brock@pinky-pie:~$ df
Filesystem     1K-blocks    Used Available Use% Mounted on
rootfs          15270068 3547752  11056676  25% /
/dev/root       15270068 3547752  11056676  25% /
devtmpfs          106076       0    106076   0% /dev
tmpfs              22052     212     21840   1% /run
tmpfs               5120       0      5120   0% /run/lock
tmpfs              44080       0     44080   0% /run/shm
/dev/mmcblk0p1     57288    9848     47440  18% /boot
brock@pinky-pie:~$ sudo dd if=/dev/mmcblk0p1 of=boot.img
114688+0 records in
114688+0 records out
58720256 bytes (59 MB) copied, 20.3459 s, 2.9 MB/s
brock@pinky-pie:~$ ls -l boot.img
-rw-r--r-- 1 root root 58720256 Mar 24 09:17 boot.img
brock@pinky-pie:~$ sudo losetup /dev/loop0
loop: can't get info on device /dev/loop0: No such device or address
brock@pinky-pie:~$ sudo losetup /dev/loop0 boot.img 
brock@pinky-pie:~$ ls -l /dev/loop0
brw-rw---T 1 root disk 7, 0 Mar 24 09:19 /dev/loop0
brock@pinky-pie:~$ mkdir /tmp/book
brock@pinky-pie:~$ grep boot /etc/fstab
/dev/mmcblk0p1  /boot           vfat    defaults          0       2
brock@pinky-pie:~$ sudo mount -t vfat -r /dev/loop0 /tmp/book
brock@pinky-pie:~$ ls /tmp/book
bootcode.bin  fixup_cd.dat  issue.txt       start_cd.elf
cmdline.txt   fixup.dat     kernel.img      start.elf
config.txt    fixup_x.dat   LICENSE.oracle  start_x.elf
brock@pinky-pie:~$ ls /boot
bootcode.bin  fixup_cd.dat  issue.txt       start_cd.elf
cmdline.txt   fixup.dat     kernel.img      start.elf
config.txt    fixup_x.dat   LICENSE.oracle  start_x.elf
brock@pinky-pie:~$ sudo umount /tmp/book
brock@pinky-pie:~$ sudo losetup -d /dev/loop0
## Take a look at the FAT documentation
## Do a man on dd
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, 0.000328991 s, 1.6 MB/s
## Do a man on od, this is a weird command
## We had a faculty member who used to use these
brock@pinky-pie:~$ od -A d -j 11 -N 2 -t d2 boot.boot-sector.img 
0000011    512
0000013
brock@pinky-pie:~$ BPB_BytesPerSector=512
brock@pinky-pie:~$ od -A d -j 13 -N 1 -t d boot.boot-sector.img 
0000013          16
0000014
brock@pinky-pie:~$ BPB_SectorsPerCluster=16
brock@pinky-pie:~$ od -A d -j 14 -N2 -t d2 boot.boot-sector.img 
0000014     16
0000016
brock@pinky-pie:~$ BPB_ReservedSectorCount=16
brock@pinky-pie:~$ od -A d -j 16 -N1 -t d boot.boot-sector.img 
0000016           2
0000017
brock@pinky-pie:~$ BPB_NumFATs=2
brock@pinky-pie:~$ od -A d -j 22 -N 2 -t d2 boot.boot-sector.img 
0000022     32
0000024
brock@pinky-pie:~$ BPB_NumSectorsPerFat=32
brock@pinky-pie:~$ od -A d -j 17 -N 2 -t d2 boot.boot-sector.img
0000017    512
0000019
brock@pinky-pie:~$ BPB_MaxRootDirectoryEntries=512
brock@pinky-pie:~$ od -A d -j 54 -N 8 -t a boot.boot-sector.img 
0000054   F   A   T   1   6  sp  sp  sp
0000062
brock@pinky-pie:~$ od -A d -j 3 -N 8 -t a boot.boot-sector.img 
0000003   m   k   d   o   s   f   s nul
0000011
