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page ,132;; Microsoft Confidential; Copyright (C) Microsoft Corporation 1983-1997; All Rights Reserved.;;; This is the standard boot record that will be shipped on all hard disks.; It contains:;; 1. Code to load (and give control to) the active boot record for 1 of 4; possible operating systems.;; 2. A partition table at the end of the boot record, followed by the; required signature.;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
VOLBOOT_ORG EQU 7c00hSIZE_SECTOR EQU 512
;; Partition table entry structure and other related stuff;Part_Active EQU 0Part_StartHead EQU 1Part_StartSector EQU 2Part_StartCylinder EQU 3Part_Type EQU 4Part_EndHead EQU 5Part_EndSector EQU 6Part_EndCylinder EQU 7Part_AbsoluteSector EQU 8Part_AbsoluteSectorH EQU 10Part_SectorCount EQU 12Part_SectorCountH EQU 14
SIZE_PART_TAB_ENT EQU 16NUM_PART_TAB_ENTS EQU 4
PART_TAB_OFF EQU (SIZE_SECTOR - 2 - (SIZE_PART_TAB_ENT * NUM_PART_TAB_ENTS))MBR_NT_OFFSET EQU (PART_TAB_OFF - 6)MBR_MSG_TABLE_OFFSET EQU (PART_TAB_OFF - 9)
;; Space we use for temp storage, beyond the end of; the active partition table entry, and thus safe.;UsingBackup EQU SIZE_PART_TAB_ENT
;; Partition types;PART_IFS EQU 07hPART_FAT32 EQU 0bhPART_FAT32_XINT13 EQU 0chPART_XINT13 EQU 0eh
;; Filesystem and pbr stuff;BOOTSECTRAILSIGH EQU 0aa55hFAT32_BACKUP EQU 6
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
RELOCATION_ORG EQU 600h
READ_RETRY_CNT EQU 5
_data segment public assume cs:_data,ds:nothing,es:nothing,ss:nothing
org 100hstart100 label near org RELOCATION_ORG
; Move ourselves so that we can load the partition boot record from; the active partition at 0:VOLBOOT_ORG without trashing our own code.;; WARNING: We are not executing at 0:RELOCATION_ORG until the far; immediate jump, below. This basically means beware of OFFSETs of; any labels until we get there. Until then, we're still executing at; 0:VOLBOOT_ORG.
reloc_delta = 1Bh
start: xor ax,ax mov ss,ax mov sp,VOLBOOT_ORG ; new stack at 0:7c00 sti push ax pop es assume es:_data push ax pop ds assume ds:_data cld mov si,VOLBOOT_ORG + reloc_delta mov di,RELOCATION_ORG + reloc_delta push ax push di mov cx,SIZE_SECTOR - reloc_delta rep movsb ; relocate to 0:0600 retf
; We are now RELOCATED and are now executing at 0:RELOCATION_ORG.;; Find the active partition. Once we find it, we also make sure that the; remaining partitions are INACTIVE, too.
.errnz reloc_delta NE $-start ; make sure reloc_delta is correct
mov bp,offset tab ; partition table mov cl,NUM_PART_TAB_ENTS ; number of table entries (CH==0)
active_loop: cmp [bp].Part_Active,ch ; is this the active entry? jl check_inactive ; yes jne display_bad ; no, it must be "00" or "8x"
add bp,SIZE_PART_TAB_ENT ; yes, go to next entry loop active_loop
int 18h ; no active entries - go to ROM BASIC
; Now make sure the remaining partitions are INACTIVE.check_inactive: mov si,bp ; si = active entryinactive_loop: add si,SIZE_PART_TAB_ENT ; point si to next entry dec cx ; # entries left jz StartLoad ; all entries look ok, start load cmp [si],ch ; all remaining entries should be zero je inactive_loop ; this one is ok
display_bad: mov al,byte ptr [m1] ; partition table is bad
display_msg:;; Al is the offset-256 of the message text. Adjust and; stick in si so lodsb below will work.; .ERRNZ RELOCATION_ORG MOD 256 mov ah,(RELOCATION_ORG / 256) + 1 mov si,ax
display_msg1: lodsb ; get a message character@@: cmp al,0 ; end of string? je @b ; yes, loop infinitely mov bx,7 mov ah,14 int 10h ; and display it jmp display_msg1 ; do the entire message
