reactos/boot/freeldr/bootsect/fat32.S

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/*
* COPYRIGHT: See COPYING in the top level directory
* PROJECT: ReactOS Bootsector
* FILE: boot/freeldr/bootsect/fat32.S
* PURPOSE:
* PROGRAMMERS: Brian Palmer
*/
/* INCLUDES ******************************************************************/
#include <asm.inc>
#include <freeldr/include/arch/pc/x86common.h>
#define BP_REL(x) [bp+x-offset start]
.code16
//ORG HEX(7c00)
start:
jmp short main
nop
OEMName:
.ASCII "FrLdr1.0"
BytesPerSector:
.word 512
SectsPerCluster:
.byte 0
ReservedSectors:
.word 32
NumberOfFats:
.byte 2
MaxRootEntries:
.word 0 // Always zero for FAT32 volumes
TotalSectors:
.word 0 // Always zero for FAT32 volumes
MediaDescriptor:
.byte HEX(0f8)
SectorsPerFat:
.word 0 // Always zero for FAT32 volumes
SectorsPerTrack:
.word 0
NumberOfHeads:
.word 0
HiddenSectors:
.long 0
TotalSectorsBig:
.long 0
// FAT32 Inserted Info
SectorsPerFatBig:
.long 0
ExtendedFlags:
.word 0
FSVersion:
.word 0
RootDirStartCluster:
.long 0
FSInfoSector:
.word 0
BackupBootSector:
.word 6
Reserved1:
.space 12, 0
// End FAT32 Inserted Info
BootDrive:
.byte 0
Reserved:
.byte 0
ExtendSig:
.byte HEX(29)
SerialNumber:
.long 0
VolumeLabel:
.ascii "NO NAME "
FileSystem:
.ascii "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, HEX(7c00)
mov sp, HEX(7c00) // Setup a stack
cmp byte ptr BP_REL(BootDrive), HEX(0ff) // If they have specified a boot drive then use it
jne CheckSectorsPerFat
mov byte ptr BP_REL(BootDrive), dl // Save the boot drive
CheckSectorsPerFat:
cmp word ptr BP_REL(SectorsPerFat), 0 // 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 ptr BP_REL(MaxRootEntries), 0 // by comparing the DWORD at offset MaxRootEntries to zero
jnz CheckFailed // If it is non-zero then exit with an error
CheckFileSystemVersion:
cmp word ptr BP_REL(FSVersion), 0 // 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, HEX(0800)
mov dl, byte ptr BP_REL(BootDrive) // Get boot drive in dl
int HEX(13) // 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, HEX(0ffff)
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, HEX(3f) // 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 dword ptr ds:[BiosCHSDriveSize], eax
LoadExtraBootCode:
// First we have to load our extra boot code at
// sector 14 into memory at [0000:7e00h]
mov eax, HEX(0e)
add eax, dword ptr BP_REL(HiddenSectors) // Add the number of hidden sectors
mov cx, 1
xor bx, bx
mov es, bx // Read sector to [0000:7e00h]
mov bx, HEX(7e00)
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:
push es
cmp eax, dword ptr ds:[BiosCHSDriveSize] // Check if they are reading a sector outside CHS range
jae ReadSectorsLBA // Yes - go to the LBA routine
// If at all possible we want to use LBA routines because
// They are optimized to read more than 1 sector per read
pushad // Save logical sector number & sector count
CheckInt13hExtensions: // Now check if this computer supports extended reads
mov ah, HEX(41) // AH = 41h
mov bx, HEX(55aa) // BX = 55AAh
mov dl, byte ptr BP_REL(BootDrive) // DL = drive (80h-FFh)
int HEX(13) // IBM/MS INT 13 Extensions - INSTALLATION CHECK
jc ReadSectorsCHS // CF set on error (extensions not supported)
cmp bx, HEX(0aa55) // BX = AA55h if installed
jne ReadSectorsCHS
test cl,1 // CX = API subset support bitmap
jz ReadSectorsCHS // Bit 0, extended disk access functions (AH=42h-44h,47h,48h) supported
popad // Restore sector count & logical sector number
ReadSectorsLBA:
pushad // Save logical sector number & sector count
cmp cx, 64 // Since the LBA calls only support 0x7F sectors at a time we will limit ourselves to 64
jbe ReadSectorsSetupDiskAddressPacket // If we are reading less than 65 sectors then just do the read
mov cx, 64 // Otherwise read only 64 sectors on this loop iteration
ReadSectorsSetupDiskAddressPacket:
mov word ptr ds:[LBASectorsRead],cx
push 0
push 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 cx // Set transfer count
push 16 // Set size of packet to 10h
mov si, sp // Setup disk address packet on stack
mov dl, byte ptr BP_REL(BootDrive) // Drive number
mov ah, HEX(42) // Int 13h, AH = 42h - Extended Read
int HEX(13) // Call BIOS
jc PrintDiskError // If the read failed then abort
add sp, 16 // Remove disk address packet from stack
popad // Restore sector count & logical sector number
push bx
mov ebx, dword ptr ds:[LBASectorsRead]
add eax, ebx // Increment sector to read
shl ebx, 5
mov dx, es
add dx, bx // Setup read buffer for next sector
mov es, dx
pop bx
sub cx, word ptr ds:[LBASectorsRead]
jnz ReadSectorsLBA // Read next sector
pop es
ret
LBASectorsRead:
.long 0
// Reads logical sectors into [ES:BX]
// EAX has logical sector number to read
// CX has number of sectors to read
ReadSectorsCHS:
popad // Get logical sector number & sector count off stack
ReadSectorsCHSLoop:
pushad
xor edx, edx
