操作系统的引导(二)

在操作系统引导（一） 一篇中，我们已经完成了MBR程序的编写。通过BOCHS调试，我们可以看到分区引导扇区被读入了07c0：0000处。

 

现在我们要完成分区引导扇区的编写。在分区引导扇区中，最主要的任务就是从FAT16文件系统中加载真正的操作系统装载程序到内存5000：7C00处。和MBR代码一样，这段代码基本参考了Singularity的代码。

 

由于我们已经掌握了基本的FAT16文件系统结构，所以对下面的程序我就不多加解释了。

 

 

;; ----------------------------------------------------------------------------
;;
;;  This boot sector code is adopt from Microsoft Singularity Operating system
;;
;; ----------------------------------------------------------------------------

;;;;;;;;;;;;;;;;;;;;
; Concerns
;  1 - there is no error checking on the int13 calls
;  2 - we assume that the disk is FAT16, and that the parameters in the BPB are correct.

;;;;;;;;;;;;;;;;;;;;
; Constants

BootSecOrigin       EQU     07c00h      ; the BIOS always puts the boot sector here
EndOfClusterMarker  EQU     0FFF8h      ; End of cluster is here or higher

; a little trickery here:  Inside the Boot Sector data structure, there several bytes that we never use.
; whether the real file system uses them or not, we don't need them, and we could use the space for locals.

FirstDataSector     EQU     OemName
RootDirSectors      EQU     OemName+2
RootDirStart        EQU     OemName+4
LBAStart            EQU     SerialNum

;;;;;;;;;;;;;;;;;;;;
; Directives

.model tiny
.686p

;;;;;;;;;;;;;;;;;;;;
; Begin Code segment

_TEXT SEGMENT use16 ; 16-bit code segment
.code
ORG     0h                          ; BIOS puts us at 07c0h:0000

start:

   ; standard boot loader proceedure is to jump over the BPB and BS data structures, then nop
   jmp short bootloader
   nop

;;; Example BPB and BS for Singularity, assuming a 64MB disk.  Installers must NEVER copy these 59 bytes - use what the HD has...

；BIOS参数块，不需要拷贝到磁盘上
BPB:
   OemName         DW      'IS'    ; setting up the first 8 bytes like this lets us address them via the hack above
                   DB      'NGULAR'
   BytesPerSector  DW      512
   SecsPerClust    DB      4
   ReservedSecs    DW      1
   NumFats         DB      2
   RootDirEntries  DW      512
   TotalSectors    DW      0
   MediaByte       DB      0F8H
   NumFatSecs      DW      128
   SecPerTrack     DW      011h
   NumHeads        DW      08h
   HiddenSecs      DD      011h
   BigTotalSecs    DD      1ff87h
   BootDrv         DB      80h
   BSReserved1     DB      0h
   ExtBootSig      DB      41
   SerialNum       DD      0
   VolumeLabel     DB      'SINGULARITY'
   FatId           DB      'FAT16   '

bootloader:
   db      0eah                    ; jmp far
   dw      OFFSET Step1            ; offset
   dw      007c0h                  ; segment
      
Step1:    ; set CS,DS,SS,ES segments to 07c0h, set stack to 07c0h:0400h

;第一步，设置段寄存器值，并设置堆栈指针。
   mov     cx, cs
   mov     ss, cx
   mov     sp, 0400h
   mov     es, cx
   mov     ds, cx
   mov     bp, sp

Step2:    ; Clear the Screen
   mov     ax, 02h
   int     010h

Step3:    ; Save the boot drive
   mov     [BootDrv], dl

Step4:    ; Ensure Int13X is supported.  (It's the only way we'll access the HD)

第四步：检查BIOS是否支持扩展INT13。 因为我们要支持大硬盘，所以扩展INT13是必须的。
   mov     ax, 4100h
   mov     bx, 055aah              ; required parameter
 
   int     13h                     ; check availability of extended int 13 (dl holds BootDrv)

   jc      HWUnsupportedError      ; CF is set on error
   cmp     bx,0aa55h
   jne     HWUnsupportedError      ; BX != 0xaa55 on error
   test    cl,1
   jz      HWUnsupportedError      ; bit 0 off on error

Step5:    ; Find the first FAT16 partition in the MBR and store the starting LBA offset.

