mirror of
https://github.com/reactos/reactos.git
synced 2024-11-07 07:00:19 +00:00
402 lines
14 KiB
NASM
402 lines
14 KiB
NASM
; FAT.ASM
|
|
; FAT12/16 Boot Sector
|
|
; Copyright (c) 1998, 2001, 2002 Brian Palmer
|
|
|
|
|
|
|
|
; This is a FAT12/16 file system boot sector
|
|
; that searches the entire root directory
|
|
; for the file freeldr.sys and loads it into
|
|
; memory.
|
|
;
|
|
; The stack is set to 0000:7BF2 so that the first
|
|
; WORD pushed will be placed at 0000:7BF0
|
|
;
|
|
; The DWORD at 0000:7BFC or BP-04h is the logical
|
|
; sector number of the start of the data area.
|
|
;
|
|
; The DWORD at 0000:7BF8 or BP-08h is the total
|
|
; sector count of the boot drive as reported by
|
|
; the computers bios.
|
|
;
|
|
; The WORD at 0000:7BF6 or BP-0ah is the offset
|
|
; of the ReadSectors function in the boot sector.
|
|
;
|
|
; The WORD at 0000:7BF4 or BP-0ch is the offset
|
|
; of the ReadCluster function in the boot sector.
|
|
;
|
|
; The WORD at 0000:7BF2 or BP-0eh is the offset
|
|
; of the PutChars function in the boot sector.
|
|
;
|
|
; When it locates freeldr.sys on the disk it will
|
|
; load the first sector of the file to 0000:8000
|
|
; With the help of this sector we should be able
|
|
; to load the entire file off the disk, no matter
|
|
; how fragmented it is.
|
|
;
|
|
; We load the entire FAT table into memory at
|
|
; 7000:0000. This improves the speed of floppy disk
|
|
; boots dramatically.
|
|
|
|
|
|
BootSectorStackTop equ 0x7bf2
|
|
DataAreaStartHigh equ 0x2
|
|
DataAreaStartLow equ 0x4
|
|
BiosCHSDriveSizeHigh equ 0x6
|
|
BiosCHSDriveSizeLow equ 0x8
|
|
BiosCHSDriveSize equ 0x8
|
|
ReadSectorsOffset equ 0xa
|
|
ReadClusterOffset equ 0xc
|
|
PutCharsOffset equ 0xe
|
|
|
|
|
|
org 7c00h
|
|
|
|
segment .text
|
|
|
|
bits 16
|
|
|
|
start:
|
|
jmp short main
|
|
nop
|
|
|
|
OEMName db 'FrLdr1.0'
|
|
BytesPerSector dw 512
|
|
SectsPerCluster db 1
|
|
ReservedSectors dw 1
|
|
NumberOfFats db 2
|
|
MaxRootEntries dw 224
|
|
TotalSectors dw 2880
|
|
MediaDescriptor db 0f0h
|
|
SectorsPerFat dw 9
|
|
SectorsPerTrack dw 18
|
|
NumberOfHeads dw 2
|
|
HiddenSectors dd 0
|
|
TotalSectorsBig dd 0
|
|
BootDrive db 0
|
|
Reserved db 0
|
|
ExtendSig db 29h
|
|
SerialNumber dd 00000000h
|
|
VolumeLabel db 'NO NAME '
|
|
FileSystem db 'FAT12 '
|
|
|
|
main:
|
|
xor ax,ax
|
|
mov ss,ax
|
|
mov bp,7c00h
|
|
mov sp,BootSectorStackTop ; Setup a stack
|
|
mov ds,ax ; Make DS correct
|
|
mov es,ax ; Make ES correct
|
|
|
|
|
|
cmp BYTE [BYTE bp+BootDrive],BYTE 0xff ; If they have specified a boot drive then use it
|
|
jne GetDriveParameters
|
|
|
|
mov [BYTE bp+BootDrive],dl ; Save the boot drive
|
|
|
|
|
|
GetDriveParameters:
|
|
mov ah,08h
|
|
mov dl,[BYTE bp+BootDrive] ; Get boot drive in dl
|
|
int 13h ; Request drive parameters from the bios
|
|
jnc CalcDriveSize ; If the call succeeded then calculate the drive size
|
|
|
|
; If we get here then the call to the BIOS failed
|
|
; so just set CHS equal to the maximum addressable
|
|
; size
|
|
mov cx,0ffffh
|
|
mov dh,cl
|
|
|
|
CalcDriveSize:
|
|
; Now that we have the drive geometry
|
|
; lets calculate the drive size
|
|
mov bl,ch ; Put the low 8-bits of the cylinder count into BL
|
|
mov bh,cl ; Put the high 2-bits in BH
|
|
shr bh,6 ; Shift them into position, now BX contains the cylinder count
|
|
and cl,3fh ; Mask off cylinder bits from sector count
|
|
; CL now contains sectors per track and DH contains head count
|
|
movzx eax,dh ; Move the heads into EAX
|
|
movzx ebx,bx ; Move the cylinders into EBX
|
|
movzx ecx,cl ; Move the sectors per track into ECX
|
|
inc eax ; Make it one based because the bios returns it zero based
|
|
inc ebx ; Make the cylinder count one based also
|
|
