Assembly 从引导加载程序进入保护模式时出现问题
此程序集代码从引导加载程序进入受保护模式,但在调用远跳转并重新启动后,无法重置CS段(或执行远跳转)。如果我删除远跳转,它将在保护模式下进入无限循环(0x66,jmp$),无需重新启动Assembly 从引导加载程序进入保护模式时出现问题,assembly,x86,nasm,bootloader,protected-mode,Assembly,X86,Nasm,Bootloader,Protected Mode,此程序集代码从引导加载程序进入受保护模式,但在调用远跳转并重新启动后,无法重置CS段(或执行远跳转)。如果我删除远跳转,它将在保护模式下进入无限循环(0x66,jmp$),无需重新启动 [bits 16] [org 0x7c00] xor ax,ax xor eax,eax add eax,ENTRY_POINT_32 ;address to plug to far jmp mov [ENTRY_OFF],eax xor eax,eax mov eax,GDT ;l
[bits 16]
[org 0x7c00]
xor ax,ax
xor eax,eax
add eax,ENTRY_POINT_32 ;address to plug to far jmp
mov [ENTRY_OFF],eax
xor eax,eax
mov eax,GDT ;load GDT label address
mov [GDTR+2],eax ; load it into address space in GDTR
lgdt [GDTR] ;load GDTR
cli ;turn off masked interrupts
in al,0x70
or al,0x80
out 0x70,al ;turn off nonmasked interrupts
in al,0x92
or al,2
out 0x92, al ;open line A20 (change address 20 to 32 bits)
mov eax,cr0
or al,1
mov cr0,eax ;switch to protected mode
db 0x66 ;prefix of opcode to change bitness
db 0xEA ;opcode of jmp far
ENTRY_OFF dd 0x0 ;32 bit offset of 32 bit instructions
dw 00001000b ; selector 1st descriptor CODE_descr,=1
ENTRY_POINT_32:
db 0x66 ;prefix of opcode to change bitness
jmp $ ;infinite jump to the same location
GDT:
NULL_descr dd 0x0,0x0 ; must be present in GDT
CODE_descr db 0xFF,0xFF,0x0,0x0,0x0,10011010b,11001111b,0x0
;descriptor of 32 bit code segment, base 0, size ffffffff
DATA_descr db 0xFF,0xFF,0x0,0x0,0x0,10010010b,11001111b,0x0
;descriptor of 32 bit data segment, base 0, size ffffffff
VIDEO_descr 0xFF,0xFF,0x0,0x80,0x0B,10010010b,01000000b,0x0
;descriptor of video buffer, base 0x000B8000, size ffff
GDT_size db $-GDT ;size of GDT table
GDTR dw GDT_size-1 ;next 3 words are size &
dd 0x0 ;address of beginning of GDT, loaded in code
times 510 - ($ - $$) db 0
dw 0xaa55
wasm.in的原始代码,稍作修改。在实模式下,所有内存操作数上都有一个隐含段。如果内存操作数不包含BP作为基,则隐含段为DS。如果内存操作数不包含BP,则隐含的基数为SS。内存操作数不使用BP,因此隐含的段是DS。具有如下内存操作数的指令:
mov [ENTRY_POINT_32],eax
bits 16
org 0x7c00
start:
xor ax,ax
mov ds,ax ; Explicitly set DS to zero
lgdt [GDTR] ; load GDTR
cli ; turn off masked interrupts
in al,0x70
or al,0x80
out 0x70,al ; turn off nonmasked interrupts
in al,0x92
or al,2
out 0x92, al ; enable A20 line
; Enter protected mode
mov eax,cr0
or al,1
mov cr0,eax ; switch to protected mode
jmp CODE32_SEL:ENTRY_POINT_32
bits 32
ENTRY_POINT_32:
mov eax, DATA32_SEL ; Set the protected mode selector
mov ds, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov esp, 0x9C000 ; Set protected mode stack below EBDA
mov eax, VIDEO32_SEL ; Set the video memory selector
mov es, ax
; Print some characters to top left of the screen in white on magenta
xor ebx, ebx
mov word [es:ebx], 0x57 << 8 | 'M'
mov word [es:ebx+2], 0x57 << 8 | 'D'
mov word [es:ebx+4], 0x57 << 8 | 'P'
jmp $ ; infinite jump to the same location
