reactos/freeldr/bootsect/fat32.asm

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; FAT32.ASM
; FAT32 Boot Sector
; Copyright (c) 1998, 2000, 2001, 2002 Brian Palmer
org 7c00h
segment .text
bits 16
start:
jmp short main
nop
OEMName db 'FrLdr1.0'
BytesPerSector dw 512
SectsPerCluster db 0
ReservedSectors dw 32
NumberOfFats db 2
MaxRootEntries dw 0 ; Always zero for FAT32 volumes
TotalSectors dw 0 ; Always zero for FAT32 volumes
MediaDescriptor db 0f8h
SectorsPerFat dw 0 ; Always zero for FAT32 volumes
SectorsPerTrack dw 0
NumberOfHeads dw 0
HiddenSectors dd 0
TotalSectorsBig dd 0
; FAT32 Inserted Info
SectorsPerFatBig dd 0
ExtendedFlags dw 0
FSVersion dw 0
RootDirStartCluster dd 0
FSInfoSector dw 0
BackupBootSector dw 6
Reserved1 times 12 db 0
; End FAT32 Inserted Info
BootDrive db 0
Reserved db 0
ExtendSig db 29h
SerialNumber dd 00000000h
VolumeLabel db 'NO NAME '
FileSystem db 'FAT32 '
main:
xor ax,ax ; Setup segment registers
mov ds,ax ; Make DS correct
mov es,ax ; Make ES correct
mov ss,ax ; Make SS correct
mov bp,7c00h
mov sp,7c00h ; Setup a stack
CheckSectorsPerFat:
cmp WORD [BYTE bp+SectorsPerFat],byte 0x00 ; Check the old 16-bit value of SectorsPerFat
jnz CheckFailed ; If it is non-zero then exit with an error
CheckTotalSectors: ; Check the old 16-bit value of TotalSectors & MaxRootEntries
cmp DWORD [BYTE bp+MaxRootEntries],byte 0x00; by comparing the DWORD at offset MaxRootEntries to zero
jnz CheckFailed ; If it is non-zero then exit with an error
CheckFileSystemVersion:
cmp WORD [BYTE bp+FSVersion],byte 0x00 ; Check the file system version word
jna GetDriveParameters ; It is zero, so continue
CheckFailed:
jmp PrintFileSystemError ; If it is not zero then exit with an error
GetDriveParameters:
mov ax,0800h
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 [BiosCHSDriveSize],eax
LoadExtraBootCode:
; First we have to load our extra boot code at
; sector 14 into memory at [0000:7e00h]
mov eax,0eh
add eax,DWORD [BYTE bp+HiddenSectors] ; Add the number of hidden sectors
mov cx,1
xor bx,bx
mov es,bx ; Read sector to [0000:7e00h]
mov bx,7e00h
call ReadSectors
jmp StartSearch
; Reads logical sectors into [ES:BX]
; EAX has logical sector number to read
; CX has number of sectors to read
ReadSectors:
cmp eax,DWORD [BiosCHSDriveSize] ; Check if they are reading a sector within CHS range
jbe ReadSectorsCHS ; Yes - go to the old CHS routine
ReadSectorsLBA:
pushad ; Save logical sector number & sector count
o32 push byte 0
push eax ; Put 64-bit logical 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
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 PrintDiskError ; If the read failed then abort
add sp,0x10 ; Remove disk address packet from stack
popad ; Restore sector count & logical sector number
inc eax ; Increment sector to read
mov dx,es
add dx,byte 20h ; Increment read buffer for next sector
mov es,dx
loop ReadSectorsLBA ; Read next sector
ret
; Reads logical sectors into [ES:BX]
; EAX has logical sector number to read
; CX has number of sectors to read
ReadSectorsCHS:
pushad
xor edx,edx
movzx ecx,WORD [BYTE bp+SectorsPerTrack]
div ecx ; Divide logical by SectorsPerTrack
inc dl ; Sectors numbering starts at 1 not 0
mov cl,dl ; Sector in CL
mov edx,eax
shr edx,16
div WORD [BYTE bp+NumberOfHeads] ; Divide logical by number of heads
mov dh,dl ; Head in DH
mov dl,[BYTE bp+BootDrive] ; Drive number in DL
mov ch,al ; Cylinder in CX