mul ecx ; Multiply heads with the sectors per track, result in edx:eax
|
|
mul ebx ; Multiply the cylinders with (heads * sectors) [stored in edx:eax already]
|
|
|
|
; We now have the total number of sectors as reported
|
|
; by the bios in eax, so store it in our variable
|
|
mov [BYTE bp-BiosCHSDriveSize],eax
|
|
|
|
|
|
; Now we must find our way to the first sector of the root directory
|
|
xor ax,ax
|
|
xor cx,cx
|
|
mov al,[BYTE bp+NumberOfFats] ; Number of fats
|
|
mul WORD [BYTE bp+SectorsPerFat] ; Times sectors per fat
|
|
add ax,WORD [BYTE bp+HiddenSectors]
|
|
adc dx,WORD [BYTE bp+HiddenSectors+2] ; Add the number of hidden sectors
|
|
add ax,WORD [BYTE bp+ReservedSectors] ; Add the number of reserved sectors
|
|
adc dx,cx ; Add carry bit
|
|
mov WORD [BYTE bp-DataAreaStartLow],ax ; Save the starting sector of the root directory
|
|
mov WORD [BYTE bp-DataAreaStartHigh],dx ; Save it in the first 4 bytes before the boot sector
|
|
mov si,WORD [BYTE bp+MaxRootEntries] ; Get number of root dir entries in SI
|
|
pusha ; Save 32-bit logical start sector of root dir
|
|
; DX:AX now has the number of the starting sector of the root directory
|
|
|
|
; Now calculate the size of the root directory
|
|
xor dx,dx
|
|
mov ax,0020h ; Size of dir entry
|
|
mul si ; Times the number of entries
|
|
mov bx,[BYTE bp+BytesPerSector]
|
|
add ax,bx
|
|
dec ax
|
|
div bx ; Divided by the size of a sector
|
|
; AX now has the number of root directory sectors
|
|
|
|
add [BYTE bp-DataAreaStartLow],ax ; Add the number of sectors of the root directory to our other value
|
|
adc [BYTE bp-DataAreaStartHigh],cx ; Now the first 4 bytes before the boot sector contain the starting sector of the data area
|
|
popa ; Restore root dir logical sector start to DX:AX
|
|
|
|
LoadRootDirSector:
|
|
mov bx,7e0h ; We will load the root directory sector
|
|
mov es,bx ; Right after the boot sector in memory
|
|
xor bx,bx ; We will load it to [0000:7e00h]
|
|
xor cx,cx ; Zero out CX
|
|
inc cx ; Now increment it to 1, we are reading one sector
|
|
xor di,di ; Zero out di
|
|
push es ; Save ES because it will get incremented by 20h
|
|
call ReadSectors ; Read the first sector of the root directory
|
|
pop es ; Restore ES (ES:DI = 07E0:0000)
|
|
|
|
SearchRootDirSector:
|
|
cmp [es:di],ch ; If the first byte of the directory entry is zero then we have
|
|
jz ErrBoot ; reached the end of the directory and FREELDR.SYS is not here so reboot
|
|
pusha ; Save all registers
|
|
mov cl,0xb ; Put 11 in cl (length of filename in directory entry)
|
|
mov si,filename ; Put offset of filename string in DS:SI
|
|
repe cmpsb ; Compare this directory entry against 'FREELDR SYS'
|
|
popa ; Restore all the registers
|
|
jz FoundFreeLoader ; If we found it then jump
|
|
dec si ; SI holds MaxRootEntries, subtract one
|
|
jz ErrBoot ; If we are out of root dir entries then reboot
|
|
add di,BYTE +0x20 ; Increment DI by the size of a directory entry
|
|
cmp di,0200h ; Compare DI to 512 (DI has offset to next dir entry, make sure we haven't gone over one sector)
|
|
jc SearchRootDirSector ; If DI is less than 512 loop again
|
|
jmp short LoadRootDirSector ; Didn't find FREELDR.SYS in this directory sector, try again
|
|
|
|
FoundFreeLoader:
|
|
; We found freeldr.sys on the disk
|
|
; so we need to load the first 512
|
|
; bytes of it to 0000:8000
|
|
; ES:DI has dir entry (ES:DI == 07E0:XXXX)
|
|
mov ax,WORD [es:di+1ah] ; Get start cluster
|
|
push ax ; Save start cluster
|
|
push WORD 800h ; Put 800h on the stack and load it
|
|
pop es ; Into ES so that we load the cluster at 0000:8000
|
|
call ReadCluster ; Read the cluster
|
|
pop ax ; Restore start cluster of FreeLoader
|
|
|
|
; Save the addresses of needed functions so
|
|
; the helper code will know where to call them.