GDT:
NULL_descr: dd 0x0,0x0 ; must be first entry in GDT
; descriptor of 32 bit code segment, base 0, size ffffffff
CODE_descr: db 0xFF,0xFF,0x0,0x0,0x0,10011010b,11001111b,0x0
; descriptor of 32 bit data segment, base 0, size ffffffff
DATA_descr: db 0xFF,0xFF,0x0,0x0,0x0,10010010b,11001111b,0x0
VIDEO_descr: db 0xFF,0xFF,0x0,0x80,0x0B,10010010b,01000000b,0x0
; descriptor of video buffer, base 0x000B8000, size ffff
GDT_END:
CODE32_SEL equ CODE_descr-GDT
DATA32_SEL equ DATA_descr-GDT
VIDEO32_SEL equ VIDEO_descr-GDT
GDTR dw GDT_END-GDT-1 ; Size of GDT (minus 1)
dd GDT ; address of beginning of GDT
times 510 - ($ - $$) db 0
dw 0xaa55
相当于:
mov [ds:ENTRY_POINT_32],eax
实模式用于到达物理内存地址。如果DS错误,您将写入错误的内存位置。20位物理地址=(段)您没有设置
DS
以便mov[ENTRY\u OFF],eax
写谁知道在哪里。你应该做一个mov ds,ax
作为零ds
的第二条指令。用一个已知的零操作数使用add
是愚蠢的,而且无论如何,你可以直接将偏移量插入ENTRY\u OFF
。显然,你可以使用一个正常的16位远跳转,你甚至不需要手动创建一个32位的过程中遇到麻烦。CS
是设置的,无论您使用16位还是32位。大小只是控制偏移量,但在这种情况下,16位显然足够了,因为您的入口点\u 32
在前64k之内。是的。每个内存访问都有一个隐式段。请参见表3-5。t中的默认段选择规则基本架构手册。请注意,说明中有8
编码(您认为dw 00001000b
是什么?)还有你的add
中的偏移量。所以你无论如何都需要更新一些东西。如果你忘了你被搞砸了。简单的解决方案不会更糟。见鬼,显然你甚至没有意识到你需要更改00001000b
LOL。通过使所有相关的源代码行彼此靠近。例如,您可以将code32_seg eq 8
放在GDT定义附近。code32_SEL eq code_descr-GDT这将为选择器分配GDT条目之间的字节大小差,而选择器是GDT中条目的线性计数。否?@user145453 GDT由8个描述符条目组成每个字节。选择器是您在保护模式下放入段寄存器的值。每个描述符的开始和GDT的开始之间的差异恰好是选择器值。因此CODE32_SEL将为8(0x08),DATA32_SEL将为16(0x10),VIDEO 32_SEL将为24(0x18)。这样做的好处是,如果您切换每个8字节描述符的顺序,选择器值将自动匹配。感谢您,解释了为什么选择器位是这样定位的。已达到粉色MDP检查点!而不是jmp dword far[bp]
您可以使用o32 retf
将远端地址放在最顶端的堆栈插槽中。@ecm:这是真的,我通常使用jmp而不是ret
技巧有一个微妙的原因,那就是在许多处理器上执行ret时,如果没有相应的调用,可能会招致惩罚,因为它会使空间,我绝对会做o32 retf
。我这样做主要是出于习惯。你会发现我的许多答案在存在选择的地方做了类似的事情。
db 0x66 ;prefix of opcode to change bitness
GDT_size db $-GDT ;size of GDT table
GDTR dw GDT_size-1 ;next 3 words are size &
GDT:
NULL_descr: dd 0x0,0x0 ; must be first entry in GDT
; descriptor of 32 bit code segment, base 0, size ffffffff
CODE_descr: db 0xFF,0xFF,0x0,0x0,0x0,10011010b,11001111b,0x0
; descriptor of 32 bit data segment, base 0, size ffffffff
DATA_descr: db 0xFF,0xFF,0x0,0x0,0x0,10010010b,11001111b,0x0
; descriptor of video buffer, base 0x000B8000, size ffff
VIDEO_descr: db 0xFF,0xFF,0x0,0x80,0x0B,10010010b,01000000b,0x0
GDT_END:
GDTR dw GDT_END-GDT-1 ; Size of GDT (minus 1)
dd 0x0 ; address of beginning of GDT, loaded in code
mov [ENTRY_OFF],eax
jmp clear_prefetch ; Clear the instruction prefetch queue
; by jumping to next instruction
clear_prefetch:
bits 16
org 0x7c00
start:
xor eax,eax
mov ds, ax ; Explicitly set DS to zero
add eax,ENTRY_POINT_32 ; address to plug to far jmp
mov [ENTRY_OFF],eax
jmp clear_prefetch ; Clear the instruction prefetch queue
; by jumping to next instruction
clear_prefetch:
xor eax,eax
mov eax,GDT ; load GDT label address
mov [GDTR+2],eax ; load it into address space in GDTR
lgdt [GDTR] ; load GDTR
