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Page ,132TITLE BOOT SECTOR 1 OF TRACK 0 - BOOT LOADER;/*; * Microsoft Confidential; * Copyright (C) Microsoft Corporation 1991; * All Rights Reserved.; */
; Rev 1.0 ChrisP, AaronR and others. 2.0 format boot;; Rev 3.0 MarkZ PC/AT enhancements; 2.50 in label; Rev 3.1 MarkZ 3.1 in label due to vagaries of SYSing to IBM drive D's; This resulted in the BPB being off by 1. So we now trust; 2.0 and 3.1 boot sectors and disbelieve 3.0.;; Rev 3.2 LeeAc Modify layout of extended BPB for >32M support; Move PHYDRV to 3rd byte from end of sector; so that it won't have to be moved again; FORMAT and SYS count on PHYDRV being in a known location;; Rev 3.3 D.C.L. Changed Sec 9 EOT field from 15 to 18. May 29, 1986.;; Rev 3.31 MarkT The COUNT value has a bogus check (JBE????) to determine; if we've loaded in all the sectors of IBMBIO. This will; cause too big of a load if the sectors per track is high; enough, causing either a stack overflow or the boot code; to be overwritten.;; Rev 4.00 J. K. For DOS 4.00 Modified to handle the extended BPB, and; 32 bit sector number calculation to enable the primary; partition be started beyond 32 MB boundary.;; Rev 7.0 JeffPar Loads WINBOOT.SYS from anywhere in the root of the boot; drive. WINBOOT.SYS must be an EXE with exactly 1 sector; of header information, followed by a series of binary; images imbedded in the EXE, as follows:;; 1. WINLOAD module (ORGed at 200h, loaded at 70:200h); 2. IO.SYS module; 3. MSDOS.SYS module;; The WINLOAD module should fit within WINLOAD_SIZE sectors,; which includes 1 sector for the EXE header, so the code; and data for the WINLOAD bootstrap should fit in 3 sectors.;; Rev 7.1 ARR Multi sector version of boot record for booting off 32-bit; BigFAT media.;; The ROM in the IBM PC starts the boot process by performing a hardware; initialization and a verification of all external devices. If all goes; well, it will then load from the boot drive the sector from track 0, head 0,; sector 1. This sector is placed at physical address 07C00h. The initial; registers are presumably set up as follows: CS=DS=ES=SS=0, IP=7C00h, and; SP=0400h. But all we rely on is the BIOS being loaded at linear 07C00h.;; If WINBOOT.SYS is not found, an error message is displayed and the user is; prompted to insert another disk. If there is a disk error during the; process, a message is displayed and things are halted.;; At the beginning of the boot sector, there is a table which describes the; MSDOS structure of the media. This is equivalent to the BPB with some; additional information describing the physical layout of the driver (heads,; tracks, sectors);;==============================================================================;REVISION HISTORY:;AN000 - New for DOS Version 4.00 - J.K.;AC000 - Changed for DOS Version 4.00 - J.K.;AN00x - PTM number for DOS Version 4.00 - J.K.;==============================================================================;AN001; d52 Make the fixed positioned variable "CURHD" to be local. 7/6/87 J.K.;AN002; d48 Change head settle at boot time. 7/7/87 J.K.;AN003; P1820 New message SKL file 10/20/87 J.K.;AN004; D304 New structrue of Boot record for OS2. 11/09/87 J.K.;AN005; Changed version to 5.0 03/08/90 E.A.;AN006; Changed to remove MSLOAD in first cluster restriction 04/23/90 J.H.;==============================================================================
