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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 ScottQ removed Winboot.sys stuff, add support for extended int 13; MSliger bootable partitions implemented by scanning the MBR.;; Rev 8.0 ScottQ re-implement winboot.sys dual-boot as JO.SYS dual boot; MSliger;; 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 IO.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);;==============================================================================;REALLY OLD REVISION HISTORY which has no meaning but hey its nostalgic;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 LOADERBIO_SEG EQU 2000H ; Destination segment of ntldrBIO_OFFSET EQU 0 ; Offset of ntldrSECTOR_SIZE EQU 512 ; Sector size in bytesDIR_ENTRY_SIZE EQU SIZE DIR_ENTRY ; Size of directory entry in bytesDSK_PARMS EQU 1EH*4 ; POINTER TO DRIVE PARMSSEC9 EQU 522h ; ADDRESS OF NEW DRIVE PARM TABLE
ROM_DISKRD EQU 2ROM_DISKRDX EQU 42h
; ==========================================================================
;; This little set of directives establishes the address; where we'll jump to once the first sector of ntldr has been; loaded. The first 3 bytes of that sector are used for non-FAT; filesystems, so we must skip over them.;SEGBIOS SEGMENT AT BIO_SEG
ORG 3NTLOAD LABEL BYTE
SEGBIOS ENDS
; ==========================================================================
; Data storage between temp. stack and start of boot sector
pReadClustersO EQU -16pReadClustersS EQU -14pReadSectorsO EQU -12pReadSectorsS EQU -10SectorBase EQU -8
DataSecL EQU -4 ; absolute sector # of first sector in data areaDataSecH EQU -2
; ==========================================================================
CODE SEGMENT ASSUME CS:CODE,DS:NOTHING,ES:NOTHING,SS:NOTHING ORG ORIGIN
Public $START$START Label byte
jmp short MainPartitionType: ;the in-memory copy of this will leave nop ;the partition type in this NOP's place ;so IO.SYS can tell if it needs x13
PartitionTypeOffset = (offset PartitionType - offset $START)
; ==========================================================================
; Start of BPB area of the boot record
OsName DB "MSWIN"OsVersion DB "4.1" ; Windows version number
CoreBpb label byteBytesPerSector DW SECTOR_SIZE ; Size of a physical sectorSecsPerClust DB 8 ; Sectors per allocation unitReservedSecs DW 1 ; Number of reserved sectorsNumFats DB 2 ; Number of fatsNumDirEntries DW 512 ; Number of direc entriesTotalSectors DW 4*17*305-1 ; Number of sectors - number of hidden ; sectors (0 when 32 bit sector number)MediaByte DB 0F8H ; MediaByte byteNumFatSecs DW 8 ; Number of fat sectorsSecPerTrack DW 17 ; Sectors per trackNumHeads DW 4 ; Number of drive heads
HiddenSecs DD 1 ; Number of hidden sectorsBigTotalSecs DD 0 ; 32 bit version of number of sectors
.errnz ($-BytesPerSector) NE SIZE BPB
BootDrv DB 80hCurrentHead DB 0h ; Current HeadExtBootSig DB 41SerialNum DD 0VolumeLabel DB 'NO NAME 'FatId DB 'FAT12 '
.errnz ($-$START) NE SIZE BOOTSEC
; =========================================================================
;; 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+pReadClustersO ASSUME SS:CODE
mov ds,cx assume DS:CODE
;; Set up es for sector reads; mov ax,BIO_SEG mov es,ax ASSUME ES:NOTHING
cld mov BP,ORIGIN
; The system is now prepared for us to begin reading.
; First we read the master boot record (MBR) if this is a; harddisk so we can get our partion type. Types E and C; need extended int13 calls.
cmp [BP].bsDriveNumber,cl ;assert cl == 0 from above jnl failed ;dont check for MBR on floppies!
