##
## References from Microsoft FAT32 File System Specification
##
##
## Copy the boot sector
## which includes the BPB (BIOS Paramter Block)
##
brock@pinky-pie:~$ dd if=boot.img of=boot.boot-sector.img bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.00834778 s, 61.3 kB/s
##
## Get some BPB variables [Pages 9-12]
## 
##
brock@pinky-pie:~$ od -A d -j 11 -N 2 -t d2 boot.boot-sector.img 
0000011    512
0000013
brock@pinky-pie:~$ BPB_BytsPerSec=512
brock@pinky-pie:~$ od -A d -j 13 -N 1 -t d1 boot.boot-sector.img 
0000013   16
0000014
brock@pinky-pie:~$ BPB_SecPerClus=16
brock@pinky-pie:~$ od -A d -j 14 -N 2 -t d2 boot.boot-sector.img 
0000014     16
0000016
brock@pinky-pie:~ $BPB_RsvdSecCnt=16
brock@pinky-pie:~$ od -A d -j 16 -N 1 -t d1 boot.boot-sector.img 
0000016    2
0000017
brock@pinky-pie:~$ BPB_NumFATs=2
brock@pinky-pie:~$ od -A d -j 17 -N 2 -t d2 boot.boot-sector.img 
0000017    512
0000019
brock@pinky-pie:~$ BPB_RootEntCnt=512
brock@pinky-pie:~$ od -A d -j 19 -N 2 -t d2 boot.boot-sector.img 
0000019      0
0000021
brock@pinky-pie:~$ BPB_TotSec16=0
##
## Because BPB_TotSec16 is 0, must check BPB_TocSec32
## 32 bit field is needed because number of sectors exceeds 65535
##
brock@pinky-pie:~$ od -A d -j 32 -N 4 -t d4 boot.boot-sector.img 
0000032      114688
0000036
brock@pinky-pie:~$ BPB_TotSec32=114688
brock@pinky-pie:~$ TotSec=$BPB_TotSec32
brock@pinky-pie:~$ echo $TotSec
114688
##
## Sanity check: Size predicted by BPB matches /boot image
##
brock@pinky-pie:~$ echo $(( $TotSec * $BPB_BytsPerSec ))
58720256
brock@pinky-pie:~$ ls -l boot.img
-rw-r--r-- 1 root root 58720256 Mar 24 09:17 boot.img
##
## Following FAT Data Structure section [pp 13-14]
##
##
## Determine count of sectors in root directory (for FAT16)
##
brock@pinky-pie:~$ RootDirSectors=$(( ( ( $BPB_RootEntCnt * 32 ) + ( $BPB_BytsPerSec - 1 ) ) / $BPB_BytsPerSec ))
brock@pinky-pie:~$ echo $RootDirSectors
32
##
## Another sanity check
##
brock@pinky-pie:~$ echo $(( $RootDirSectors * 32 ))
1024
brock@pinky-pie:~$ echo $(( $RootDirSectors * $BPB_BytsPerSec ))
16384
brock@pinky-pie:~$ echo $(( $BPB_RootEntCnt * 32 ))
16384
##
## Determine size of FAT
##
brock@pinky-pie:~$ od -A d -j 22 -N 2 -t d2 boot.boot-sector.img 
0000022     32
0000024
brock@pinky-pie:~$ BPB_FATSz16=32
brock@pinky-pie:~$ FATSz=$BPB_FATSz16
##
## Find sector address of first data cluster
##
brock@pinky-pie:~$ FirstDataSector=$(( $BPB_RsvdSecCnt + ( $BPB_NumFATs * $FATSz ) + $RootDirSectors ))
brock@pinky-pie:~$ echo $FirstDataSector
112
##
## Find number of data sectors
## 
brock@pinky-pie:~$ DataSec=$(( $TotSec - ( $BPB_RsvdSecCnt + ( $BPB_NumFATs * $FATSz ) + $RootDirSectors ) ))
##
## More sanity checks
##
brock@pinky-pie:~$ echo $DataSec
114576
brock@pinky-pie:~$ echo $FirstDataSector
112
brock@pinky-pie:~$ echo $TotSec
114688
brock@pinky-pie:~$ echo $(( $TotSec * $BPB_BytsPerSec ))
58720256
brock@pinky-pie:~$ brock@pinky-pie:~$ ls -l boot.img
-rw-r--r-- 1 root root 58720256 Mar 24 09:17 boot.img
##
## Official MS-sanctioned way to determine FAT type, FAT12, FAT16 or FAT32
##
brock@pinky-pie:~$ CountofClusters=$(( $DataSec / $BPB_SecPerClus ))
brock@pinky-pie:~$ echo $CountofClusters
7161
##
## Count is more than 2^12 and less than or equal to 2^16
## This is a FAT16 file system
##
## Copy the root directory
##
brock@pinky-pie:~$ echo $(( $BPB_RsvdSecCnt + $BPB_NumFATs * $FATSz ))