.xlist include bpb.inc include bootsec.inc include dirent.inc ;include version.inc .list
; ==========================================================================ORIGIN EQU 7C00H ; Origin of bootstrap LOADER
BIO_SEG EQU 2000H ; Destination segment of BIOSBIO_OFFSET EQU 0000H ; Offset of bios
SECTOR_SIZE EQU 512 ; Sector size in bytesDIR_ENTRY_SIZE EQU SIZE DIR_ENTRY ; Size of directory entry in bytes
ROM_DISKRD EQU 2
SIZEBIGBOOTINSEC EQU 3
; ==========================================================================
SEGBIOS SEGMENT AT BIO_SEG
; Define the destination segment of the BIOS, including the ; initialization label ORG BIO_OFFSETWINLOAD LABEL BYTE
SEGBIOS ENDS
; ==========================================================================
; Local (on stack) Data storage between temp stack and start of ; boot sector
CurrBuffFatSecL EQU -12CurrBuffFatSecH EQU -10Int13Sectors EQU -8DataSecL EQU -4DataSecH EQU -2
; ==========================================================================
CODE SEGMENT ASSUME CS:CODE,DS:NOTHING,ES:NOTHING,SS:NOTHING ORG ORIGIN
Public $START$START Label byte
jmp short MainGotXint13: nop ; used to store xint13 flag
GotXint13Offset = (offset GotXint13 - offset $START)
; ==========================================================================
; Start of BPB area of the boot record
OsName DB "MSWIN"OsVersion DB "4.1" ; Windows version number
BytesPerSector DW SECTOR_SIZE ; Size of a physical sectorSecsPerClust DB 2 ; Sectors per allocation unitReservedSecs DW 8 ; Number of reserved sectorsNumFats DB 1 ; Number of fatsNumDirEntries DW 1024 ; Number of direc entriesTotalSectors DW 0 ; Number of sectors - number of hidden ; sectors (0 when 32 bit sector number)MediaByte DB 0F8H ; MediaByte byteNumFatSecs DW 0 ; Number of fat sectors (0 when 32 bit)SecPerTrack DW 17 ; Sectors per trackNumHeads DW 4 ; Number of drive heads
HiddenSecs DD 1 ; Number of hidden sectorsBigTotalSecs DD 00200000h ; 32 bit version of number of sectorsBigNumFatSecs DD 00001FE0h ; 32 bit version of number of FAT sectorsExtFlags DW 0BPBReserved1 DW 0RootStrtClus DD 0FSInfoSec dw ((FSInfoSecSig - $START) / SECTOR_SIZE)BkUpBootSec dw MBR_BOOTFAILBACKUPBPBReserved2 DD 3 DUP (0) .errnz ($-BytesPerSector) NE SIZE BIGFATBPB
BootDrv DB 80hCurrentHead DB 0h ; Current HeadExtBootSig DB 41SerialNum DD 0VolumeLabel DB 'NO NAME 'FatId DB 'FAT32 '
.errnz ($-$START) NE SIZE BIGFATBOOTSEC
; =========================================================================
;; First thing is to reset the stack to a better and more known; place. The ROM may change, but we'd like to get the stack; in the correct place.;Main: xor CX,CX mov SS,CX ;Work in stack just below this routine mov SP,ORIGIN+CurrBuffFatSecL mov es,cx mov ds,cx ; DS = ES = SS = 0 ASSUME DS:CODE,ES:CODE,SS:CODE mov BP,ORIGIN