mov ax,cx ;read sector zero cwd
call DoReadOne ;note! assume cx==0 for DoReadOne jc failed sub bx,(42h - 12 +4) ;bx comes back point it end of sector, ;we want hidden sector field of first ;partition entry.386 mov eax,dword ptr HiddenSecs;get the hidden sectors for usscan: cmp eax,dword ptr es:[bx] ;is it the same as this partition entry?.286 mov dl,byte ptr es:[bx-4] ;put the entry's type in dl
jne short dontgotit ;its our partition, go write it down or dl,2 ;turn C's into E's mov [bp+2],dl ;put our partition type at byte 2's nopdontgotit: add bl,10h ; 8CA - 8DA - 8EA - 8FA - 90A : carry jnc short scan ; we scan 4 partition entries
failed: ;if we failed, leave nop alone; we ;will use old int13 and so will IO.SYS xor cx,cx ;later code needs CX to still be zero
.286
; Next, determine logical sector numbers of the start of the; directory and the start of the data area.;
DirRead: mov AL,[BP].bsBPB.BPB_NumberOfFATs ; Determine sector dir starts on (NumFats) cbw ;
mul [BP].bsBPB.BPB_SectorsPerFAT ; DX:AX (NumFatSecs) add AX,[BP].bsBPB.BPB_HiddenSectors ; (HiddenSecs) adc DX,[BP].bsBPB.BPB_HiddenSectorsHigh add AX,[BP].bsBPB.BPB_ReservedSectors;(ReservedSecs) adc DX,CX;; DX:AX = NumFats * NumFatSecs + ReservedSecs + cSecHid; mov SI,[BP].bsBPB.BPB_RootEntries pusha
mov [BP].DataSecL,AX mov [BP].DataSecH,DX
mov AX,DIR_ENTRY_SIZE ; bytes per directory entry mul SI ; convert to bytes in directory (NumDirEntries)
mov BX,[BP].bsBPB.BPB_BytesPerSector; add in sector size add AX,BX dec AX ; decrement so that we round up div BX ; convert to sector number add [BP].DataSecL,AX ; Start sector # of Data area adc [BP].DataSecH,CX ; popa
DirSector: mov DI,BIO_OFFSET ; address in DI for comparisons
call DoReadOne ;NOTE assumes CX==0!
DiskErrorJump: jc DiskError ; if errors try to recover
DirEntry: cmp byte ptr es:[di],ch ; empty directory entry? je MissingFile ; yes, that's the end pusha mov cl,11 mov si,offset BootFilename
repz cmpsb ; see if the same popa
jz DoLoad ; if so, continue booting
dec SI ; decrement # root entries jz MissingFile ; hmmm, file doesn't exist in rootnext_entry: add DI,DIR_ENTRY_SIZE
cmp DI,BX ; exhausted this root dir sector yet? jb DirEntry ; no, check next entry jmp DirSector ; yes, check next sector
MissingFile: mov al,byte ptr [MSGOFF_NOSYS] ; point to no system file message
;; There has been some recoverable error. Display a message; and wait for a keystroke. 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 cbw ; make 00->0000, FF->FFFF inc ax ; end of sub-message? (0xFF?) jz DisplayWait ; if so, get tail dec ax ; end of message? (0x00?) jz WaitForKey ; if so, get key
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
DiskError: mov al,byte ptr [MSGOFF_IOERROR] jmp short DisplayError
WaitForKey: ;we know ax is zero, thats how we got here int 16h ; get character from keyboard int 19h ; Continue in loop till good disk;; We now load the first sector of ntldr.;; All we have to do is multiply the file's starting cluster; (whose directory entry is at ES:DI-11) by sectors per cluster and; add that to the disk's starting data sector.;; Note that after we're read the sector into 2000:0 the directory sector; will get blown away. So we need to save the starting cluster number.;DoLoad:
mov dx,es:[di].dir_first ; dx = ntldr starting cluster push dx ; save for later mov al,1 ; 1 sector mov bx,BIO_OFFSET ; into 2000:0 call NtldrClusterRead jc DiskError
; =========================================================================;; NTLDR requires the following input conditions:;; BX == starting cluster number of NTLDR; DL == int13 unit number of boot drive; DS:SI -> BPB; DS:DI -> arg structure;; =========================================================================
pop bx ; starting cluster number mov dl,[bp].bsDriveNumber mov si,OFFSET CoreBpb mov di,sp ; we should be at TOS now
mov [bp].pReadClustersO,OFFSET NtldrClusterRead mov [bp].pReadSectorsO,OFFSET NtldrSectorRead mov cx,ds mov [bp].pReadClustersS,cx mov [bp].pReadSectorsS,cx