cli ; turn off masked interrupts
in al,0x70
or al,0x80
out 0x70,al ; turn off nonmasked interrupts
in al,0x92
or al,2
out 0x92, al ; enable A20 line
mov eax,cr0
or al,1
mov cr0,eax ; switch to protected mode
db 0x66 ; prefix of opcode to change bitness
db 0xEA ; opcode of jmp far
ENTRY_OFF:
dd 0x0 ; 32 bit offset of 32 bit instructions
dw 00001000b ; selector 1st descriptor CODE_descr,=1
bits 32
ENTRY_POINT_32:
jmp $ ; infinite jump to the same location
GDT:
NULL_descr: dd 0x0,0x0 ; must be first entry in GDT
; descriptor of 32 bit code segment, base 0, size ffffffff
CODE_descr: db 0xFF,0xFF,0x0,0x0,0x0,10011010b,11001111b,0x0
; descriptor of 32 bit data segment, base 0, size ffffffff
DATA_descr: db 0xFF,0xFF,0x0,0x0,0x0,10010010b,11001111b,0x0
; descriptor of video buffer, base 0x000B8000, size ffff
VIDEO_descr: db 0xFF,0xFF,0x0,0x80,0x0B,10010010b,01000000b,0x0
GDT_END:
GDTR dw GDT_END-GDT-1 ; Size of GDT (minus 1)
dd 0x0 ; address of beginning of GDT, loaded in code
times 510 - ($ - $$) db 0
dw 0xaa55
bits 16
org 0x7c00
start:
xor ax,ax
mov ds,ax ; Explicitly set DS to zero
lgdt [GDTR] ; load GDTR
cli ; turn off masked interrupts
in al,0x70
or al,0x80
out 0x70,al ; turn off nonmasked interrupts
in al,0x92
or al,2
out 0x92, al ; enable A20 line
; Enter protected mode
mov eax,cr0
or al,1
mov cr0,eax ; switch to protected mode
jmp CODE32_SEL:ENTRY_POINT_32
bits 32
ENTRY_POINT_32:
mov eax, DATA32_SEL ; Set the protected mode selector
mov ds, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov esp, 0x9C000 ; Set protected mode stack below EBDA
mov eax, VIDEO32_SEL ; Set the video memory selector
mov es, ax
; Print some characters to top left of the screen in white on magenta
xor ebx, ebx
mov word [es:ebx], 0x57 << 8 | 'M'
mov word [es:ebx+2], 0x57 << 8 | 'D'
mov word [es:ebx+4], 0x57 << 8 | 'P'
jmp $ ; infinite jump to the same location
GDT:
NULL_descr: dd 0x0,0x0 ; must be first entry in GDT
; descriptor of 32 bit code segment, base 0, size ffffffff
CODE_descr: db 0xFF,0xFF,0x0,0x0,0x0,10011010b,11001111b,0x0
; descriptor of 32 bit data segment, base 0, size ffffffff
DATA_descr: db 0xFF,0xFF,0x0,0x0,0x0,10010010b,11001111b,0x0
VIDEO_descr: db 0xFF,0xFF,0x0,0x80,0x0B,10010010b,01000000b,0x0
; descriptor of video buffer, base 0x000B8000, size ffff
GDT_END:
CODE32_SEL equ CODE_descr-GDT
DATA32_SEL equ DATA_descr-GDT
VIDEO32_SEL equ VIDEO_descr-GDT
GDTR dw GDT_END-GDT-1 ; Size of GDT (minus 1)
dd GDT ; address of beginning of GDT
times 510 - ($ - $$) db 0
dw 0xaa55
; Assemble with NASM as
; nasm -f bin enterpm.asm -o enterpm.com
STACK32_TOP EQU 0x200000
CODE32_REL EQU 0x110000
VIDEOMEM EQU 0x0b8000
use16
; COM program CS=DS=SS
org 100h
call check_pmode ; Check if we are already in protected mode
; This may be the case if we are in a VM8086 task.
