+;
+; Win2k FAT32 Boot Sector
+;
+; Brian Palmer <brianp@sginet.com>
+;
+
+;
+; The BP register is initialized to 0x7c00, the start of
+; the boot sector. The SP register is initialized to
+; 0x7bf4, leaving 12 bytes of data storage space above
+; the stack.
+;
+; The DWORD that gets stored at 0x7bf4 is 0xffffffff ??
+;
+; The DWORD that gets stored at 0x7bf8 is the count of
+; total sectors of the volume, calculated from the BPB.
+;
+; The DWORD that gets stored at 0x7bfc is the logical
+; sector number of the start of the data area.
+;
+
segment .text
bits 16
main:
00007C5A 33C9 xor cx,cx
-00007C5C 8ED1 mov ss,cx
-00007C5E BCF47B mov sp,0x7bf4
+00007C5C 8ED1 mov ss,cx ; Setup the stack
+00007C5E BCF47B mov sp,0x7bf4 ; Give us 12 bytes of space above the stack
00007C61 8EC1 mov es,cx
00007C63 8ED9 mov ds,cx
00007C65 BD007C mov bp,0x7c00
-00007C68 884E02 mov [bp+0x2],cl
+00007C68 884E02 mov [bp+0x2],cl ; Zero out the nop instruction?? (3rd byte of the boot sector)
00007C6B 8A5640 mov dl,[bp+BootDrive]
00007C6E B408 mov ah,0x8
00007C70 CD13 int 0x13 ; Int 13, func 8 - Get Drive Parameters
-00007C72 7305 jnc 0x7c79 ; If no error jmp
-
-00007C74 B9FFFF mov cx,0xffff
-00007C77 8AF1 mov dh,cl
-
-00007C79 660FB6C6 movzx eax,dh
-00007C7D 40 inc ax
-00007C7E 660FB6D1 movzx edx,cl
-00007C82 80E23F and dl,0x3f
-00007C85 F7E2 mul dx
-00007C87 86CD xchg cl,ch
-00007C89 C0ED06 shr ch,0x6
-00007C8C 41 inc cx
+00007C72 7305 jnc drive_param_ok ; If no error jmp
+
+drive_param_error:
+00007C74 B9FFFF mov cx,0xffff ; We couldn't determine the drive parameters
+00007C77 8AF1 mov dh,cl ; So just set the CHS to 0xff
+
+drive_param_ok:
+00007C79 660FB6C6 movzx eax,dh ; Store the number of heads in eax
+00007C7D 40 inc ax ; Make it one-based because the bios returns it zero-based
+00007C7E 660FB6D1 movzx edx,cl ; Store the sectors per track in edx
+00007C82 80E23F and dl,0x3f ; Mask off the cylinder bits
+00007C85 F7E2 mul dx ; Multiply the sectors per track with the heads, result in dx:ax
+00007C87 86CD xchg cl,ch ; Switch the cylinder with the sectors
+00007C89 C0ED06 shr ch,0x6 ; Move the top two cylinder bits down where they should be
+00007C8C 41 inc cx ; Make it one-based because the bios returns it zero-based
00007C8D 660FB7C9 movzx ecx,cx
-00007C91 66F7E1 mul ecx
-00007C94 668946F8 mov [bp-0x8],eax
-00007C98 837E1600 cmp word [bp+TotalSectors],byte +0x0
-00007C9C 7538 jnz print_ntldr_error_message
+00007C91 66F7E1 mul ecx ; Multiply the cylinders with (heads * sectors) [stored in dx:ax already]
+00007C94 668946F8 mov [bp-0x8],eax ; This value is the number of total sectors on the disk, so save it for later
+00007C98 837E1600 cmp word [bp+TotalSectors],byte +0x0 ; Check the old 16-bit value of TotalSectors
+00007C9C 7538 jnz print_ntldr_error_message ; If it is non-zero then exit with an error
-00007C9E 837E2A00 cmp word [bp+FSVersion],byte +0x0
-00007CA2 7732 ja print_ntldr_error_message
+00007C9E 837E2A00 cmp word [bp+FSVersion],byte +0x0 ; Check the file system version word
+00007CA2 7732 ja print_ntldr_error_message ; If it is not zero then exit with an error
-00007CA4 668B461C mov eax,[bp+0x1c]
-00007CA8 6683C00C add eax,byte +0xc
-00007CAC BB0080 mov bx,0x8000
-00007CAF B90100 mov cx,0x1
-00007CB2 E82B00 call read_sectors
-00007CB5 E94803 jmp 0x8000
+
+ ;
+ ; We are now ready to load our second sector of boot code
+ ; But first, a bit of undocumented information about how
+ ; Win2k stores it's second sector of boot code.