;; Determine the number of sectors addressable via; conventional int13. If we can't get drive params for some reason; then something is very wrong -- we'll try to force the caller; to use conventional int13 by maxing out the sector count.; mov [bp].GotXint13Offset,cl ; no xint13 yet mov dl,[bp].bgbsDriveNumber ; int13 unit number mov ah,8 ; get drive params int 13h ; call BIOS jnc @f ; no error, procede mov cx,-1 ; strange case, fake registers to force mov dh,cl ; use of standard int13 (set all vals to max)@@:.386 movzx eax,dh ; eax = max head # (0-255) inc ax ; eax = heads (1-256) movzx edx,cl ; edx = sectors per track + cyl bits and dl,3fh ; edx = sectors per track (1-63) mul dx ; eax = sectors per cylinder, edx = 0 xchg cl,ch shr ch,6 ; cx = max cylinder # (0-1023) inc cx ; cx = cylinders (1-1024) movzx ecx,cx ; ecx = cylinders (1-1024) mul ecx ; eax = sectors visible via int13, edx = 0 mov [bp].Int13Sectors,eax ; save # sectors addressable via int13.8086
;; The MBR (or boot ROM) only reads one boot sector. Thus the first order; of business is to read the rest of ourself in by reading the second; boot sector of the 2-sector boot record.;; The second sector in the NT case is at sector 12. This preserves; the bootsect.dos logic and eliminates a special case for fat32.;ReadBoot: cmp [BP].bgbsBPB.oldBPB.BPB_SectorsPerFAT,0 ; FAT32 BPB? jne short NoSysMsg ; No, invalid, messed up cmp [BP].bgbsBPB.BGBPB_FS_Version,FAT32_Curr_FS_Version ja short NoSysMsg ; boot code too old for this volume.386 mov eax,dword ptr [BP].bgbsBPB.oldBPB.BPB_HiddenSectors add eax,12 ; read in the second boot sector.8086 mov BX,ORIGIN + (SECTOR_SIZE * 2) mov cx,1
call DoRead ; doesn't return if err
jmp DirRead ; no error, continue boot in sector 2
DiskError: mov al,byte ptr [MSGOFF_IOERROR]
;; Al is the offset - 256 of the message within the boot sector.; So we first calculate the real segment-based offset to the message; and stick it in si so lodsb will work later.;DisplayError: .ERRNZ ORIGIN MOD 256
mov ah,(ORIGIN / 256) + 1 mov si,ax
DisplayError1: lodsb ; get next character test AL,AL ; end of message? jz WaitForKey ; yes cmp AL,0FFh ; end of sub-message? je DisplayWait ; yes, switch to final message now mov AH,14 ; write character & attribute mov BX,7 ; attribute (white char on black) int 10h ; print the character jmp short DisplayError1
DisplayWait: mov al,byte ptr [MSGOFF_COMMON] jmp short DisplayError
NoSysMsg: mov al,byte ptr [MSGOFF_NOSYS] ; point to no system file message jmp short DisplayError
WaitForKey: cbw ;warning assumes al is zero! int 16h ; get character from keyboard int 19h ; Continue in loop till good disk
; =========================================================================;; Read disk sector(s). This routine cannot transfer more than 64K!;; Inputs: EAX == physical sector #; CL == # sectors (CH == 0); ES:BX == transfer address;; Outputs: EAX next physical sector #; CX == 0; ES:BX -> byte after last byte of read; Does not return if error;; Reads sectors, switching to extended int13 if necessary and; available. The note below is for the conventional int13 case.;; Notes: Reads sectors one at a time in case they straddle a; track boundary. Performs full 32-bit division on the; first decomposition (of logical sector into track+sector); but not on the second (of track into cylinder+head),; since (A) we don't have room for it, and (B) the results; of that division must yield a quotient < 1024 anyway, because; the CHS-style INT 13h interface can't deal with cylinders; larger than that.;; =========================================================================
