09 主引导程序控制权的转移

 

参考

https://www.cnblogs.com/wanmeishenghuo/tag/%E6%93%8D%E4%BD%9C%E7%B3%BB%E7%BB%9F/

https://blog.51cto.com/13475106/category6.html

这一节，我们来真正的读取文件中的内容到内存中，首先来看一下内存布局是什么样的，如下所示：

　　Boot占用了512字节，Fat Table占用了4KB，而真正的文件中的内容，我们把它存在0x9000开始的内存地址处。

　　加载文件内容的过程如下：

　　实验步骤如下：

　　1、在虚拟软盘中创建体积较大的文本文件，使之内容大小超过一个扇区。

　　2、将文件的内容加载到BaseOfLoader地址处。

　　3、打印加载的内容，判断是否加载完全。

org 0x7c00

jmp short start
nop

define:
    BaseOfStack      equ 0x7c00
    BaseOfLoader     equ 0x9000
    RootEntryOffset  equ 19
    RootEntryLength  equ 14
    EntryItemLength  equ 32
    FatEntryOffset   equ 1
    FatEntryLength   equ 9

header:
    BS_OEMName     db "D.T.Soft"
    BPB_BytsPerSec dw 512
    BPB_SecPerClus db 1
    BPB_RsvdSecCnt dw 1
    BPB_NumFATs    db 2
    BPB_RootEntCnt dw 224
    BPB_TotSec16   dw 2880
    BPB_Media      db 0xF0
    BPB_FATSz16    dw 9
    BPB_SecPerTrk  dw 18
    BPB_NumHeads   dw 2
    BPB_HiddSec    dd 0
    BPB_TotSec32   dd 0
    BS_DrvNum      db 0
    BS_Reserved1   db 0
    BS_BootSig     db 0x29
    BS_VolID       dd 0
    BS_VolLab      db "D.T.OS-0.01"
    BS_FileSysType db "FAT12   "

start:
    mov ax, cs
    mov ss, ax
    mov ds, ax
    mov es, ax
    mov sp, BaseOfStack

    mov ax, RootEntryOffset
    mov cx, RootEntryLength
    mov bx, Buf

    call ReadSector

    mov si, Target
    mov cx, TarLen
    mov dx, 0

    call FindEntry

    cmp dx, 0
    jz output

    mov si, bx
    mov di, EntryItem
    mov cx, EntryItemLength

    call MemCpy

    mov ax, FatEntryLength
    mov cx, [BPB_BytsPerSec]
    mul cx
    mov bx, BaseOfLoader
    sub bx, ax

    mov ax, FatEntryOffset
    mov cx, FatEntryLength

    call ReadSector

    mov dx, [EntryItem + 0x1A]
    mov si, BaseOfLoader

loading:
    mov ax, dx
    add ax, 31
    mov cx, 1
    push dx
    push bx
    mov bx, si
    call ReadSector
    pop bx
    pop cx
    call FatVec
    cmp dx, 0xFF7
    jnb output
    add si, 512
    jmp loading

output:
    mov bp, BaseOfLoader
    mov cx, [EntryItem + 0x1C]
    call Print

last:
    hlt
    jmp last


; cx --> index
; bx --> fat table address
;
; return:
;     dx --> fat[index]
FatVec:
    mov ax, cx
    mov cl, 2
    div cl

    push ax

    mov ah, 0
    mov cx, 3
    mul cx
    mov cx, ax

    pop ax

    cmp ah, 0
    jz even
    jmp odd

even:    ; FatVec[j] = ( (Fat[i+1] & 0x0F) << 8 ) | Fat[i];
    mov dx, cx
    add dx, 1
    add dx, bx
    mov bp, dx
    mov dl, byte [bp]
    and dl, 0x0F
    shl dx, 8
    add cx, bx
    mov bp, cx
    or  dl, byte [bp]
    jmp return

