reactos/freeldr/bootsect/fat.asm

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; 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
BiosCHSDriveSize equ 0x6
BiosCHSDriveSizeHigh equ 0x6
BiosCHSDriveSizeLow 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
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
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 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
; 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]
call ReadSectors
ret
; Displays an error message
; And reboots
ErrBoot:
mov si,msgFreeLdr ; 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
; Displays a bad boot message
; And reboots
BadBoot:
mov si,msgDiskError ; Bad boot disk message
call PutChars ; Display it
mov si,msgAnyKey ; Press any key message
call PutChars ; Display it
jmp Reboot
; Reads logical sectors into [ES:BX]
; DX:AX has logical sector number to read
; CX has number of sectors to read
ReadSectors:
cmp dx,WORD [BYTE bp-BiosCHSDriveSizeHigh]; Check if they are reading a sector within CHS range
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:
pushad ; 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,0x10 ; Remove disk address packet from stack
popad ; Restore sector count & logical sector number
inc ax ; Increment Sector to Read
jnz NoCarry
inc dx
NoCarry:
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]
; DX:AX has logical sector number to read
; CX has number of sectors to read
; CarryFlag set on error
ReadSectorsCHS:
pushad
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,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 BadBoot
popad
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 ReadSectorsCHS ; 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 510-($-$$) db 0 ; Pad to 510 bytes
dw 0aa55h ; BootSector signature