;; Now attempt to read the sector.;; We store a data byte indicating whether this is the backup; sector, at the byte just beyond the end of the partition table entry.; We know there's nothing there we care about any more, so this; is harmless.;; BP is the address of the active partition entry.; AX and CX are currently zero.;StartLoad: mov byte ptr [bp].UsingBackup,cl ; not backup sector call ReadSector jnc CheckPbrtrybackup: inc byte ptr [bp].UsingBackup
;; Switch to backup sector for NTFS and FAT32.;if 0;; (tedm) for NTFS this code is actually worthless since other code,; such as ntldr and the filesystem itself won't all do the right thing.; For example the filesystem won't recognize the volume if sector 0; can be read but is bad.; cmp byte ptr [bp].Part_Type,PART_IFS jne tryfat32;; We assume here that type 7 is NTFS.; Back up to end of partition by using the end CHS and subtracting 1; from the sector #. There is no check for the case where we underflow; and wrap a head -- it is assumed that partitions are at least head-aligned.; There is also no check for the case where the start sector and sector count; are both maxdword and so adding them overflows.; mov al,[bp].Part_EndHead ; make start head same as end head mov [bp].Part_StartHead,al mov ax,[bp].Part_EndSector ; ax = end sector and cylinder dec al ; start sector = end sector minus 1 mov [bp].Part_StartSector,ax mov ax,[bp].Part_SectorCount mov dx,[bp].Part_SectorCountH add [bp].Part_AbsoluteSector,ax adc [bp].Part_AbsoluteSectorH,dx sub word ptr [bp].Part_AbsoluteSector,1 sbb word ptr [bp].Part_AbsoluteSectorH,0 jmp short RestartLoadendif
tryfat32: cmp byte ptr [bp].Part_Type,PART_FAT32 je fat32backup cmp byte ptr [bp].Part_Type,PART_FAT32_XINT13 je fat32backup mov al,byte ptr [m2] ; unknown fs, so no backup sector jne display_msg
fat32backup:;; There is no check for the case where adding to the sector value; in the start CHS overflows and wraps to the next head. It is assumed; that partitions are at least head-aligned and that hard drives have; at least FAT32_BACKUP+1 sectors per track.; add byte ptr [bp].Part_StartSector,FAT32_BACKUP add word ptr [bp].Part_AbsoluteSector,FAT32_BACKUP adc word ptr [bp].Part_AbsoluteSectorH,0
RestartLoad: call ReadSector jnc CheckPbr mov al,byte ptr [m2] ; can't load, we're done. jmp short display_msg
CheckPbr: cmp word ptr ds:[VOLBOOT_ORG + SIZE_SECTOR - 2],BOOTSECTRAILSIGH je done;; Not a valid filesystem boot sector. Switch to backup if this; isn't already the backup.; cmp byte ptr [bp].UsingBackup,0 je trybackup mov al,byte ptr [m3] jmp short display_msg
;; Jump to PBR. We pass a pointer to the table entry that booted us; in bp. If we used the backup boot sector, then that table entry; will have been altered, but neither NTFS not FAT32 use the pointer; in BP, and no other fs type will have been loaded via the backup; boot sector, so this isn't an issue.;done: mov di,sp ; DI -> start of PBR push ds ; prepare for RETF, which is push di ; smaller than JMP 0:VOLBOOT_ORG mov si,bp ; pass boot partition table entry address retf ; start executing PBR
ReadSector proc near
mov di,5 ; retry count;; Calculate the maximum sector # that can be addressed via; conventional int13. Note that the max # of heads is 256; and the maximum # of sectors per track is 63. Thus the maximum; # of sectors per cylinder is something less than a 16-bit quantity.; mov dl,[bp].Part_Active ; mov ah,8 ; get disk params int 13h jc nonxint13 ; strange case, fall back to std int13
mov al,cl and al,3fh ; al = # of sectors per track cbw ; ax = # of sectors per track (ah=0)