odd:     ; FatVec[j+1] = (Fat[i+2] << 4) | ( (Fat[i+1] >> 4) & 0x0F );
    mov dx, cx
    add dx, 2
    add dx, bx
    mov bp, dx
    mov dl, byte [bp]
    mov dh, 0
    shl dx, 4
    add cx, 1
    add cx, bx
    mov bp, cx
    mov cl, byte [bp]
    shr cl, 4
    and cl, 0x0F
    mov ch, 0
    or  dx, cx

return:
    ret

; ds:si --> source
; es:di --> destination
; cx    --> length
MemCpy:

    cmp si, di

    ja btoe

    add si, cx
    add di, cx
    dec si
    dec di

    jmp etob

btoe:
    cmp cx, 0
    jz done
    mov al, [si]
    mov byte [di], al
    inc si
    inc di
    dec cx
    jmp btoe

etob:
    cmp cx, 0
    jz done
    mov al, [si]
    mov byte [di], al
    dec si
    dec di
    dec cx
    jmp etob

done:
    ret

; es:bx --> root entry offset address
; ds:si --> target string
; cx    --> target length
;
; return:
;     (dx !=0 ) ? exist : noexist
;        exist --> bx is the target entry
FindEntry:
    push cx

    mov dx, [BPB_RootEntCnt]
    mov bp, sp

find:
    cmp dx, 0
    jz noexist
    mov di, bx
    mov cx, [bp]
    push si
    call MemCmp
    pop si
    cmp cx, 0
    jz exist
    add bx, 32
    dec dx
    jmp find

exist:
noexist:
    pop cx

    ret

; ds:si --> source
; es:di --> destination
; cx    --> length
;
; return:
;        (cx == 0) ? equal : noequal
MemCmp:

compare:
    cmp cx, 0
    jz equal
    mov al, [si]
    cmp al, byte [di]
    jz goon
    jmp noequal
goon:
    inc si
    inc di
    dec cx
    jmp compare

equal:
noequal:

    ret

; es:bp --> string address
; cx    --> string length
Print:
    mov dx, 0
    mov ax, 0x1301
    mov bx, 0x0007
    int 0x10
    ret

; no parameter
ResetFloppy:

    mov ah, 0x00
    mov dl, [BS_DrvNum]
    int 0x13

    ret

; ax    --> logic sector number
; cx    --> number of sector
; es:bx --> target address
ReadSector:

    call ResetFloppy

    push bx
    push cx

    mov bl, [BPB_SecPerTrk]
    div bl
    mov cl, ah
    add cl, 1
    mov ch, al
    shr ch, 1
    mov dh, al
    and dh, 1
    mov dl, [BS_DrvNum]

    pop ax
    pop bx

    mov ah, 0x02

read:
    int 0x13
    jc read

    ret

MsgStr db  "No LOADER ..."
MsgLen equ ($-MsgStr)
Target db  "LOADER     "
TarLen equ ($-Target)
EntryItem times EntryItemLength db 0x00
Buf:
    times 510-($-$$) db 0x00
    db 0x55, 0xaa

95行跳转到加载文件内容的地址处，96行为文件的长度。运行bochs，可以看到文件内容全部打印出来了，结果如下：

　以上我们加载的文件只是文本内容，下面我们在文件中写入程序，并编译成可执行程序，然后将这个可执行程序文件放到虚拟软盘中，并加载到指定地址，然后跳转到这个地址执行。

　　待加载文件中的内容如下：

org 0x9000

begin:
    mov si, msg

print:
    mov al, [si]
    add si, 1
    cmp al, 0x00
    je end
    mov ah, 0x0E
    mov bx, 0x0F
    int 0x10
    jmp print

end:
    hlt
    jmp end

msg:
    db 0x0a, 0x0a
    db "Hello, D.T.OS!"
    db 0x0a, 0x0a
    db 0x00

为了方便起见，我们修改makefile，如下：

.PHONY : all clean rebuild

BOOT_SRC := boot.asm
BOOT_OUT := boot

LOADER_SRC := loader.asm
LOADER_OUT := loader

IMG := data.img
IMG_PATH := /mnt/hgfs

RM := rm -fr

all : $(IMG) $(BOOT_OUT) $(LOADER_OUT)
	@echo "Build Success ==> D.T.OS!"