|
|
mov WORD [BYTE bp-ReadSectorsOffset],ReadSectors ; Save the address of ReadSectors
|
|
mov WORD [BYTE bp-ReadClusterOffset],ReadCluster ; Save the address of ReadCluster
|
|
mov WORD [BYTE bp-PutCharsOffset],PutChars ; Save the address of PutChars
|
|
|
|
; Now AX has start cluster of FreeLoader and we
|
|
; have loaded the helper code in the first 512 bytes
|
|
; of FreeLoader to 0000:8000. Now transfer control
|
|
; to the helper code. Skip the first three bytes
|
|
; because they contain a jump instruction to skip
|
|
; over the helper code in the FreeLoader image.
|
|
;jmp 0000:8003h
|
|
jmp 8003h
|
|
|
|
|
|
|
|
|
|
; Displays an error message
|
|
; And reboots
|
|
ErrBoot:
|
|
mov si,msgFreeLdr ; FreeLdr not found message
|
|
call PutChars ; Display it
|
|
|
|
Reboot:
|
|
mov si,msgAnyKey ; Press any key message
|
|
call PutChars ; Display it
|
|
xor ax,ax
|
|
int 16h ; Wait for a keypress
|
|
int 19h ; Reboot
|
|
|
|
PutChars:
|
|
lodsb
|
|
or al,al
|
|
jz short Done
|
|
mov ah,0eh
|
|
mov bx,07h
|
|
int 10h
|
|
jmp short PutChars
|
|
Done:
|
|
retn
|
|
|
|
; Displays a bad boot message
|
|
; And reboots
|
|
BadBoot:
|
|
mov si,msgDiskError ; Bad boot disk message
|
|
call PutChars ; Display it
|
|
|
|
jmp short Reboot
|
|
|
|
|
|
; Reads cluster number in AX into [ES:0000]
|
|
ReadCluster:
|
|
; StartSector = ((Cluster - 2) * SectorsPerCluster) + ReservedSectors + HiddenSectors;
|
|
dec ax ; Adjust start cluster by 2
|
|
dec ax ; Because the data area starts on cluster 2
|
|
xor ch,ch
|
|
mov cl,BYTE [BYTE bp+SectsPerCluster]
|
|
mul cx ; Times sectors per cluster
|
|
add ax,[BYTE bp-DataAreaStartLow] ; Add start of data area
|
|
adc dx,[BYTE bp-DataAreaStartHigh] ; Now we have DX:AX with the logical start sector of OSLOADER.SYS
|
|
xor bx,bx ; We will load it to [ES:0000], ES loaded before function call
|
|
;mov cl,BYTE [BYTE bp+SectsPerCluster]; Sectors per cluster still in CX
|
|
;call ReadSectors
|
|
;ret
|
|
|
|
|
|
|
|
; Reads logical sectors into [ES:BX]
|
|
; DX:AX has logical sector number to read
|
|
; CX has number of sectors to read
|
|
ReadSectors:
|
|
|
|
; We can't just check if the start sector is
|
|
; in the BIOS CHS range. We have to check if
|
|
; the start sector + length is in that range.
|
|
pusha
|
|
dec cx
|
|
add ax,cx
|
|
adc dx,byte 0
|
|
|
|
cmp dx,WORD [BYTE bp-BiosCHSDriveSizeHigh] ; Check if they are reading a sector within CHS range
|
|
ja ReadSectorsLBA ; No - go to the LBA routine
|
|
jb ReadSectorsCHS ; Yes - go to the old CHS routine
|
|
cmp ax,WORD [BYTE bp-BiosCHSDriveSizeLow] ; Check if they are reading a sector within CHS range
|
|
jbe ReadSectorsCHS ; Yes - go to the old CHS routine
|
|
|
|
ReadSectorsLBA:
|
|
popa
|
|
ReadSectorsLBALoop:
|
|
pusha ; Save logical sector number & sector count
|
|
|
|
o32 push byte 0
|
|
push dx ; Put 64-bit logical
|
|
push ax ; block address on stack
|
|
push es ; Put transfer segment on stack
|
|
push bx ; Put transfer offset on stack
|
|
push byte 1 ; Set transfer count to 1 sector
|
|
push byte 0x10 ; Set size of packet to 10h
|
|
mov si,sp ; Setup disk address packet on stack
|
|
|
|
; We are so totally out of space here that I am forced to
|
|
; comment out this very beautifully written piece of code
|
|
; It would have been nice to have had this check...
