我正在编写一个简单的引导程序，其主要任务是加载内核，并将处理器切换到虚拟模式。我的问题是当我打开虚拟模式时，处理器会崩溃。以下是我的代码（部分代码使用了MikeOS）。我使用NASM。
BITS 16
jmp short bootloader_start ; Jump past disk description section
nop ; Pad out before disk description
; ------------------------------------------------------------------
; Disk description table, to make it a valid floppy
; Note: some of these values are hard-coded in the source!
; Values are those used by IBM for 1.44 MB, 3.5" diskette
OEMLabel db "16DOSRUN" ; Disk label
BytesPerSector dw 512 ; Bytes per sector
SectorsPerCluster db 1 ; Sectors per cluster
ReservedForBoot dw 1 ; Reserved sectors for boot record
NumberOfFats db 2 ; Number of copies of the FAT
RootDirEntries dw 224 ; Number of entries in root dir
; (224 * 32 = 7168 = 14 sectors to read)
LogicalSectors dw 2880 ; Number of logical sectors
MediumByte db 0F0h ; Medium descriptor byte
SectorsPerFat dw 9 ; Sectors per FAT
SectorsPerTrack dw 18 ; Sectors per track (36/cylinder)
Sides dw 2 ; Number of sides/heads
HiddenSectors dd 0 ; Number of hidden sectors
LargeSectors dd 0 ; Number of LBA sectors
DriveNo dw 0 ; Drive No: 0
Signature db 41 ; Drive signature: 41 for floppy
VolumeID dd 00000000h ; Volume ID: any number
VolumeLabel db "16DOS "; Volume Label: any 11 chars
FileSystem db "FAT12 " ; File system type: don't change!
; ------------------------------------------------------------------
; Main bootloader code
bootloader_start:
xor ax, ax ; make it zero
mov ds, ax ; DS=0
mov ss, ax ; stack starts at seg 0
mov sp, 0x9c00 ; 2000h past code start,
; making the stack 7.5k in size
;***********HERE I TRY TO SWITCH INTO "UNREAL" MODE***********;
cli ; no interrupts
push ds ; save real mode
lgdt [gdtinfo] ; load gdt register
mov eax, cr0 ; switch to pmode by
or al,1 ; set pmode bit
mov cr0, eax
jmp $+2 ; tell 386/486 to not crash
mov bx, 0x08 ; select descriptor 1
mov ds, bx ; 8h = 1000b
and al,0xFE ; back to realmode
mov cr0, eax ; by toggling bit again
pop ds ; get back old segment
sti
;***********END***********;
mov ax, 07C0h ; Set up 4K of stack space above buffer
add ax, 544 ; 8k buffer = 512 paragraphs + 32 paragraphs (loader)
cli ; Disable interrupts while changing stack
mov ss, ax
mov sp, 4096
sti ; Restore interrupts
mov ax, 07C0h ; Set data segment to where we're loaded
mov ds, ax
; NOTE: A few early BIOSes are reported to improperly set DL
cmp dl, 0
je no_change
mov [bootdev], dl ; Save boot device number
mov ah, 8 ; Get drive parameters
int 13h
jc fatal_disk_error
and cx, 3Fh ; Maximum sector number
mov [SectorsPerTrack], cx ; Sector numbers start at 1
movzx dx, dh ; Maximum head number
add dx, 1 ; Head numbers start at 0 - add 1 for total
mov [Sides], dx
no_change:
mov eax, 0 ; Needed for some older BIOSes
; First, we need to load the root directory from the disk. Technical details:
; Start of root = ReservedForBoot + NumberOfFats * SectorsPerFat = logical 19
; Number of root = RootDirEntries * 32 bytes/entry / 512 bytes/sector = 14
; Start of user data = (start of root) + (number of root) = logical 33
floppy_ok: ; Ready to read first block of data
mov ax, 19 ; Root dir starts at logical sector 19
call l2hts
mov si, buffer ; Set ES:BX to point to our buffer (see end of code)
mov bx, ds
mov es, bx
mov bx, si
mov ah, 2 ; Params for int 13h: read floppy sectors
mov al, 14 ; And read 14 of them
pusha ; Prepare to enter loop
read_root_dir:
popa ; In case registers are altered by int 13h
pusha
stc ; A few BIOSes do not set properly on error
int 13h ; Read sectors using BIOS
jnc search_dir ; If read went OK, skip ahead
call reset_floppy ; Otherwise, reset floppy controller and try again
jnc read_root_dir ; Floppy reset OK?