DoRead:.386 pushad
;; Determine if the sector we're about to read is available via; conventional int13.; cmp eax,[bp].Int13Sectors ; determine if standard int13 is ok jb stdint13
;; Need extended int13. First set up parameter packet on stack.; Then, if we don't know whether xint13 is available for the drive yet,; find out. If not, error out since we know we can't read the sector; we need.; db 66h ; hand-coded 32-bit push of 8-bit immediate push 0 ; high 32 bits of sector # push eax ; low 32 bits of sector # push es push bx ; transfer address push dword ptr 10010h ; transfer 1 sector, packet size = 16
cmp byte ptr [bp].GotXint13Offset,0 ; have xint13? jnz xint13ok ; yes, do the read mov ah,41h mov bx,055aah mov dl,[bp].bgbsDriveNumber int 13h ; check availability jc xint13err ; error from int13 means no xint13 cmp bx,0aa55h ; absence of sig means no xint13 jne xint13err test cl,1 ; bit 0 off means no xint13 jz xint13err inc byte ptr [bp].GotXint13Offset ; have xint13, remember for next time
xint13ok: mov ah,42h ; extended read mov dl,[bp].bgbsDriveNumber ; dl = int13 unit # mov si,sp ; ds:si -> param packet int 13h ; perform the read db 0b0h ; HACK: avoid stc by making next ; byte part of mov al,xx instructionxint13err: stc ; this instruction MUST follow previous byte! pop eax ; throw away param packet without pop eax ; clobbering carry flag pop eax pop eax
jmp short did_read
stdint13:;; Read via conventional int13; xor edx,edx ; edx:eax = absolute sector number movzx ecx,[bp].bgbsBPB.oldBPB.BPB_SectorsPerTrack ; ecx = sectors per track div ecx ; eax = track, edx = sector within track (0-62) inc dl ; dl = sector within track (1-63) mov cl,dl ; cl = sector within track mov edx,eax shr edx,16 ; dx:ax = track div [bp].bgbsBPB.oldBPB.BPB_Heads ; ax = cylinder (0-1023), dx = head (0-255) xchg dl,dh ; dh = head mov dl,[bp].bgbsDriveNumber ; dl = int13 unit # mov ch,al ; ch = bits 0-7 of cylinder shl ah,6 or cl,ah ; bits 6-7 of cl = bits 8-9 of cylinder mov ax,201h ; read 1 sector int 13h
did_read: popad
jc DiskError
add bx,SECTOR_SIZE ; advance transfer address inc eax ; next sector number dec cx ; loop instruction is out of range, jnz DoRead ; have to do it manually ret.8086
Public WinBoot ; System boot file (11 bytes)
WinBoot DB "NTLDR "
;; Message table.;; We put English messages here as a placeholder only, so that in case; anyone uses bootf32.h without patching new messages in, things will; still be correct (in English, but at least functional).;
.errnz ($-$START) GT 1ACH ORG ORIGIN + 01ACH ; shift message to coincide with that in FAT ; this will help driver to think the MBR ; is empty when dealing with FAT32 superfloppy include msgstub.inc
;; Now build a table with the low byte of the offset to each message.; Code that patches the boot sector messages updates this table.; .errnz ($-$START) GT (SECTOR_SIZE-7) ORG ORIGIN + SECTOR_SIZE - 7MSGOFF_NOSYS: db OFFSET (MSG_NOSYS - ORIGIN) - 256MSGOFF_IOERROR: db OFFSET (MSG_IOERROR - ORIGIN) - 256MSGOFF_COMMON: db OFFSET (MSG_COMMON - ORIGIN) - 256
