另一种切换到32位保护模式的问题。我目前正在编写一个安全的操作系统（仅限文本）并且已经研究了有关切换模式的教程，但要么我缺少某些内容，要么列出的代码没有详细信息。我有一个简单的启动加载器，它加载此代码然后运行它。然后，此代码将加载内核，并可能切换到64位长模式。代码在mov cr0，eax行出错。我现在真的很难过，所以非常感谢有人看一看。如果它有帮助，我将它作为VirtualBox中的VM运行，并且在调试器中它出现' u32：错误：DBGCCmdHlpVarToDbgfAddr在'0000：00009362 L a'：VERR_INVALID_SELECTOR '失败时我尝试解开。代码中有一些部分借用了其他部分，所以我试图在可以的地方承认这一点。
;;
;; 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
最后几行代码应该在屏幕上写'A'。 此致
Bipman
答案 0 :(得分：0)
感谢Margaret我在移动[BITS 32]时得到了答案，并且还生成了远程跳转作为一系列字节而不是使用32位远跳，这当然不起作用。希望64位长模式不是这个令人困惑的！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 $