編寫主引導扇區
在看《x86組合語言從真實模式到保護模式》這本書時,因為我是用fedora進行學習的,所以書上的很多工具是用不了的,只能用另外的方法來建立硬碟了:
nasm mbr.asm
qemu-img create -f qcow disk.img 128M
dd if=mbr of=./disk.img
qemu disk.img
我們知道處理器加電或復位之後,如果硬碟是首選的啟動裝置,那麼,ROM-BIOS將試圖讀取硬碟的0面0道1扇區。傳統上,這就是主引導扇區。
主引導扇區的資料有512位元組,ROM-BIOS程式將他載入到邏輯地址0x0000:0x7c00處,也就是實體地址0x07c00處,然後判斷它是否有效。
一個有效的主引導扇區,其最後兩個字應該是0x55和0xaa。ROM-BIOS程式首先檢測這兩個標誌,如果主引導扇區有效,則跳到0x0000:0x7c00執行。
編寫一個硬碟主引導扇區
eg:
mov ax,0xb800
mov es,ax
mov byte[es:0x00],'L'
mov byte[es:0x01],0x07
mov byte[es:0x02],'a'
mov byte[es:0x03],0x07
mov byte[es:0x04],'b'
mov byte[es:0x05],0x07
mov byte[es:0x06],'e'
mov byte[es:0x07],0x07
mov byte[es:0x08],'l'
mov byte[es:0x09],0x07
mov byte[es:0x0a],' '
mov byte[es:0x0b],0x07
mov byte[es:0x0c],'o'
mov byte[es:0x0d],0x07
mov byte[es:0x0e],'f'
mov byte[es:0x0f],0x07
mov byte[es:0x10],'f'
mov byte[es:0x11],0x07
mov byte[es:0x12],'s'
mov byte[es:0x13],0x07
mov byte[es:0x14],'e'
mov byte[es:0x15],0x07
mov byte[es:0x16],'t'
mov byte[es:0x17],0x07
mov byte[es:0x18],':'
mov byte[es:0x19],0x07
mov ax,number
mov bx,10
mov cx,cs
mov ds,cx
mov dx,0
div bx
mov [0x7c00+number+0x00],dl
xor dx,dx
div bx
mov [0x7c00+number+0x01],dl
xor dx,dx
div bx
mov [0x7c00+number+0x02],dl
xor dx,dx
div bx
mov [0x7c00+number+0x03],dl
xor dx,dx
div bx
mov [0x7c00+number+0x04],dl
mov al,[0x7c00+number+0x04]
add al,0x30
mov [es:0x1a],al
mov byte [es:0x1b],0x04
mov al,[0x7c00+number+0x03]
add al,0x30
mov [es:0x1c],al
mov byte [es:0x1d],0x04
mov al,[0x7c00+number+0x02]
add al,0x30
mov [es:0x1e],al
mov byte [es:0x1f],0x04
mov al,[0x7c00+number+0x01]
add al,0x30
mov [es:0x20],al
mov byte [es:0x21],0x04
mov al,[0x7c00+number+0x00]
add al,0x30
mov [es:0x22],al
mov byte [es:0x23],0x04
mov byte [es:0x24],'D'
mov byte [es:0x25],0x04
infi:
jmp near infi
number db 0,0,0,0,0
times 203 db 0
db 0x55,0xaa