|
|
;CheckInt13hExtensions: ; Now make sure this computer supports extended reads
|
|
; mov ah,0x41 ; AH = 41h
|
|
; mov bx,0x55aa ; BX = 55AAh
|
|
; mov dl,[BYTE bp+BootDrive] ; DL = drive (80h-FFh)
|
|
; int 13h ; IBM/MS INT 13 Extensions - INSTALLATION CHECK
|
|
; jc PrintDiskError ; CF set on error (extensions not supported)
|
|
; cmp bx,0xaa55 ; BX = AA55h if installed
|
|
; jne PrintDiskError
|
|
; test cl,1 ; CX = API subset support bitmap
|
|
; jz PrintDiskError ; Bit 0, extended disk access functions (AH=42h-44h,47h,48h) supported
|
|
|
|
|
|
; Good, we're here so the computer supports LBA disk access
|
|
; So finish the extended read
|
|
mov dl,[BYTE bp+BootDrive] ; Drive number
|
|
mov ah,42h ; Int 13h, AH = 42h - Extended Read
|
|
int 13h ; Call BIOS
|
|
jc BadBoot ; If the read failed then abort
|
|
|
|
add sp,byte 0x10 ; Remove disk address packet from stack
|
|
|
|
popa ; Restore sector count & logical sector number
|
|
|
|
inc ax ; Increment Sector to Read
|
|
adc dx,byte 0
|
|
|
|
push bx
|
|
mov bx,es
|
|
add bx,byte 20h ; Increment read buffer for next sector
|
|
mov es,bx
|
|
pop bx
|
|
|
|
loop ReadSectorsLBALoop ; Read next sector
|
|
|
|
ret
|
|
|
|
|
|
; Reads logical sectors into [ES:BX]
|
|
; DX:AX has logical sector number to read
|
|
; CX has number of sectors to read
|
|
; CarryFlag set on error
|
|
ReadSectorsCHS:
|
|
popa
|
|
ReadSectorsCHSLoop:
|
|
pusha
|
|
xchg ax,cx
|
|
xchg ax,dx
|
|
xor dx,dx
|
|
div WORD [BYTE bp+SectorsPerTrack]
|
|
xchg ax,cx
|
|
div WORD [BYTE bp+SectorsPerTrack] ; Divide logical by SectorsPerTrack
|
|
inc dx ; Sectors numbering starts at 1 not 0
|
|
xchg cx,dx
|
|
div WORD [BYTE bp+NumberOfHeads] ; Number of heads
|
|
mov dh,dl ; Head to DH, drive to DL
|
|
mov dl,[BYTE bp+BootDrive] ; Drive number
|
|
mov ch,al ; Cylinder in CX
|
|
ror ah,2 ; Low 8 bits of cylinder in CH, high 2 bits
|
|
; in CL shifted to bits 6 & 7
|
|
or cl,ah ; Or with sector number
|
|
mov ax,0201h
|
|
int 13h ; DISK - READ SECTORS INTO MEMORY
|
|
; AL = number of sectors to read, CH = track, CL = sector
|
|
; DH = head, DL = drive, ES:BX -> buffer to fill
|
|
; Return: CF set on error, AH = status (see AH=01h), AL = number of sectors read
|
|
|
|
jc BadBoot
|
|
|
|
popa
|
|
inc ax ;Increment Sector to Read
|
|
jnz NoCarryCHS
|
|
inc dx
|
|
|
|
|
|
NoCarryCHS:
|
|
push bx
|
|
mov bx,es
|
|
add bx,byte 20h
|
|
mov es,bx
|
|
pop bx
|
|
; Increment read buffer for next sector
|
|
loop ReadSectorsCHSLoop ; Read next sector
|
|
|
|
ret
|
|
|
|
|
|
msgDiskError db 'Disk error',0dh,0ah,0
|
|
msgFreeLdr db 'freeldr.sys not found',0dh,0ah,0
|
|
; Sorry, need the space...
|
|
;msgAnyKey db 'Press any key to restart',0dh,0ah,0
|
|
msgAnyKey db 'Press any key',0dh,0ah,0
|
|
filename db 'FREELDR SYS'
|
|
|
|
times 509-($-$$) db 0 ; Pad to 509 bytes
|
|
|
|
BootPartition:
|
|
db 0
|
|
|
|
BootSignature:
|
|
dw 0aa55h ; BootSector signature
|