+ ;
+ ; The FAT32 filesystem was designed so that you can store
+ ; multiple sectors of boot code. The boot sector of a FAT32
+ ; volume is actually three sectors long. Microsoft extended
+ ; the BPB so much that you can't fit enough code in the
+ ; boot sector to make it work. So they extended it. Sector 0
+ ; is the traditional boot sector, sector 1 is the FSInfo sector,
+ ; and sector 2 is used to store extra boot code to make up
+ ; for the lost space the BPB takes.
+ ;
+ ; Now this creates an interesting problem. Suppose for example
+ ; that the user has Win98 and Win2k installed. The Win2k
+ ; boot sector is stored at sector 0 and the Win98 boot sector is
+ ; stored as BOOTSECT.DOS on the file system. Now if Win2k were
+ ; to store it's second sector of boot code in sector 2 like
+ ; the fat spec says to do then when you try to dual boot back
+ ; to Win98 the Win98 boot sector will load Win2k's second
+ ; sector of boot code. Understand? ;-)
+ ;
+ ; To get around this problem Win2k stores it's second sector
+ ; of boot code elsewhere. This sector is always stored at sector 13
+ ; on the file system. Now don't ask me what happens when you don't
+ ; have enough reserved sectors to store it, but I've never seen a
+ ; FAT32 volume that didn't have at least 32 reserved sectors.
+ ;
+
+00007CA4 668B461C mov eax,[bp+HiddenSectors] ; Get the count of hidden sectors
+00007CA8 6683C00C add eax,byte +0xc ; Add 12 to that value so that we are loading the 13th sector of the volume
+00007CAC BB0080 mov bx,0x8000 ; Read the sector to address 0x8000
+00007CAF B90100 mov cx,0x1 ; Read just one sector
+00007CB2 E82B00 call read_sectors ; Read it
+00007CB5 E94803 jmp 0x8000 ; Jump to the next sector of boot code
print_disk_error_message:
00007CB8 A0FA7D mov al,[DISK_ERR_offset_from_0x7d00]
RESTART_ERR_offset_from_0x7d00 db 0cch
dw 0
- dw 0aa55h
\ No newline at end of file
+ dw 0aa55h
+
+
+
+
+
+
+;
+; And that ends the code that makes up the traditional boot sector
+; From here on out is a disassembly of the extra sector of boot
+; code required for a FAT32 volume. Win2k stores this code at
+; sector 13 on the file system.
+;
+
+
+
+
+00008000 660FB64610 movzx eax,byte [bp+NumberOfFats] ; Put the number of fats into eax
+00008005 668B4E24 mov ecx,[bp+SectorsPerFatBig] ; Put the count of sectors per fat into ecx
+00008009 66F7E1 mul ecx ; Multiply them, edx:eax = (eax * ecx)
+0000800C 6603461C add eax,[bp+HiddenSectors] ; Add the hidden sectors to eax
+00008010 660FB7560E movzx edx,word [bp+ReservedSectors] ; Put the count of reserved sectors into edx
+00008015 6603C2 add eax,edx ; Add it to eax
+00008018 668946FC mov [bp-0x4],eax ; eax now contains the start of the data area, so save it for later
+0000801C 66C746F4FFFFFFFF mov dword [bp-0xc],0xffffffff ; Save 0xffffffff for later??