movzx ecx, word ptr BP_REL(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 ptr BP_REL(NumberOfHeads) // Divide logical by number of heads
mov dh, dl // Head in DH
mov dl, byte ptr BP_REL(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, HEX(0201)
int HEX(13) // 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, 32 // Increment read buffer for next sector
mov es, dx
loop ReadSectorsCHSLoop // Read next sector
pop es
ret
// Displays a disk error message
// And reboots
PrintDiskError:
mov si, offset msgDiskError // Bad boot disk message
call PutChars // Display it
jmp Reboot
// Displays a file system error message
// And reboots
PrintFileSystemError:
mov si, offset msgFileSystemError // FreeLdr not found message
call PutChars // Display it
Reboot:
mov si, offset msgAnyKey // Press any key message
call PutChars // Display it
xor ax, ax
int HEX(16) // Wait for a keypress
int HEX(19) // Reboot
PutChars:
lodsb
or al, al
jz short Done
mov ah, HEX(0e)
mov bx, 7
int HEX(10)
jmp short PutChars
Done:
ret
BiosCHSDriveSize:
.long 0
msgDiskError:
.ascii "Disk error", CR, LF, NUL
msgFileSystemError:
.ascii "File system error", CR, LF, NUL
msgAnyKey:
.ascii "Press any key to restart", CR, LF, NUL
.org 509 // Pad to 509 bytes
BootPartition:
.byte 0
BootSignature:
.word HEX(0aa55) // 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 ptr BP_REL(RootDirStartCluster)
cmp eax, HEX(0ffffff8) // 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, HEX(2000)
mov es, bx // Read cluster to [2000:0000h]
call ReadCluster // Read the cluster
// Now we have to find our way through the root directory to
// The FREELDR.SYS file
xor bx,bx
mov bl, byte ptr BP_REL(SectsPerCluster)
shl bx, 4 // BX = BX * 512 / 32
mov ax, HEX(2000) // We loaded at 2000:0000
mov es, ax
xor di, di
mov si, offset filename
mov cx, 11
repe 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, offset filename
mov cx, 11
repe 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 ptr BP_REL(RootDirStartCluster)
call GetFatEntry
mov dword ptr BP_REL(RootDirStartCluster), eax
jmp StartSearch
FoundFile:
// Display "Loading FreeLoader..." message
mov si, offset msgLoading // Loading message
call PutChars // Display it
xor di, di // ES:DI has dir entry
xor dx, dx
mov ax, word ptr es:[di+20] // Get start cluster high word
shl eax, 16
mov ax, word ptr es:[di+26] // 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, HEX(0ffffff8) // 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, FREELDR_BASE / 16
mov es, bx
LoadFile:
cmp eax, HEX(0ffffff8) // 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:F800h
push es
call ReadCluster
pop es
xor bx, bx
mov bl, byte ptr BP_REL(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 ptr BP_REL(BootDrive) // Load boot drive into DL
mov dh, byte ptr ds:[BootPartition] // Load boot partition into DH
/* Transfer execution to the bootloader */
ljmp16 0, FREELDR_BASE
// 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 ptr BP_REL(BytesPerSector)
push ebx
div ebx // FAT Sector Number = EAX / BytesPerSector
movzx ebx, word ptr BP_REL(ReservedSectors)
add eax, ebx // FAT Sector Number += ReservedSectors
mov ebx, dword ptr BP_REL(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 ptr BP_REL(ExtendedFlags) // Get extended flags and put into ebx
and bx, HEX(0f) // 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 ptr BP_REL(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, dword ptr BP_REL(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
mov bx, HEX(9000) // We will load it to [9000:0000h]
mov es, bx
// EAX holds logical FAT sector number
// Check if we have already loaded it
cmp eax, dword ptr ds:[FatSectorInCache]
je LoadFatSectorAlreadyLoaded
mov dword ptr ds:[FatSectorInCache], eax
xor bx, bx
mov cx, 1
call ReadSectors
LoadFatSectorAlreadyLoaded:
pop ecx
mov eax, dword ptr es:[ecx] // Get FAT entry
and eax, HEX(0fffffff) // Mask off reserved bits
ret
FatSectorInCache: // This variable tells us which sector we currently have in memory
.long HEX(0ffffffff) // There is no need to re-read the same sector if we don't have to
// 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 ptr BP_REL(SectsPerCluster)
mul ebx
push eax
xor edx, edx
movzx eax, byte ptr BP_REL(NumberOfFats)
mul dword ptr BP_REL(SectorsPerFatBig)
movzx ebx, word ptr BP_REL(ReservedSectors)
add eax, ebx
add eax, dword ptr BP_REL(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 ptr BP_REL(SectsPerCluster)
call ReadSectors
ret
// Displays a file not found error message
// And reboots
PrintFileNotFound:
mov si, offset msgFreeLdr // FreeLdr not found message
call PutChars // Display it
jmp Reboot
msgFreeLdr:
.ascii "freeldr.sys not found", CR, LF, NUL
filename:
.ascii "FREELDR SYS"
msgLoading:
.ascii "Loading FreeLoader...", CR, LF, NUL
.org 1022 // Pad to 1022 bytes
.word HEX(0aa55) // BootSector signature
.endcode16
END