$(IMG) :
	bximage $@ -q -fd -size=1.44
	
$(BOOT_OUT) : $(BOOT_SRC)
	nasm $^ -o $@
	dd if=$@ of=$(IMG) bs=512 count=1 conv=notrunc
	
$(LOADER_OUT) : $(LOADER_SRC)
	nasm $^ -o $@
	sudo mount -o loop $(IMG) $(IMG_PATH)
	sudo cp $@ $(IMG_PATH)/$@
	sudo umount $(IMG_PATH)
	
clean :
	$(RM) $(IMG) $(BOOT_OUT) $(LOADER_OUT)
	
rebuild :
	@$(MAKE) clean
	@$(MAKE) all

　　makefile完成编译boot.asm，编译loader.asm，写入boot.bin到第一个扇区，挂载a.img到/mnt/hgfs目录，写入loader可执行文件，卸载a.img等。

　　我们修改boot.asm，程序如下：

org 0x7c00

jmp short start
nop

define:
    BaseOfStack      equ 0x7c00
    BaseOfLoader     equ 0x9000
    RootEntryOffset  equ 19
    RootEntryLength  equ 14
    EntryItemLength  equ 32
    FatEntryOffset   equ 1
    FatEntryLength   equ 9

header:
    BS_OEMName     db "D.T.Soft"
    BPB_BytsPerSec dw 512
    BPB_SecPerClus db 1
    BPB_RsvdSecCnt dw 1
    BPB_NumFATs    db 2
    BPB_RootEntCnt dw 224
    BPB_TotSec16   dw 2880
    BPB_Media      db 0xF0
    BPB_FATSz16    dw 9
    BPB_SecPerTrk  dw 18
    BPB_NumHeads   dw 2
    BPB_HiddSec    dd 0
    BPB_TotSec32   dd 0
    BS_DrvNum      db 0
    BS_Reserved1   db 0
    BS_BootSig     db 0x29
    BS_VolID       dd 0
    BS_VolLab      db "D.T.OS-0.01"
    BS_FileSysType db "FAT12   "

start:
    mov ax, cs
	mov ss, ax
	mov ds, ax
	mov es, ax
	mov sp, BaseOfStack
	
	mov ax, RootEntryOffset
	mov cx, RootEntryLength
	mov bx, Buf
	
	call ReadSector
	
	mov si, Target
	mov cx, TarLen
	mov dx, 0
	
	call FindEntry
	
	cmp dx, 0
	jz output
	
	mov si, bx
	mov di, EntryItem
	mov cx, EntryItemLength
	
	call MemCpy
	
	mov ax, FatEntryLength
	mov cx, [BPB_BytsPerSec]
	mul cx
	mov bx, BaseOfLoader
	sub bx, ax
	
	mov ax, FatEntryOffset
	mov cx, FatEntryLength
	
	call ReadSector
	
	mov dx, [EntryItem + 0x1A]
	mov si, BaseOfLoader
	
loading:
    mov ax, dx
    add ax, 31
    mov cx, 1
    push dx
    push bx
    mov bx, si
    call ReadSector
    pop bx
    pop cx
    call FatVec
    cmp dx, 0xFF7
    jnb BaseOfLoader
    add si, 512
    jmp loading
	
output:	
    mov bp, MsgStr
    mov cx, MsgLen
	call Print
	
last:
    hlt
	jmp last	


; cx --> index
; bx --> fat table address
;
; return:
;     dx --> fat[index]
FatVec:
    mov ax, cx
    mov cl, 2
    div cl
    
    push ax
    
    mov ah, 0
    mov cx, 3
    mul cx
    mov cx, ax
    
    pop ax
    
    cmp ah, 0
    jz even
    jmp odd

even:    ; FatVec[j] = ( (Fat[i+1] & 0x0F) << 8 ) | Fat[i];
    mov dx, cx
    add dx, 1
    add dx, bx
    mov bp, dx
    mov dl, byte [bp]
    and dl, 0x0F
    shl dx, 8
    add cx, bx
    mov bp, cx
    or  dl, byte [bp]
    jmp return
    