+00008024 668B462C mov eax,[bp+RootDirStartCluster] ; Put the starting cluster of the root directory into eax
+00008028 6683F802 cmp eax,byte +0x2 ; Check and see if the root directory starts at cluster 2 or above
+0000802C 0F82A6FC jc near print_ntldr_error_message ; If not exit with error
+00008030 663DF8FFFF0F cmp eax,0xffffff8 ; Check and see if the root directory start cluster is and end of cluster chain indicator
+00008036 0F839CFC jnc near print_ntldr_error_message ; If so exit with error
+
+search_root_directory_cluster:
+0000803A 6650 push eax ; Save root directory start cluster on stack
+0000803C 6683E802 sub eax,byte +0x2 ; Adjust it because the first two fat entries are unused so the third entry marks the first data area cluster
+00008040 660FB65E0D movzx ebx,byte [bp+SectsPerCluster] ; Put the number of sectors per cluster in ebx
+00008045 8BF3 mov si,bx ; Now store it also in si register
+00008047 66F7E3 mul ebx ; Multiply sectors per cluster with root directory start cluster
+0000804A 660346FC add eax,[bp-0x4] ; Add the start sector of the data area
+
+read_directory_sector:
+0000804E BB0082 mov bx,0x8200 ; We now have the start sector of the root directory, so load it to 0x8200
+00008051 8BFB mov di,bx ; Put the address of the root directory sector in di also
+00008053 B90100 mov cx,0x1 ; Read one sector
+00008056 E887FC call read_sectors ; Perform the read
+
+check_entry_for_ntldr:
+00008059 382D cmp [di],ch ; Check the first byte of the root directory entry for zero
+0000805B 741E jz ntldr_not_found ; If so then NTLDR is missing so exit with error
+0000805D B10B mov cl,0xb ; Put the value 11 in cl so we can compare an 11-byte filename
+0000805F 56 push si ; Save si (which contains the number of sectors per cluster)
+00008060 BE707D mov si,NTLDR ;0x7d70 ; Check and see if "NTLDR" is the first file entry
+00008063 F3A6 repe cmpsb ; Do the compare
+00008065 5E pop si ; Restore sectors per cluster into si
+00008066 741B jz ntldr_found ; If we found it then continue, else check next entry
+00008068 03F9 add di,cx ; Add 0 to di? the next entry is 0x15 bytes away
+0000806A 83C715 add di,byte +0x15 ; Add 0x15 to di
+0000806D 3BFB cmp di,bx ; Check to see if we have reached the end of our sector we loaded, read_sectors sets bx = end address of data loaded
+0000806F 72E8 jc check_entry_for_ntldr ; If we haven't reached the end then check the next entry
+00008071 4E dec si ; decrement si, si holds the number of sectors per cluster
+00008072 75DA jnz read_directory_sector ; If it's not zero then search the next sector for NTLDR
+00008074 6658 pop eax ; If we got here that means we didn't find NTLDR in the previous root directory cluster, so restore eax with the start cluster
+00008076 E86500 call get_fat_entry ; Get the next cluster in the fat chain
+00008079 72BF jc search_root_directory_cluster ; If we reached end-of-file marker then don't jump, otherwise continue search
+
+ntldr_not_found:
+0000807B 83C404 add sp,byte +0x4
+0000807E E955FC jmp print_ntldr_error_message
+
+ntldr_load_segment_address dw 0x2000
+
+ntldr_found:
+00008083 83C404 add sp,byte +0x4 ; Adjust stack to remove root directory start cluster
+00008086 8B7509 mov si,[di+0x9] ; Put start cluster high word in si
+00008089 8B7D0F mov di,[di+0xf] ; Put start cluster low word in di
+0000808C 8BC6 mov ax,si ; Put high word in ax
+0000808E 66C1E010 shl eax,0x10 ; Shift it into position
+00008092 8BC7 mov ax,di ; Put low word in ax, now eax contains start cluster of NTLDR
+00008094 6683F802 cmp eax,byte +0x2 ; Check and see if the start cluster of NTLDR starts at cluster 2 or above
+00008098 0F823AFC jc near print_ntldr_error_message ; If not exit with error
+0000809C 663DF8FFFF0F cmp eax,0xffffff8 ; Check and see if the start cluster of NTLDR is and end of cluster chain indicator