第五步，找到第一个FAT16分区，只有在从分区表中，我们才能得到分区的位置
   xor     eax, eax
   mov     bx, 4000h
   mov     es, bx
   mov     cx, 1
   call    ReadDisk                ; load the MBR to 4000:0000

   mov     cx, 4                   ; check up to 4 entries in the partition table
   mov     di, 446                 ; the offset of the table within the MBR

   mov     eax, 00eh               ; 00eh is the signature for a Fat16 LBA-accessible partition
FindFat16Partition:
   cmp     al, es:[di+4]
   je      FoundFat16Partition
   add     di, 16
   dec     cx
   jz      PartitionNotFoundError
   jmp     FindFat16Partition

FoundFat16Partition:
   mov     eax, es:[di+8]
   mov     [LBAStart], eax         ; save the starting lba

Step6:    ; Compute the First Data Sector of the FAT

；计算数据块开始的位置，用我们前面提到的共式
   movzx   eax, [RootDirEntries]
   shl     eax, 5                  ; (assume no overflow)
   movzx   ecx, [BytesPerSector]
   add     eax, ecx
   dec     eax
   div     ecx                     ; (discard remainder)
   mov     [RootDirSectors], ax    ; RootDirSectors = ((RootDirEntries * 32) + (BytesPerSector - 1)) / BytesPerSector

   movzx   eax, [SecsPerClust]
   mul     ecx
   shr     eax, 4
   push    ax                      ; save the cluster size on the stack

   movzx   eax, [NumFats]
   mul     [NumFatSecs]            ; (assume no overflow)
   add     ax, [ReservedSecs]
   mov     [RootDirStart], ax      ; RootDirStart = ReservedSecs + (NumFats * NumFatSecs)

   add     ax, [RootDirSectors]
   mov     [FirstDataSector], ax   ; FirstDataSector = RootDirStart + RootDirSectors

Step7:    ; Search the Root Directory for 'RMOSLDR ', find its starting cluster
；在根目录中查找RMOSLDR文件
   mov     dx, [RootDirSectors]    ; set counter for # sectors in root dir

   mov     ax, 4000h
   mov     es, ax                  ; set up ES for a disk transfer

   movzx   eax, [RootDirStart]
   add     eax, [LBAStart]         ; compute true first sector of root dir

LoadNextSector:
   mov     ecx, 1
   call    ReadDisk                ; read 1 sector (eax has sector #) to es:0000h

   mov     bx, [BytesPerSector]    ; set a counter for how many directory entries we will check (32 bytes per entry)
   xor     di, di                  ; prep for comparison between [ds:si] (search string) and [es:di] (sector data)

CompareDirEntries:
   mov     si, OFFSET Stage2File   ; [ds:si] points to the string we are looking for...
   mov     cx, 11                  ; we are matching an 11-character file name
   repe    cmpsb
   je      FoundMatch
   add     di, cx
   add     di, 21                  ; move to next dir entry, but first make sure we aren't at the last entry!
   sub     bx,32
   jnz     CompareDirEntries

   inc     eax                     ; prep to read next entry
   dec     edx
   jz      FileNotFoundError
   jmp     LoadNextSector

FoundMatch:
                                   ; es:[di] points to 12th byte of the 32-byte
                                   ; directory entry for the 2nd stage boot loader file
   mov     ax, es:[di+15]          ; read the cluster at offset 26

Step8:    ; Load the file whose first cluster is ax to 57c0:0000

；找到了，将其读到57C0：0000
   mov     cx, 057c0h
   mov     es, cx

DoRead:
   call    ReadCluster
   push    es
  
   mov     cx, 04000h              ; use different segment to find next cluster
   mov     es, cx
   call    CalcNextCluster
   pop     cx
   add     cx, [bp-2]
   mov     es, cx
   cmp     ax, EndOfClusterMarker
   jb      DoRead

Step9:    ; Transfer Control to the boot loader file, which we stored at 5000:7c00
；跳转
   movzx   edx, [BootDrv]
   push    edx                     ; SINGLDR will need to know the boot drive #
   pushd   04806h                  ; H6 boot signature = HD, Fat16
      
   db      09Ah                    ; emit a long call to 5000:7c00
   dd      50007c00h
      
   mov     al, "*"
   jmp     PrintError
      
;;;;;;;;;;;;;;;;;;;;
; CalcNextCluster
;
; Inputs:       ax      = current cluster
;               es      = target segment for disk read (es:00)
; Operation:    Read the FAT, lookup the value for the cluster in ax, and return it as ax