odd:     ; FatVec[j+1] = (Fat[i+2] << 4) | ( (Fat[i+1] >> 4) & 0x0F );
    mov dx, cx
    add dx, 2
    add dx, bx
    mov bp, dx
    mov dl, byte [bp]
    mov dh, 0
    shl dx, 4
    add cx, 1
    add cx, bx
    mov bp, cx
    mov cl, byte [bp]
    shr cl, 4
    and cl, 0x0F
    mov ch, 0
    or  dx, cx

return: 
    ret

; ds:si --> source
; es:di --> destination
; cx    --> length
MemCpy:
    
    cmp si, di
    
    ja btoe
    
    add si, cx
    add di, cx
    dec si
    dec di
    
    jmp etob
    
btoe:
    cmp cx, 0
    jz done
    mov al, [si]
    mov byte [di], al
    inc si
    inc di
    dec cx
    jmp btoe
    
etob: 
    cmp cx, 0
    jz done
    mov al, [si]
    mov byte [di], al
    dec si
    dec di
    dec cx
    jmp etob

done:   
    ret

; es:bx --> root entry offset address
; ds:si --> target string
; cx    --> target length
;
; return:
;     (dx !=0 ) ? exist : noexist
;        exist --> bx is the target entry
FindEntry:
    push cx
    
    mov dx, [BPB_RootEntCnt]
    mov bp, sp
    
find:
    cmp dx, 0
    jz noexist
    mov di, bx
    mov cx, [bp]
    push si
    call MemCmp
    pop si
    cmp cx, 0
    jz exist
    add bx, 32
    dec dx
    jmp find

exist:
noexist: 
    pop cx
       
    ret

; ds:si --> source
; es:di --> destination
; cx    --> length
;
; return:
;        (cx == 0) ? equal : noequal
MemCmp:

compare:
    cmp cx, 0
    jz equal
    mov al, [si]
    cmp al, byte [di]
    jz goon
    jmp noequal
goon:
    inc si
    inc di
    dec cx
    jmp compare
    
equal: 
noequal:   

    ret

; es:bp --> string address
; cx    --> string length
Print:
    mov dx, 0
    mov ax, 0x1301
	mov bx, 0x0007
	int 0x10
    ret

; no parameter
ResetFloppy:
    
    mov ah, 0x00
    mov dl, [BS_DrvNum]
    int 0x13
    
    ret

; ax    --> logic sector number
; cx    --> number of sector
; es:bx --> target address
ReadSector:
    
    call ResetFloppy
    
    push bx
    push cx
    
    mov bl, [BPB_SecPerTrk]
    div bl
    mov cl, ah
    add cl, 1
    mov ch, al
    shr ch, 1
    mov dh, al
    and dh, 1
    mov dl, [BS_DrvNum]
    
    pop ax
    pop bx
    
    mov ah, 0x02

read:    
    int 0x13
    jc read
    
    ret

MsgStr db  "No LOADER ..."	
MsgLen equ ($-MsgStr)
Target db  "LOADER     "
TarLen equ ($-Target)
EntryItem times EntryItemLength db 0x00
Buf:
	times 510-($-$$) db 0x00
	db 0x55, 0xaa

第90行改为跳转到文件内容加载地址处，而不再跳转到output。第309行文件的名字也改为LOADER。

　　执行make，然后运行bochs，结果如下：

　　可以看到，文件中的可执行程序被加载并成功执行了，成功打印出字符串。此时的打印是文件中的可执行程序中print函数实现的，而不是我们的加载程序中的Print函数实现的。

　　我们将data.img拷贝到windows下，在真正的机器上试一下，设置如下：

　　启动虚拟机，我们看到了以下结果：

　　我们的加载程序和可执行文件都成功运行了。