+000080A2 0F8330FC jnc near print_ntldr_error_message ; If so exit with error
+
+load_next_ntldr_cluster:
+000080A6 6650 push eax ; Save NTLDR start cluster for later
+000080A8 6683E802 sub eax,byte +0x2 ; Adjust it because the first two fat entries are unused so the third entry marks the first data area cluster
+000080AC 660FB64E0D movzx ecx,byte [bp+SectsPerCluster] ; Put the sectors per cluster into ecx
+000080B1 66F7E1 mul ecx ; Multiply sectors per cluster by the start cluster, we now have the logical start sector
+000080B4 660346FC add eax,[bp-0x4] ; Add the start of the data area logical sector
+000080B8 BB0000 mov bx,0x0 ; Load NTLDR to offset zero
+000080BB 06 push es ; Save es
+000080BC 8E068180 mov es,[ntldr_load_segment_address] ; Get the segment address to load NTLDR to
+000080C0 E81DFC call read_sectors ; Load the first cluster
+000080C3 07 pop es ; Restore es
+000080C4 6658 pop eax ; Restore eax to NTLDR start cluster
+000080C6 C1EB04 shr bx,0x4 ; bx contains the amount of data we transferred, so divide it by 16
+000080C9 011E8180 add [ntldr_load_segment_address],bx ; Add that value to the segment
+000080CD E80E00 call get_fat_entry ; Get the next cluster in eax
+000080D0 0F830200 jnc near jump_to_ntldr ; If we have reached the end of file then lets get to NTLDR
+000080D4 72D0 jc load_next_ntldr_cluster ; If not, then load another cluster
+
+jump_to_ntldr:
+000080D6 8A5640 mov dl,[bp+BootDrive] ; Put the boot drive in dl
+000080D9 EA00000020 jmp 0x2000:0x0 ; Jump to NTLDR
+
+get_fat_entry:
+000080DE 66C1E002 shl eax,0x2 ; Multiply cluster by 4
+000080E2 E81100 call load_fat_sector ; Load the fat sector
+000080E5 26668B01 mov eax,[es:bx+di] ; Get the fat entry
+000080E9 6625FFFFFF0F and eax,0xfffffff ; Mask off the most significant 4 bits
+000080EF 663DF8FFFF0F cmp eax,0xffffff8 ; Compare it to end of file marker to set the flags correctly
+000080F5 C3 ret ; Return to caller
+
+load_fat_sector:
+000080F6 BF007E mov di,0x7e00 ; We will load the fat sector to 0x7e00
+000080F9 660FB74E0B movzx ecx,word [bp+SectsPerCluster] ; Get the sectors per cluster
+000080FE 6633D2 xor edx,edx ; We will divide (cluster * 4) / sectorspercluster
+00008101 66F7F1 div ecx ; eax is already set before we get to this routine
+00008104 663B46F4 cmp eax,[bp-0xc] ; Compare eax to 0xffffffff (initially, we set this value later)
+00008108 743A jz load_fat_sector_end ; If it is the same return
+0000810A 668946F4 mov [bp-0xc],eax ; Update that value
+0000810E 6603461C add eax,[bp+HiddenSectors] ; Add the hidden sectors
+00008112 660FB74E0E movzx ecx,word [bp+ReservedSectors] ; Add the reserved sectors
+00008117 6603C1 add eax,ecx ; To the hidden sectors + the value we computed earlier
+0000811A 660FB75E28 movzx ebx,word [bp+ExtendedFlags] ; Get extended flags and put into ebx
+0000811F 83E30F and bx,byte +0xf ; Mask off everything but the active fat bits
+00008122 7416 jz load_fat_sector_into_memory ; If active fat is first fat skip fat size calcs
+00008124 3A5E10 cmp bl,[bp+NumberOfFats] ; Compare bl to number of fats
+00008127 0F83ABFB jnc near print_ntldr_error_message ; If bl is bigger than numfats exit with error
+0000812B 52 push dx ; Save dx
+0000812C 668BC8 mov ecx,eax ; Put the current fat sector offset into ecx
+0000812F 668B4624 mov eax,[bp+SectorsPerFatBig] ; Get the number of sectors occupied by one fat
+00008133 66F7E3 mul ebx ; Multiplied by the active fat index
+00008136 6603C1 add eax,ecx ; Add the current fat sector offset
+00008139 5A pop dx ; Restore dx
+load_fat_sector_into_memory:
+0000813A 52 push dx ; Save dx, what is so important in dx??
+0000813B 8BDF mov bx,di ; Put 0x7e00 in bx
+0000813D B90100 mov cx,0x1 ; Load one sector
+00008140 E89DFB call read_sectors ; Perform the read
+00008143 5A pop dx ; Restore dx
+load_fat_sector_end:
+00008144 8BDA mov bx,dx ; Put it into bx, what is this value??