ror ah,1 ; Low 8 bits of cylinder in CH, high 2 bits
ror ah,1 ; 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 PrintDiskError ; If the read failed then abort
popad
inc eax ; Increment Sector to Read
mov dx,es
add dx,byte 20h ; Increment read buffer for next sector
mov es,dx
loop ReadSectorsCHS ; Read next sector
ret
; Displays a disk error message
; And reboots
PrintDiskError:
mov si,msgDiskError ; Bad boot disk message
call PutChars ; Display it
mov si,msgAnyKey ; Press any key message
call PutChars ; Display it
jmp Reboot
; Displays a file system error message
; And reboots
PrintFileSystemError:
mov si,msgFileSystemError ; FreeLdr not found message
call PutChars ; Display it
mov si,msgAnyKey ; Press any key message
call PutChars ; Display it
Reboot:
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
BiosCHSDriveSize dd 0
msgDiskError db 'Disk error',0dh,0ah,0
msgFileSystemError db 'File system error',0dh,0ah,0
msgAnyKey db 'Press any key to restart',0dh,0ah,0
times 510-($-$$) db 0 ; Pad to 510 bytes
dw 0aa55h ; BootSector signature
; End of bootsector
;
; Now starts the extra boot code that we will store
; at sector 14 on a FAT32 volume
;
; To remain multi-boot compatible with other operating
; systems we must not overwrite anything other than
; the bootsector which means we will have to use
; a different sector like 14 to store our extra boot code
StartSearch:
; Now we must get the first cluster of the root directory
mov eax,DWORD [BYTE bp+RootDirStartCluster]
cmp eax,0ffffff8h ; Check to see if this is the last cluster in the chain
jb ContinueSearch ; If not continue, if so then we didn't find freeldr.sys
jmp PrintFileNotFound
ContinueSearch:
mov bx,800h
mov es,bx ; Read cluster to [0000:8000h]
call ReadCluster ; Read the cluster
; Now we have to find our way through the root directory to
; The OSLOADER.SYS file
xor bx,bx
mov bl,[BYTE bp+SectsPerCluster]
shl bx,4 ; BX = BX * 512 / 32
mov ax,800h ; We loaded at 0800:0000
mov es,ax
xor di,di
mov si,filename
mov cx,11
rep cmpsb ; Compare filenames
jz FoundFile ; If same we found it
dec bx
jnz FindFile
jmp PrintFileNotFound
FindFile:
mov ax,es ; We didn't find it in the previous dir entry
add ax,2 ; So lets move to the next one
mov es,ax ; And search again
xor di,di
mov si,filename
mov cx,11
rep cmpsb ; Compare filenames
jz FoundFile ; If same we found it
dec bx ; Keep searching till we run out of dir entries
jnz FindFile ; Last entry?
; Get the next root dir cluster and try again until we run out of clusters
mov eax,DWORD [BYTE bp+RootDirStartCluster]
call GetFatEntry
mov [BYTE bp+RootDirStartCluster],eax
jmp StartSearch
FoundFile:
; Display "Loading FreeLoader..." message
mov si,msgLoading ; Loading message
call PutChars ; Display it
popad
xor di,di ; ES:DI has dir entry
xor dx,dx
mov ax,WORD [es:di+14h] ; Get start cluster high word
shl eax,16
mov ax,WORD [es:di+1ah] ; Get start cluster low word
CheckStartCluster:
cmp eax,2 ; Check and see if the start cluster starts at cluster 2 or above
jnb CheckEndCluster ; If so then continue
jmp PrintFileSystemError ; If not exit with error
CheckEndCluster:
cmp eax,0ffffff8h ; Check and see if the start cluster is and end of cluster chain indicator
jb InitializeLoadSegment ; If not then continue
jmp PrintFileSystemError ; If so exit with error
InitializeLoadSegment:
mov bx,800h
mov es,bx
LoadFile:
cmp eax,0ffffff8h ; Check to see if this is the last cluster in the chain
