Win2k FAT32 Boot Sector Disassembly
authorBrian Palmer <brianp@sginet.com>
Wed, 20 Jun 2001 20:31:35 +0000 (20:31 +0000)
committerBrian Palmer <brianp@sginet.com>
Wed, 20 Jun 2001 20:31:35 +0000 (20:31 +0000)
svn path=/trunk/; revision=1992

freeldr/bootsect/win2k-12.asm [deleted file]
freeldr/bootsect/win2k.asm

diff --git a/freeldr/bootsect/win2k-12.asm b/freeldr/bootsect/win2k-12.asm
deleted file mode 100644 (file)
index d68a6fb..0000000
+++ /dev/null
@@ -1,197 +0,0 @@
-00008000  660FB64610        movzx eax,byte [bp+NumberOfFats]
-00008005  668B4E24          mov ecx,[bp+SectorsPerFatBig]
-00008009  66F7E1            mul ecx
-0000800C  6603461C          add eax,[bp+HiddenSectors]
-00008010  660FB7560E        movzx edx,word [bp+ReservedSectors]
-00008015  6603C2            add eax,edx
-00008018  668946FC          mov [bp-0x4],eax
-0000801C  66C746F4FFFFFFFF  mov dword [bp-0xc],0xffffffff
-00008024  668B462C          mov eax,[bp+RootDirStartCluster]
-00008028  6683F802          cmp eax,byte +0x2
-0000802C  0F82A6FC          jc near 0x7cd6
-00008030  663DF8FFFF0F      cmp eax,0xffffff8
-00008036  0F839CFC          jnc near 0x7cd6
-0000803A  6650              push eax
-0000803C  6683E802          sub eax,byte +0x2
-00008040  660FB65E0D        movzx ebx,byte [bp+SectsPerCluster]
-00008045  8BF3              mov si,bx
-00008047  66F7E3            mul ebx
-0000804A  660346FC          add eax,[bp-0x4]
-0000804E  BB0082            mov bx,0x8200
-00008051  8BFB              mov di,bx
-00008053  B90100            mov cx,0x1
-00008056  E887FC            call 0x7ce0
-00008059  382D              cmp [di],ch
-0000805B  741E              jz 0x807b
-0000805D  B10B              mov cl,0xb
-0000805F  56                push si
-00008060  BE707D            mov si,0x7d70
-00008063  F3A6              repe cmpsb
-00008065  5E                pop si
-00008066  741B              jz 0x8083
-00008068  03F9              add di,cx
-0000806A  83C715            add di,byte +0x15
-0000806D  3BFB              cmp di,bx
-0000806F  72E8              jc 0x8059
-00008071  4E                dec si
-00008072  75DA              jnz 0x804e
-00008074  6658              pop eax
-00008076  E86500            call 0x80de
-00008079  72BF              jc 0x803a
-0000807B  83C404            add sp,byte +0x4
-0000807E  E955FC            jmp 0x7cd6
-00008081  0020              add [bx+si],ah
-00008083  83C404            add sp,byte +0x4
-00008086  8B7509            mov si,[di+0x9]
-00008089  8B7D0F            mov di,[di+0xf]
-0000808C  8BC6              mov ax,si
-0000808E  66C1E010          shl eax,0x10
-00008092  8BC7              mov ax,di
-00008094  6683F802          cmp eax,byte +0x2
-00008098  0F823AFC          jc near 0x7cd6
-0000809C  663DF8FFFF0F      cmp eax,0xffffff8
-000080A2  0F8330FC          jnc near 0x7cd6
-000080A6  6650              push eax
-000080A8  6683E802          sub eax,byte +0x2
-000080AC  660FB64E0D        movzx ecx,byte [bp+0xd]
-000080B1  66F7E1            mul ecx
-000080B4  660346FC          add eax,[bp-0x4]
-000080B8  BB0000            mov bx,0x0
-000080BB  06                push es
-000080BC  8E068180          mov es,[0x8081]
-000080C0  E81DFC            call 0x7ce0
-000080C3  07                pop es
-000080C4  6658              pop eax
-000080C6  C1EB04            shr bx,0x4
-000080C9  011E8180          add [0x8081],bx
-000080CD  E80E00            call 0x80de
-000080D0  0F830200          jnc near 0x80d6
-000080D4  72D0              jc 0x80a6
-000080D6  8A5640            mov dl,[bp+0x40]
-000080D9  EA00000020        jmp 0x2000:0x0
-000080DE  66C1E002          shl eax,0x2
-000080E2  E81100            call 0x80f6
-000080E5  26668B01          mov eax,[es:bx+di]
-000080E9  6625FFFFFF0F      and eax,0xfffffff
-000080EF  663DF8FFFF0F      cmp eax,0xffffff8
-000080F5  C3                ret
-000080F6  BF007E            mov di,0x7e00
-000080F9  660FB74E0B        movzx ecx,word [bp+0xb]
-000080FE  6633D2            xor edx,edx
-00008101  66F7F1            div ecx
-00008104  663B46F4          cmp eax,[bp-0xc]
-00008108  743A              jz 0x8144
-0000810A  668946F4          mov [bp-0xc],eax
-0000810E  6603461C          add eax,[bp+0x1c]
-00008112  660FB74E0E        movzx ecx,word [bp+0xe]
-00008117  6603C1            add eax,ecx
-0000811A  660FB75E28        movzx ebx,word [bp+0x28]
-0000811F  83E30F            and bx,byte +0xf
-00008122  7416              jz 0x813a
-00008124  3A5E10            cmp bl,[bp+0x10]
-00008127  0F83ABFB          jnc near 0x7cd6
-0000812B  52                push dx
-0000812C  668BC8            mov ecx,eax
-0000812F  668B4624          mov eax,[bp+0x24]
-00008133  66F7E3            mul ebx
-00008136  6603C1            add eax,ecx
-00008139  5A                pop dx
-0000813A  52                push dx
-0000813B  8BDF              mov bx,di
-0000813D  B90100            mov cx,0x1
-00008140  E89DFB            call 0x7ce0
-00008143  5A                pop dx
-00008144  8BDA              mov bx,dx
-00008146  C3                ret
-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
index 466ad7e..6a2798c 100644 (file)
@@ -1,3 +1,24 @@
+;
+; 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
@@ -37,43 +58,76 @@ FileSystem                  db 'FAT12   '
 
 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]
@@ -171,4 +225,240 @@ DISK_ERR_offset_from_0x7d00                                               db 0bfh
 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