ORG ORIGIN + (SECTOR_SIZE - 4) DD BOOTSECTRAILSIG ; Boot sector signature (4 bytes)
.errnz ($-$START) NE SECTOR_SIZESecndSecStart label byte;; The second boot sector contains nothing but data. This sector is re-written; by MS-DOS with a fairly high frequency due to changes made to the fsinfo; structure. We don't want the actual boot code to get accidentally corrupted.;FSInfoSecSig label byte .errnz ($-SecndSecStart) NE 0 DD SECONDBOOTSECSIG
db (SECTOR_SIZE - ($-FSInfoSecSig) - 4 - (SIZE BIGFATBOOTFSINFO)) DUP (0)
.errnz ($-SecndSecStart) NE (OFFSETFSINFOFRMSECSTRT)
fsinfo BIGFATBOOTFSINFO <FSINFOSIG,0FFFFFFFFh,000000002h>
.errnz ($-FSInfoSecSig) NE OFFSETTRLSIG DD BOOTSECTRAILSIG ; Boot sector signature (4 bytes)
.errnz ($-$START) NE (SECTOR_SIZE * 2)StrtThirdBootSector LABEL BYTE
DirRead:.386 movzx eax,[BP].bgbsBPB.oldBPB.BPB_NumberOfFATs ; Determine sector dir starts on (NumFats) mov ecx,dword ptr [BP].bgbsBPB.BGBPB_BigSectorsPerFat mul ecx ; EAX = (NumFatSecs) add EAX,dword ptr [BP].bgbsBPB.oldBPB.BPB_HiddenSectors ; (HiddenSecs) movzx edx,[BP].bgbsBPB.oldBPB.BPB_ReservedSectors ;(ReservedSecs) add EAX,EDX;; EAX = NumFats * NumFatSecs + ReservedSecs + cSecHid; (first physical sector of cluster area); mov dword ptr [BP].DataSecL,EAX mov dword ptr [BP].CurrBuffFatSecL,0FFFFFFFFhDirReRead: mov eax,dword ptr [BP].bgbsBPB.BGBPB_RootDirStrtClus cmp eax,2 jb NoSysMsg cmp eax,00FFFFFF8h jae NoSysMsg
; EAX is starting cluster of root directory
DirCluster: push eax ; save starting cluster number sub eax,2 ; Convert to 0 based cluster # movzx EBX,[BP].bgbsBPB.oldBPB.BPB_SectorsPerCluster mov si,bx ; Sector count to SI for sector loop mul EBX ; compute logical sector in EAX add EAX,dword ptr [BP].DataSecL ; Add data start biasDirSector: mov BX,ORIGIN+(SIZEBIGBOOTINSEC*SECTOR_SIZE) mov DI,BX ; save address in DI for comparisons mov CX,1 call DoRead ; doesn't return if error ; relies on return cx=0DirEntry: cmp byte ptr [di],ch ; empty, NUL directory entry? je short MissingFile ; yes, that's the end
mov CL,11 push SI mov si,offset WinBoot repz cmpsb ; see if the same pop SI jz short DoLoad ; if so, continue booting
add DI,CX ; Finish advance to end of name field add DI,DIR_ENTRY_SIZE-11 ; Next dir entry cmp DI,BX ; exhausted this root dir sector yet? jb DirEntry ; no, check next entry dec SI ; decrement # dir sectors jnz DirSector ; More dir sectors in this cluster pop eax ; recover current root dir cluster call GetNextFatEntry jc DirCluster ; Do next Root dir clusterMissingFile: add sp,4 ; Discard EAX saved on stack jmp NoSysMsg
CurrentSegment dw BIO_SEG;; We now load NTLDR;; All we have to do is multiply the file's starting cluster; (whose directory entry is at DS:DI-11) by sectors per cluster and; add that to the disk's starting data sector. We read ntldr into; 2000:0, and begin execution there.;DoLoad:
add sp,4 ; Discard DX:AX saved on stack above mov si,[DI-11].DIR_FIRSTHIGH mov di,[DI-11].DIR_FIRST ; SI:DI = NTLDR starting cluster mov ax,si shl eax,16 mov ax,di ; EAX = NTLDR starting cluster cmp eax,2 ; valid cluster #? jb NoSysMsg ; NO! cmp eax,00FFFFFF8h jae NoSysMsg ; NO!