+00008146 C3 ret ; Return
+
+
+00008147 0000 add [bx+si],al
+00008149 0000 add [bx+si],al
+0000814B 0000 add [bx+si],al
+0000814D 0000 add [bx+si],al
+0000814F 0000 add [bx+si],al
+00008151 0000 add [bx+si],al
+00008153 0000 add [bx+si],al
+00008155 0000 add [bx+si],al
+00008157 0000 add [bx+si],al
+00008159 0000 add [bx+si],al
+0000815B 0000 add [bx+si],al
+0000815D 0000 add [bx+si],al
+0000815F 0000 add [bx+si],al
+00008161 0000 add [bx+si],al
+00008163 0000 add [bx+si],al
+00008165 0000 add [bx+si],al
+00008167 0000 add [bx+si],al
+00008169 0000 add [bx+si],al
+0000816B 0000 add [bx+si],al
+0000816D 0000 add [bx+si],al
+0000816F 0000 add [bx+si],al
+00008171 0000 add [bx+si],al
+00008173 0000 add [bx+si],al
+00008175 0000 add [bx+si],al
+00008177 0000 add [bx+si],al
+00008179 0000 add [bx+si],al
+0000817B 0000 add [bx+si],al
+0000817D 0000 add [bx+si],al
+0000817F 0000 add [bx+si],al
+00008181 0000 add [bx+si],al
+00008183 0000 add [bx+si],al
+00008185 0000 add [bx+si],al
+00008187 0000 add [bx+si],al
+00008189 0000 add [bx+si],al
+0000818B 0000 add [bx+si],al
+0000818D 0000 add [bx+si],al
+0000818F 0000 add [bx+si],al
+00008191 0000 add [bx+si],al
+00008193 0000 add [bx+si],al
+00008195 0000 add [bx+si],al
+00008197 0000 add [bx+si],al
+00008199 0000 add [bx+si],al
+0000819B 0000 add [bx+si],al
+0000819D 0000 add [bx+si],al
+0000819F 0000 add [bx+si],al
+000081A1 0000 add [bx+si],al
+000081A3 0000 add [bx+si],al
+000081A5 0000 add [bx+si],al
+000081A7 0000 add [bx+si],al
+000081A9 0000 add [bx+si],al
+000081AB 0000 add [bx+si],al
+000081AD 0000 add [bx+si],al
+000081AF 0000 add [bx+si],al
+000081B1 0000 add [bx+si],al
+000081B3 0000 add [bx+si],al
+000081B5 0000 add [bx+si],al
+000081B7 0000 add [bx+si],al
+000081B9 0000 add [bx+si],al
+000081BB 0000 add [bx+si],al
+000081BD 0000 add [bx+si],al
+000081BF 0000 add [bx+si],al
+000081C1 0000 add [bx+si],al
+000081C3 0000 add [bx+si],al
+000081C5 0000 add [bx+si],al
+000081C7 0000 add [bx+si],al
+000081C9 0000 add [bx+si],al
+000081CB 0000 add [bx+si],al
+000081CD 0000 add [bx+si],al
+000081CF 0000 add [bx+si],al
+000081D1 0000 add [bx+si],al
+000081D3 0000 add [bx+si],al
+000081D5 0000 add [bx+si],al
+000081D7 0000 add [bx+si],al
+000081D9 0000 add [bx+si],al
+000081DB 0000 add [bx+si],al
+000081DD 0000 add [bx+si],al
+000081DF 0000 add [bx+si],al
+000081E1 0000 add [bx+si],al
+000081E3 0000 add [bx+si],al
+000081E5 0000 add [bx+si],al
+000081E7 0000 add [bx+si],al
+000081E9 0000 add [bx+si],al
+000081EB 0000 add [bx+si],al
+000081ED 0000 add [bx+si],al
+000081EF 0000 add [bx+si],al
+000081F1 0000 add [bx+si],al
+000081F3 0000 add [bx+si],al
+000081F5 0000 add [bx+si],al
+000081F7 0000 add [bx+si],al
+000081F9 0000 add [bx+si],al
+000081FB 0000 add [bx+si],al
+000081FD 0055AA add [di-0x56],dl ; We can't forget the infamous boot signature
projects / reactos.git / blobdiff