CalcNextCluster PROC NEAR
   and     eax, 0FFFFh             ; clear top half of eax
   shl     ax, 1
   xor     dx, dx                  ; clear top half of dividend
   div     [BytesPerSector]
   push    dx                      ; save remainder for later - it is the offset
   add     ax, [ReservedSecs]
   add     eax, [LBAStart]         ; eax has the sector to read

   ; Call ReadDisk for 1 sector to es:0000
   mov     cx, 1
   call    ReadDisk

   ; read cluster# at the offset
   pop     bx                      ; pop the offset
   mov     ax, es:[bx]             ; get the value, put it in ax
   ret
CalcNextCluster ENDP

;;;;;;;;;;;;;;;;;;;;
; ReadCluster
;
; Inputs:       ax      = cluster number to read
;               es      = segment into which we will read (at es:0000)
; Operation:    Read a cluster from the HD, using ReadDisk as a subroutine

ReadCluster PROC NEAR
   pushad

   ; compute the sector that starts the cluster
   and     eax, 0FFFFh             ; clear top half of eax for safety.
   sub     eax, 2
   movzx   ecx, [SecsPerClust]     ; cx = sectors to load (this always read full clusters)
   mul     ecx
   add     ax, [FirstDataSector]
   add     eax, [LBAStart]         ; eax holds the sector to read
   call    ReadDisk

   popad
   ret
ReadCluster ENDP

;;;;;;;;;;;;;;;;;;;;
; ReadDisk
;
; Inputs:       eax     = LBA sector to read
;               cx      = number of sectors to read (limit to 64 or less)
;               es:bx   = destination address (always read to offset 0)
; Assumptions:  1 - assumes request will not cause overflow of es (limit on # sectors)
;               2 - assumes int13 extensions available
;               3 - LBA is limited to 32-bit values.  If the disk is > 2 TB, we have a problem

ReadDisk PROC NEAR
   pushad
   mov     dl, [BootDrv]           ; set the drive
              
   pushd   00
   push    eax                     ; push 64-bit sector address (top half always null)

   push    es
   pushw   00                      ; push transfer address

   push    cx                      ; # sectors
   pushw   0010h                   ; this request packet is 16 bytes
   mov     ah,42h                  ; extended read
   mov     si,sp                   ; ds:si = address of params
   int     13h                     ; perform the read

   add     sp, 10h                 ; clean the stack and return
   popad
   ret
ReadDisk ENDP

;;;;;;;;;;;;;;;;;;;;
; Error Routines (these are jump points that never return)

PartitionNotFoundError:
   mov     al, "P"       
   jmp     PrintError
FileNotFoundError:
   mov     al, "F"
   jmp     PrintError
HWUnsupportedError:
   mov     al, "H"
PrintError:
   mov     bx, 07h                 ; normal attribute
   mov     ah, 0eh                 ; default print 1 char
   mov     cx, 6
   mov     si, offset ErrorMsg
   int     10h
printnext:
   lodsb
   or      al, al
   jz      infloop
   int     10h
   jmp     printnext
infloop:
   jmp     infloop

;;;;;;;;;;;;;;;;;;;;
; String Constants

   ErrorMsg    DB  " Error "
   Stage2File  DB  "RMOSLDR    "   ; spaces are important for FAT file name determination.  Compare all 11 bytes!
   DB  0
      
;;;;;;;;;;;;;;;;;;;;
; Boot Sector Signature

   ORG     510
   DW      0AA55h
  
end start

 

我们可以通过
    ML  /nologo /c /omf /Cp /AT  /Fl"FAT16.lst" /Fo"FAT16.obj" FAT16_BOOT.ASM
    Link16 /nologo /tiny FAT16.obj,FAT16.bin

来生成bin格式的引导扇区代码

 

我们可以写一个程序将第FAT16.bin的0x3e-0x199字节写入硬盘镜像的引导扇区中即可在BOCHS中看到你的RMOSLDR被加载了。祝你好运