search_dir:
popa
mov ax, ds ; Root dir is now in [buffer]
mov es, ax ; Set DI to this info
mov di, buffer
mov cx, word [RootDirEntries] ; Search all (224) entries
mov ax, 0 ; Searching at offset 0
next_root_entry:
xchg cx, dx ; We use CX in the inner loop...
mov si, kern_filename ; Start searching for kernel filename
mov cx, 11
rep cmpsb
je found_file_to_load ; Pointer DI will be at offset 11
add ax, 32 ; Bump searched entries by 1 (32 bytes per entry)
mov di, buffer ; Point to next entry
add di, ax
xchg dx, cx ; Get the original CX back
loop next_root_entry
mov si, file_not_found ; If kernel is not found, bail out
call print_string
found_file_to_load: ; Fetch cluster and load FAT into RAM
mov ax, word [es:di+0Fh] ; Offset 11 + 15 = 26, contains 1st cluster
mov word [cluster], ax
mov ax, 1 ; Sector 1 = first sector of first FAT
call l2hts
mov di, buffer ; ES:BX points to our buffer
mov bx, di
mov ah, 2 ; int 13h params: read (FAT) sectors
mov al, 9 ; All 9 sectors of 1st FAT
pusha ; Prepare to enter loop
read_fat:
popa ; In case registers are altered by int 13h
pusha
stc
int 13h ; Read sectors using the BIOS
jnc read_fat_ok ; If read went OK, skip ahead
call reset_floppy ; Otherwise, reset floppy controller and try again
jnc read_fat ; Floppy reset OK?
; ******************************************************************
fatal_disk_error:
; ******************************************************************
mov si, disk_error
read_fat_ok:
popa
mov ax, 2000h ; Segment where we'll load the kernel
mov es, ax
mov bx, 0
mov ah, 2 ; int 13h floppy read params
mov al, 1
push ax ; Save in case we (or int calls) lose it
; Now we must load the FAT from the disk. Here's how we find out where it starts:
; FAT cluster 0 = media descriptor = 0F0h
; FAT cluster 1 = filler cluster = 0FFh
; Cluster start = ((cluster number) - 2) * SectorsPerCluster + (start of user)
; = (cluster number) + 31
load_file_sector:
mov ax, word [cluster] ; Convert sector to logical
add ax, 31
call l2hts ; Make appropriate params for int 13h
mov ax, 2000h ; Set buffer past what we've already read
mov es, ax
mov bx, word [pointer]
pop ax ; Save in case we (or int calls) lose it
push ax
stc
int 13h
jnc calculate_next_cluster ; If there's no error...
call reset_floppy ; Otherwise, reset floppy and retry
jmp load_file_sector
; In the FAT, cluster values are stored in 12 bits, so we have to
; do a bit of maths to work out whether we're dealing with a byte
; and 4 bits of the next byte -- or the last 4 bits of one byte
; and then the subsequent byte!