mov bl,dh ; bl = max head # (0-based) mov bh,ah ; bh = 0 inc bx ; bx = # of heads
mul bx ; ax = sectors per cylinder, dx = 0
mov dx,cx ; dx = cylinder/sector in int13 format xchg dl,dh ; dl = low 8 bits of cylinder mov cl,6 shr dh,cl ; dx = max cylinder # (0-based) inc dx ; dx = # cylinders
mul dx ; dx:ax = # sectors visible via int13
;; If the sector # we're reading is less than the than number of; addressable sectors, use conventional int 13; cmp [bp].Part_AbsoluteSectorH,dx ja xint13 jb nonxint13 cmp [bp].Part_AbsoluteSector,ax jae xint13
nonxint13: mov ax,201h mov bx,VOLBOOT_ORG ; es:bx = read address (0:7c00) mov cx,[bp].Part_StartSector mov dx,[bp].Part_Active int 13h jnc endread dec di ; retry? (does not affect carry) jz endread ; carry set for return xor ah,ah ; ah = 0 (reset disk system) mov dl,[bp].Part_Active ; dl = int13 unit # int 13h jmp short nonxint13
xint13:;; We want to avoid calling xint13 unless we know it's available; since we don't trust all BIOSes to not hang if we attempt an xint13 read.; If xint13 isn't supported we won't be able to boot, but at least; we'll give an error message instead of just a black screen.; mov dl,[bp].Part_Active ; unit #.286 pusha mov bx,055AAh ; signature mov ah,41h ; perform X13 interrogation int 13h ; jc endread1 ; call failed cmp bx,0AA55h ; did our signature get flipped? jne endread1 ; no test cl,1 ; is there X13 support? jz endread1 ; no popa
doxint13: pusha ; save regs push 0 ; push dword of 0 push 0 ; (high 32 bits of sector #) push [bp].Part_AbsoluteSectorH push [bp].Part_AbsoluteSector ; low 32 bits of sector # push 0 push VOLBOOT_ORG ; transfer address push 1 ; 1 sector push 16 ; packet size and reserved byte
mov ah,42h ; extended read mov si,sp ; ds:si points to parameter packet int 13h ; dl already set from above
popa ; pop param packet off stack popa ; get real registers back jnc endread dec di ; retry? (does not affect carry) jz endread ; carry set for return xor ah,ah ; ah = 0 (reset disk system) mov dl,[bp].Part_Active ; dl = int13 unit # int 13h jmp short doxint13
endread1: popa stc ; this is the error case.8086endread: retReadSector endp
;; Message table.;; We put English messages here as a placeholder only, so that in case; anyone uses bootmbr.h without patching new messages in, things will; still be correct (in English, but at least functional).;_m1: db "Invalid partition table",0_m2: db "Error loading operating system",0_m3: db "Missing operating system",0
;; 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 MBR_MSG_TABLE_OFFSET org RELOCATION_ORG + MBR_MSG_TABLE_OFFSET
m1: db (OFFSET _m1 - RELOCATION_ORG) - 256m2: db (OFFSET _m2 - RELOCATION_ORG) - 256m3: db (OFFSET _m3 - RELOCATION_ORG) - 256
.errnz ($ - start) NE MBR_NT_OFFSET dd 0 ; NT disk administrator signature dw 0
.errnz ($ - start) GT PART_TAB_OFF
org RELOCATION_ORG + PART_TAB_OFFtab: ;partition table dw 0,0 ;partition 1 begin dw 0,0 ;partition 1 end dw 0,0 ;partition 1 relative sector (low, high parts) dw 0,0 ;partition 1 # of sectors (low, high parts) dw 0,0 ;partition 2 begin dw 0,0 ;partition 2 end dw 0,0 ;partition 2 relative sector dw 0,0 ;partition 2 # of sectors dw 0,0 ;partition 3 begin dw 0,0 ;partition 3 end dw 0,0 ;partition 3 relative sector dw 0,0 ;partition 3 # of sectors dw 0,0 ;partition 4 begin dw 0,0 ;partition 4 end dw 0,0 ;partition 4 relative sector dw 0,0 ;partition 4 # of sectors
.errnz ($ - tab) NE (SIZE_PART_TAB_ENT * NUM_PART_TAB_ENTS) .errnz ($ - start) NE (SIZE_SECTOR - 2)
signa dw BOOTSECTRAILSIGH ;signature
.errnz ($ - start) NE SIZE_SECTOR
_data ends
end start100
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