jae LoadFileDone ; If so continue, if not then read the next one
push eax
xor bx,bx ; Load ROSLDR starting at 0000:8000h
push es
call ReadCluster
pop es
xor bx,bx
mov bl,[BYTE bp+SectsPerCluster]
shl bx,5 ; BX = BX * 512 / 16
mov ax,es ; Increment the load address by
add ax,bx ; The size of a cluster
mov es,ax
pop eax
push es
call GetFatEntry ; Get the next entry
pop es
jmp LoadFile ; Load the next cluster (if any)
LoadFileDone:
mov dl,[BYTE bp+BootDrive]
xor ax,ax
push ax ; We loaded at 0000:8000
push WORD 8000h ; We will do a far return to 0000:8000h
retf ; Transfer control to ROSLDR
; Returns the FAT entry for a given cluster number
; On entry EAX has cluster number
; On return EAX has FAT entry for that cluster
GetFatEntry:
shl eax,2 ; EAX = EAX * 4 (since FAT32 entries are 4 bytes)
mov ecx,eax ; Save this for later in ECX
xor edx,edx
movzx ebx,WORD [BYTE bp+BytesPerSector]
push ebx
div ebx ; FAT Sector Number = EAX / BytesPerSector
movzx ebx,WORD [BYTE bp+ReservedSectors]
add eax,ebx ; FAT Sector Number += ReservedSectors
mov ebx,DWORD [BYTE bp+HiddenSectors]
add eax,ebx ; FAT Sector Number += HiddenSectors
pop ebx
dec ebx
and ecx,ebx ; FAT Offset Within Sector = ECX % BytesPerSector
; EAX holds logical FAT sector number
; ECX holds FAT entry offset
; Now we have to check the extended flags
; to see which FAT is the active one
; and use it, or if they are mirrored then
; no worries
movzx ebx,WORD [BYTE bp+ExtendedFlags] ; Get extended flags and put into ebx
and bx,0x0f ; Mask off upper 8 bits, now we have active fat in bl
jz LoadFatSector ; If fat is mirrored then skip fat calcs
cmp bl,[BYTE bp+NumberOfFats] ; Compare bl to number of fats
jb GetActiveFatOffset
jmp PrintFileSystemError ; If bl is bigger than numfats exit with error
GetActiveFatOffset:
push eax ; Save logical FAT sector number
mov eax,[BYTE bp+SectorsPerFatBig] ; Get the number of sectors occupied by one fat in eax
mul ebx ; Multiplied by the active FAT index we have in ebx
pop edx ; Get logical FAT sector number
add eax,edx ; Add the current FAT sector offset
LoadFatSector:
push ecx
; EAX holds logical FAT sector number
mov bx,7000h
mov es,bx
xor bx,bx ; We will load it to [7000:0000h]
mov cx,1
call ReadSectors
mov bx,7000h
mov es,bx
pop ecx
mov eax,DWORD [es:ecx] ; Get FAT entry
and eax,0fffffffh ; Mask off reserved bits
ret
; Reads cluster number in EAX into [ES:0000]
ReadCluster:
; StartSector = ((Cluster - 2) * SectorsPerCluster) + ReservedSectors + HiddenSectors;
dec eax
dec eax
xor edx,edx
movzx ebx,BYTE [BYTE bp+SectsPerCluster]
mul ebx
push eax
xor edx,edx
movzx eax,BYTE [BYTE bp+NumberOfFats]
mul DWORD [BYTE bp+SectorsPerFatBig]
movzx ebx,WORD [BYTE bp+ReservedSectors]
add eax,ebx
add eax,DWORD [BYTE bp+HiddenSectors]
pop ebx
add eax,ebx ; EAX now contains the logical sector number of the cluster
xor bx,bx ; We will load it to [ES:0000], ES loaded before function call
movzx cx,BYTE [BYTE bp+SectsPerCluster]
call ReadSectors
ret
; Displays a file not found error message
; And reboots
PrintFileNotFound:
mov si,msgFreeLdr ; FreeLdr not found message
call PutChars ; Display it
mov si,msgAnyKey ; Press any key message
call PutChars ; Display it
jmp Reboot
msgFreeLdr db 'FREELDR.SYS not found',0dh,0ah,0
filename db 'FREELDR SYS'
msgLoading db 'Loading FreeLoader...',0dh,0ah,0
times 1022-($-$$) db 0 ; Pad to 1022 bytes
dw 0aa55h ; BootSector signature