calculate_next_cluster:
mov ax, [cluster]
mov dx, 0
mov bx, 3
mul bx
mov bx, 2
div bx ; DX = [cluster] mod 2
mov si, buffer
add si, ax ; AX = word in FAT for the 12 bit entry
mov ax, word [ds:si]
or dx, dx ; If DX = 0 [cluster] is even; if DX = 1 then it's odd
jz even ; If [cluster] is even, drop last 4 bits of word
; with next cluster; if odd, drop first 4 bits
odd:
shr ax, 4 ; Shift out first 4 bits (they belong to another entry)
jmp short next_cluster_cont
even:
and ax, 0FFFh ; Mask out final 4 bits
next_cluster_cont:
mov word [cluster], ax ; Store cluster
cmp ax, 0FF8h ; FF8h = end of file marker in FAT12
jae end
add word [pointer], 512 ; Increase buffer pointer 1 sector length
jmp load_file_sector
end: ; We've got the file to load!
pop ax ; Clean up the stack (AX was pushed earlier)
mov dl, byte [bootdev] ; Provide kernel with boot device info
jmp 2000h:0000h ; Jump to entry point of loaded kernel!
; ------------------------------------------------------------------
; BOOTLOADER SUBROUTINES
print_string: ; Output string in SI to screen
pusha
mov ah, 0Eh ; int 10h teletype function
.repeat:
lodsb ; Get char from string
cmp al, 0
je .done ; If char is zero, end of string
int 10h ; Otherwise, print it
jmp short .repeat
.done:
popa
ret
reset_floppy: ; IN: [bootdev] = boot device; OUT: carry set on error
push ax
push dx
mov ax, 0
mov dl, byte [bootdev]
stc
int 13h
pop dx
pop ax
ret
l2hts: ; Calculate head, track and sector settings for int 13h
; IN: logical sector in AX, OUT: correct registers for int 13h
push bx
push ax
mov bx, ax ; Save logical sector
mov dx, 0 ; First the sector
div word [SectorsPerTrack]
add dl, 01h ; Physical sectors start at 1
mov cl, dl ; Sectors belong in CL for int 13h
mov ax, bx
mov dx, 0 ; Now calculate the head
div word [SectorsPerTrack]
mov dx, 0
div word [Sides]
mov dh, dl ; Head/side
mov ch, al ; Track
pop ax
pop bx
mov dl, byte [bootdev] ; Set correct device
ret
; ------------------------------------------------------------------
; STRINGS AND VARIABLES
kern_filename db "KERNEL SYS" ; MikeOS kernel filename
disk_error db "Error.", 0
file_not_found db "Error.", 0
bootdev db 0 ; Boot device number
cluster dw 0 ; Cluster of the file we want to load
pointer dw 0 ; Pointer into Buffer, for loading kernel
gdtinfo:
dw gdt_end - gdt - 1 ;last byte in table
dd gdt ;start of table
gdt dd 0,0 ; entry 0 is always unused
flatdesc db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0
gdt_end:
; ------------------------------------------------------------------
; END OF BOOT SECTOR AND BUFFER START
times 510-($-$$) db 0 ; Pad remainder of boot sector with zeros
dw 0AA55h ; Boot signature (DO NOT CHANGE!)
buffer: ; Disk buffer begins (8k after this, stack starts)
; ==================================================================
那么如何修复这段代码呢？如果在我的情况下切换到“虚幻”模式是不可能的，那么我该如何在实模式下访问整个内存（4GiB会持续），我已经在内核代码中打开了A20。 几年后：发现SmallerC支持进入虚幻模式，因此实际上并不需要所有汇编代码，我可以直接用C语言编写。
-f bin选项（或者根本不指定格式），它将默认原点为 0x0000。在 PC 引导加载程序中，如果你打算将段设置为 0（你的代码确实这样做了），则需要一个原点为 0x7c00。在代码顶部添加ORG 0x7c00指令。 - Michael Petchgdtinfo偏移量错误。 - Michael Petchgdtinfo需要进行修改，以将gdt的地址转换为线性地址（而不是段：偏移）。修订后的代码可以在此处找到：http://www.capp-sysware.com/misc/stackoverflow/40223669/boot.asm - Michael Petch