ReadAcluster: push eax ; save cluster number sub eax,2 ; Subtract first 2 reserved clusters movzx ecx,[BP].bgbsBPB.oldBPB.BPB_SectorsPerCluster ; ECX = Sectors per cluster (SecsPerClust) mul ecx ; EAX = logical sector #
add eax,dword ptr [BP].DataSecL ; EAX = physical sector #
mov bx,BIO_OFFSET push es mov es,CurrentSegment ; ES:BX = destination for read call DoRead ; read all sectors in cluster, doesn't return if error pop es pop eax ; recover current 0-based cluster# shr bx,4 ; updated offset -> paragraphs add CurrentSegment,bx ; update segment for next read
call GetNextFatEntry ; get 2-based successor cluster in EAX jnc StartItUp ; if end of cluster chain reached jc ReadACluster ; keep sucking up clusters.8086
;; NTLDR requires the following input conditions:;; DL = boot drive #;
StartItUp: mov DL,[BP].bgbsDriveNumber jmp FAR PTR WINLOAD ; CRANK UP THE WINDOWS NT BOOT LOADER
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; GetNextFatEntry;; Given the last cluster found, this will return the next cluster; of a chain of clusters. If the last cluster is (ffff)(f)ff8 - (ffff)(f)fff,; then the final cluster has been loaded.;; INPUTS:; EAX = CurrentCluster (0 based cluster #);; OUTPUTS:; EAX = Next cluster (2 based cluster #); Carry CLEAR if all done, SET if not;; USES:; EAX,EBX,ECX,EDX,ESI,DI es;
GetNextFatEntry PROC NEAR
; NOTE For following... FAT32 cluster numbers are 28 bits not 32, ; so we know the following multiply (shl DX:AX by 2) will never ; overflow into carry..386 shl eax,2 call GetFatSector mov EAX,dword ptr ES:[DI+BX] and EAX,0FFFFFFFh ; Mask to valid FAT32 cluster # bits cmp EAX,00FFFFFF8h ; carry CLEAR if all done, SET if not.8086 ret
GetNextFatEntry ENDP
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; GetFatSector;; Read the corresponding FAT sector into the second boot sector;; INPUTS:; EAX == offset (from FAT entry 0) of entry to find;; OUTPUTS:; ES:DI+BX -> corresponding FAT entry in the FAT sector;; USES:; BX,EAX,ECX,EDX,DI;
GetFatSector PROC NEAR.386 mov DI,ORIGIN + SECTOR_SIZE movzx ECX,[BP].bgbsBPB.oldBPB.BPB_BytesPerSector xor edx,edx div ECX ; EAX = Sector number, (E)DX = Offset cmp EAX,dword ptr [BP].CurrBuffFatSecL ; The same fat sector? je short SetRet ; Don't need to read it again mov dword ptr [BP].CurrBuffFatSecL,EAX add EAX,dword ptr [BP].bgbsBPB.oldBPB.BPB_HiddenSectors movzx ecx,[BP].bgbsBPB.oldBPB.BPB_ReservedSectors add eax,ecx ; Point at 1st (0th) FAT movzx ebx,[BP].bgbsBPB.BGBPB_ExtFlags and bx,BGBPB_F_ACTIVEFATMSK jz short GotFatSec cmp bl,[BP].bgbsBPB.oldBPB.BPB_NumberOfFATs jae NoSysMsg push dx ; Save offset of cluster in the FAT sec mov ecx,eax ; Save FAT sector # in 0th FAT mov eax,dword ptr [BP].bgbsBPB.BGBPB_BigSectorsPerFat mul ebx ; EAX = Sector offset to active FAT ; from 0th FAT add eax,ecx pop dxGotFatSec: push dx ; Save offset of cluster in the FAT sec mov BX,DI mov CX,1 call DoRead ; do the disk read, doesn't return if error pop dxSetRet: mov BX,DX ; set BX to the offset of the cluster.8086 ret
GetFatSector ENDP
db ((SECTOR_SIZE - ($-StrtThirdBootSector)) - 4) DUP (0)
.errnz ($-StrtThirdBootSector) NE OFFSETTRLSIG DD BOOTSECTRAILSIG ; Boot sector signature (4 bytes)
.errnz ($-$START) NE (SECTOR_SIZE * 3) .errnz SIZEBIGBOOTINSEC NE 3
$BigEnd label byte
CODE ENDS
END
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