of_gdt:
toc: 
dw end_of_gdt - gdt_data    ; limit (Size of GDT)
dd gdt_data             ; base of GDT

CODE_SEG equ gdt_code - gdt_data
DATA_SEG equ gdt_dat - gdt_data

最后几行代码应该将“A”写入屏幕。问候

比普曼

Answer 1

感谢玛格丽特，我得到了移动 [BITS 32] 的答案，并且还生成了一系列字节的远跳转，而不是使用 32 位远跳转，这当然是行不通的。希望 64 位长模式不会这么困惑!代码如下。

lgdt [toc]          ; load GDT into GDTR
mov eax, cr0        ; set bit 0 in cr0--enter pmode
or  eax, 1
mov cr0, eax
jmp clear_prefetch
nop
nop
clear_prefetch:
mov ax, DATA_SEG
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax 
mov    esp, 0x090000  ; Set stack to grown downwards from 0x30000

;;mov ax,cs     ;
;;push ax           ;
;;mov eax,Really32  ;
;;push eax          ;
;;retf          ;

db 0x66
db 0xEA
dd Really32
dw 0x0008


Really32:

;
;test 32 bit mode
;
[BITS 32]

mov eax,0xb8000
mov word [Eax],0x4141
jmp $