;; We do something nasty. Ntldr is expecting the readcluster and readsector; routines to do a far return. Since ntldr is now the only guy who will be; reading via those routines, we patch the return instruction from a near; return to a far return.; mov byte ptr DoReadExit,0cbh ; retf jmp FAR PTR NTLOAD ; Crank up NTLDR
;; Sector read routine required by ntldr;; Read al sectors into es:bx starting from sector SectorBase; (SectorBase is logical sector # from start of partition);NtldrSectorRead label near.386 movzx cx,al mov eax,[bp].SectorBase add eax,dword ptr [bp].bsBPB.BPB_HiddenSectors mov edx,eax shr edx,16.286 jmp short DoReadMore
;; Cluster read routine required by ntldr;; Read al sectors into es:bx starting from cluster dx;NtldrClusterRead label near.386 movzx cx,al ; cx = # of sectors to read.286 dec dx dec dx ; adjust for reserved clusters 0 and 1 mov al,[BP].bsBPB.BPB_SectorsPerCluster xor ah,ah mul dx ; DX:AX = starting sector number add ax,[bp].DataSecL ; adjust for FATs, root dir, boot sec. adc dx,[bp].DataSecHDoPush: jmp short DoReadMore
; =========================================================================;; Read disk sector(s).;; Inputs: DX:AX == logical sector #; CL == # sectors (CH == 0); ES:BX == transfer address;; Outputs: DX:AX next logical sector #; CX == 0 (assuming no errors); ES:BX -> byte after last byte of read; Carry set if error, else clear if success;; Preserves: BP, SI, DI;; New Notes: This function will now use extended int 13 if; necessary. The next note is correct for standard int 13;; 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.;; =========================================================================
switchx13: ;setup for x13 read, and switch to old if not needed
push dx ; push ax ; block number push es push bx ; transfer address push 1 ; count of one, because we're looping push 16 ; packet size
xchg AX,CX ; AX -> CX
mov ax,[bp].bsBPB.BPB_SectorsPerTrack ; get sectors/track xchg ax,si ; save for divides
xchg AX,DX ; DX -> AX xor DX,DX ; divide 0:AX div si ; AX = high word of track
xchg AX,CX ; save AX in CX and restore old AX div si ; CX:AX = track, DX = sector
inc DX ; sector is 1-based xchg CX,DX ; CX = sector, DX = high word of track div [BP].bsBPB.BPB_Heads ; AX = cylinder, DX = head
mov DH,DL ; head # < 255
mov CH,AL ; CH = cyl # ror AH,2 ; move bits 8,9 of AX to bits 14,15 ; (the rest has to be zero, since ; cyls cannot be more than 10 bits) or CL,AH ; CL = sector # plus high bits of cyl # mov AX,(ROM_DISKRD SHL 8)+1 ; disk read 1 sector
cmp byte ptr [bp].PartitionTypeOffset,0Eh ;set flag to be tested jne short doio ;use extended calls if E (or C) mov ah,ROM_DISKRDX ;x13, we're ready to rock mov si,sp ; DS:SI -> X13 packetdoio: mov dl,[bp].bsDriveNumber ; DL == drive # int 13h
popa ; throw away packet on stack (8 words) popa ; get real registers back
jc DoReadExit ; disk error
inc ax jnz DoReadNext inc dx
DoReadNext: ; Adjust buffer address add BX,[BP].bsBPB.BPB_BytesPerSector
dec cx jnz DoReadMore clc
DoReadExit:
ret
DoReadOne: inc cx ;assumes cx == 0! to set to 1 sector readDoRead: mov bx,BIO_OFFSETDoReadMore:
pusha.386 db 66h ;push a dword of 0 on the stack push 0 ;hand coded for 386.286 jmp switchx13
;; This string can go anywhere. NT and NT Setup are not picky about it.; Public BootFilename
BootFilename db "NTLDR ";
;; Message table.;; We put English messages here as a placeholder only, so that in case; anyone uses bootfat.h without patching new messages in, things will; still be correct (in English, but at least functional).; 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-5) ORG ORIGIN + SECTOR_SIZE - 5MSGOFF_NOSYS: db OFFSET (MSG_NOSYS - ORIGIN) - 256MSGOFF_IOERROR: db OFFSET (MSG_IOERROR - ORIGIN) - 256MSGOFF_COMMON: db OFFSET (MSG_COMMON - ORIGIN) - 256
.errnz ($-$START) NE (SECTOR_SIZE-2) DB 55h,0AAh ; Boot sector signature
CODE ENDS
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