; EMM386 and other expanded memory manager often
; run DOS in a VM8086 task. DOS extenders will have
; the same effect
jz not_prot_mode ; If not in protected mode proceed to switch
mov dx, in_pmode_str; otherwise print an error and exit back to DOS
mov ah, 0x9
int 0x21 ; Print Error
ret
not_prot_mode:
call a20_on ; Enable A20 gate (uses Fast method as proof of concept)
cli
; Compute linear address of label gdt_start
; Using (segment << 4) + offset
mov eax,cs ; EAX = CS
shl eax,4 ; EAX = (CS << 4)
mov ebx,eax ; Make a copy of (CS << 4)
add [gdtr+2],eax ; Add base linear address to gdt_start address
; in the gdtr
lgdt [gdtr] ; Load gdt
; Compute linear address of label code_32bit
; Using (segment << 4) + offset
add ebx,code_32bit ; EBX = (CS << 4) + code_32bit
push dword 0x08 ; CS Selector
push ebx ; Linear offset of code_32bit
mov bp, sp ; m16:32 address on top of stack, point BP to it
mov eax,cr0
or eax,1
mov cr0,eax ; Set protected mode flag
jmp dword far [bp] ; Indirect m16:32 FAR jmp with
; m16:32 constructed at top of stack
; DWORD allows us to use a 32-bit offset in 16-bit code
; 16-bit functions that run in real mode
; Check if protected mode is enabled, effectively checking if we are
; in in a VM8086 task. Set ZF to 1 if in protected mode
check_pmode:
smsw ax
test ax, 0x1
ret
; Enable a20 (fast method). This may not work on all hardware
a20_on:
cli
in al, 0x92 ; Read System Control Port A
test al, 0x02 ; Test current a20 value (bit 1)
jnz .skipfa20 ; If already 1 skip a20 enable
or al, 0x02 ; Set a20 bit (bit 1) to 1
and al, 0xfe ; Always write a zero to bit 0 to avoid
; a fast reset into real mode
out 0x92, al ; Enable a20
.skipfa20:
sti
ret
in_pmode_str: db "Processor already in protected mode - exiting",0x0a,0x0d,"$"
align 4
gdtr:
dw gdt_end-gdt_start-1
dd gdt_start
gdt_start:
; First entry is always the Null Descriptor
dd 0
dd 0
gdt_code:
; 4gb flat r/w/executable code descriptor
dw 0xFFFF ; limit low
dw 0 ; base low
db 0 ; base middle
db 0b10011010 ; access
db 0b11001111 ; granularity
db 0 ; base high
gdt_data:
; 4gb flat r/w data descriptor
dw 0xFFFF ; limit low
dw 0 ; base low
db 0 ; base middle
db 0b10010010 ; access
db 0b11001111 ; granularity
db 0 ; base high
gdt_end:
; Code that will run in 32-bit protected mode
; Align code to 4 byte boundary. code_32bit label is
; relative to the origin point 100h
align 4
code_32bit:
use32
; Set virtual memory address of pm code/data to CODE32_REL
; We will be relocating this section from low memory where DOS
; originally loaded it.
section protectedmode vstart=CODE32_REL, valign=4
start_32:
cld ; Direction flag forward
mov eax,0x10 ; 0x10 is flat selector for data
mov ds,eax
mov es,eax
mov fs,eax
mov gs,eax
mov ss,eax
mov esp,STACK32_TOP ; Should set ESP to a usable memory location
; Stack will be grow down from this location
mov edi,start_32 ; EDI = linear address where PM code will be copied
mov esi,ebx ; ESI = linear address of code_32bit
mov ecx,PMSIZE_LONG ; ECX = number of DWORDs to copy
rep movsd ; Copy all code/data from code_32bit to CODE32_REL
jmp 0x08:.relentry ; Absolute jump to relocated code
.relentry:
mov ah, 0x57 ; Attribute white on magenta
; Print a string to display
mov esi,str ; ESI = address of string to print
mov edi,VIDEOMEM ; EDI = base address of video memory
call print_string_attr
cli
endloop:
hlt ; Halt CPU with infinite loop
jmp endloop
print_string_attr:
push ecx
xor ecx,ecx ; ECX = 0 current video offset
jmp .loopentry
.printloop:
mov [edi+ecx*2],ax ; Copy attr and character to display
inc ecx ; Next word position
.loopentry:
mov al,[esi+ecx] ; Get next character to print
test al,al
jnz .printloop ; If it's not NUL continue
.endprint:
pop ecx
ret
str: db "Protected Mode",0
PMSIZE_LONG equ ($-$$+3)>>2
; Number of DWORDS that the protected mode
; code and data takes up (rounded up)