80
brock@pinky-pie:~$ echo $RootDirSectors
32
brock@pinky-pie:~$ dd if=boot.img of=boot.rootdir.img bs=512 count=32 skip=80
32+0 records in
32+0 records out
16384 bytes (16 kB) copied, 0.00549986 s, 3.0 MB/s
##
## Looking at FAT Directory structure [pp 22-...]
##
## Sanity check of first directory entry
## Looks OK
##
brock@pinky-pie:~$ od -A d -j 0 -N 32 -t a boot.rootdir.img
0000000   b   o   o   t  sp  sp  sp  sp  sp  sp  sp  bs nul nul   U   &
0000016   '   D   '   D nul nul   U   &   '   D nul nul nul nul nul nul
0000032
##
## The dreaded long directory entry for bootcode.bin
##
brock@pinky-pie:~$ od -A d -j 32 -N 32 -t a boot.rootdir.img
0000032   A   b nul   o nul   o nul   t nul   c nul  si nul enq   o nul
0000048   d nul   e nul   . nul   b nul   i nul nul nul   n nul nul nul
0000064
##
## Old-style directory entry for BOOTCODE.BIN
##
brock@pinky-pie:~$ od -A d -j 64 -N 32 -t a boot.rootdir.img
0000064   B   O   O   T   C   O   D   E   B   I   N  sp nul   d   T   %
0000080   9   E   9   E nul nul   T   %   9   E etx nul   0   E nul nul
0000096
##
## Determine file size from directory entry
##
brock@pinky-pie:~$ od -A d -j $(( 64 + 28 )) -N 4 -t d4 boot.rootdir.img
0000092       17840
0000096
##
## Sanity check
##
brock@pinky-pie:~$ ls -l /boot/bootcode.bin 
-rwxr-xr-x 1 root root 17840 Sep 25  2014 /boot/bootcode.bin
##
## Determine first cluster address for BOOTCODE.BIN
## Looks like it's 3
##
brock@pinky-pie:~$ od -A d -j $(( 64 + 26 )) -N 2 -t d2 boot.rootdir.img
0000090      3
0000092
brock@pinky-pie:~$ BOOTCODEBINCLUSTER=3
##
## Copy the entire FAT
##
brock@pinky-pie:~$ dd if=boot.img of=boot.fat.img bs=512 skip=16 count=32
32+0 records in
32+0 records out
16384 bytes (16 kB) copied, 0.0144706 s, 1.1 MB/s
##
## Look at the FAT's linked list structure
## Our file seems to be in clusters 3, 4, t
##
brock@pinky-pie:~$ od -A d -j 0 -N 32 -t d2 boot.fat.img 
0000000     -8     -1      0      4      5     -1     -1     -1
0000016     -1     10     -1     12     13     14     15     16
0000032
##
## Sanity check: Cf file length and number of clusters
##
brock@pinky-pie:~$ echo $*(( $BPB_SecPerClus * $BPB_BytsPerSec ))
(( 16 * 512 ))
brock@pinky-pie:~$ echo $(( $BPB_SecPerClus * $BPB_BytsPerSec ))
8192
brock@pinky-pie:~$ echo $(( ( 17840 - 1) / 8192 + 1 ))
3
##
## Another look at our clusters
##
brock@pinky-pie:~$ od -A d -j 6 -N 6 -t d2 boot.fat.img 
0000006      4      5     -1
0000012
##
## Get sector address of clusters
##
brock@pinky-pie:~$ FirstSectorofCluster3=$(((  ( 3 - 2 ) * $BPB_SecPerClus ) + $FirstDataSector ))
brock@pinky-pie:~$ FirstSectorofCluster4=$(((  ( 4 - 2 ) * $BPB_SecPerClus ) + $FirstDataSector ))
brock@pinky-pie:~$ FirstSectorofCluster5=$(((  ( 5 - 2 ) * $BPB_SecPerClus ) + $FirstDataSector ))
brock@pinky-pie:~$ echo $FirstSectorofCluster3
128
brock@pinky-pie:~$ echo $FirstSectorofCluster4
144
brock@pinky-pie:~$ echo $FirstSectorofCluster5
160
##
## Read the three clusters
##
brock@pinky-pie:~$ dd if=boot.img of=boot.cluster3.img bs=512 skip=128 count=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.00104097 s, 7.9 MB/s
brock@pinky-pie:~$ dd if=boot.img of=boot.cluster4.img bs=512 skip=144 count=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.00104697 s, 7.8 MB/s
brock@pinky-pie:~$ dd if=boot.img of=boot.cluster5.img bs=512 skip=160 count=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.00118497 s, 6.9 MB/s
##
## Concatentate the three clusters and truncate to 17840 bytes
##
brock@pinky-pie:~$ cat boot.cluster3.img boot.cluster4.img boot.cluster5.img > boot.clusters3-5.img
brock@pinky-pie:~$ ls -l boot.cluster*
-rw-r--r-- 1 brock brock  8192 Apr  7 00:02 boot.cluster3.img
-rw-r--r-- 1 brock brock  8192 Apr  7 00:02 boot.cluster4.img
-rw-r--r-- 1 brock brock  8192 Apr  7 00:02 boot.cluster5.img
-rw-r--r-- 1 brock brock 24576 Apr  7 00:03 boot.clusters3-5.img
brock@pinky-pie:~$ dd if=boot.clusters3-5.img of=boot.bootcode.bin.img bs=17840 count=1
1+0 records in
1+0 records out
17840 bytes (18 kB) copied, 0.0110697 s, 1.6 MB/s
##
## We did it!
##
brock@pinky-pie:~$ ls -l boot.bootcode.bin.img 
-rw-r--r-- 1 brock brock 17840 Apr  7 00:04 boot.bootcode.bin.img
brock@pinky-pie:~$ ls -l /boot/bootcode.bin 
-rwxr-xr-x 1 root root 17840 Sep 25  2014 /boot/bootcode.bin
brock@pinky-pie:~$ cmp boot.bootcode.bin.img /boot/bootcode.bin 
brock@pinky-pie:~$ echo $?
0