brock@pinky-pie:~$ od -A d -j 22 -N 2 -t d2 boot.boot-sector.img 
0000022     32
0000024
brock@pinky-pie:~$ BPB_FATSize=32
brock@pinky-pie:~$ od -A d -j 32 -N 4 -t d4 boot.boot-sector.img 
0000032      114688
0000036
brock@pinky-pie:~$ ls -l boot.img
-rw-r--r-- 1 root root 58720256 Mar 24 09:17 boot.img
brock@pinky-pie:~$ dc
512 114688 * f
58720256
brock@pinky-pie:~$ od -A d -j 14 -N 2 -t d2 boot.boot-sector.img 
0000014     16
0000016
brock@pinky-pie:~$ BPB_ReservedSectors=16
brock@pinky-pie:~$ echo $BPB_NumFATs
2
brock@pinky-pie:~$ echo $BPB_NumSectorsPerFat
32
brock@pinky-pie:~$ dd if=boot.img of=boot.img.rootdir bs=512 count=1 skip=80
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.00613083 s, 83.5 kB/s
brock@pinky-pie:~$ dd if=boot.img of=boot.img.fat bs=512 skip=16 count=32
32+0 records in
32+0 records out
16384 bytes (16 kB) copied, 0.0163896 s, 1000 kB/s
brock@pinky-pie:~$ od -A d -j 0 -N 11 -t a boot.img.rootdir 
0000000   b   o   o   t  sp  sp  sp  sp  sp  sp  sp
0000011
brock@pinky-pie:~$ od -A d -j 32 -N 11 -t a boot.img.rootdir 
0000032   A   b nul   o nul   o nul   t nul   c nul
0000043
brock@pinky-pie:~$ od -A d -j 32 -N 11 -t a boot.img.rootdir 
0000032   A   b nul   o nul   o nul   t nul   c nul
0000043
brock@pinky-pie:~$ ls /booit
bootcode.bin  fixup_cd.dat  issue.txt       start_cd.elf
cmdline.txt   fixup.dat     kernel.img      start.elf
config.txt    fixup_x.dat   LICENSE.oracle  start_x.elf
brock@pinky-pie:~$ od -A d -j 32 -N 32 -t a boot.img.rootdir 
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
brock@pinky-pie:~$ od -A d -j 64 -N 32 -t a boot.img.rootdir 
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
brock@pinky-pie:~$ od -A d -j 96 -N 32 -t a boot.img.rootdir 
0000096   A   c nul   m nul   d nul   l nul   i nul  si nul  si   n nul
0000112   e nul   . nul   t nul   x nul   t nul nul nul nul nul del del
0000128
brock@pinky-pie:~$ od -A d -j 128 -N 32 -t a boot.img.rootdir 
0000128   C   M   D   L   I   N   E  sp   T   X   T  sp nul nul   H   )
0000144   '   D   '   D nul nul   2   )   '   D ack nul  so nul nul nul
0000160
brock@pinky-pie:~$ dd if=boot.img.rootdir of=boot.img.cmdline bs=32 count=1 skip=4
1+0 records in
1+0 records out
32 bytes (32 B) copied, 0.000353991 s, 90.4 kB/s
brock@pinky-pie:~$ od -A d -j 0 -N 11 -t a boot.img.cmdline 
0000000   C   M   D   L   I   N   E  sp   T   X   T
0000011
brock@pinky-pie:~$ od -A d -j 20 -N 2 -t d2 boot.img.cmdline 
0000020      0
0000022
brock@pinky-pie:~$ od -A d -j 26 -N 2 -t d2 boot.img.cmdline 
0000026      6
0000028
brock@pinky-pie:~$ BPB_CMDLINE_CLUSTER=6
brock@pinky-pie:~$ echo $BPB_SectorsPerCluster
16
brock@pinky-pie:~$ echo $BPB_BytesPerSector
512
brock@pinky-pie:~$ od -A d -j 28 -N 4 -t d4 boot.img.cmdline 
0000028         142
0000032
brock@pinky-pie:~$ ls -l /boot/cmdline.txt 
-rwxr-xr-x 1 root root 142 Jan  7  2014 /boot/cmdline.txt
brock@pinky-pie:~$ echo $(( $BPB_CMDLINE_LUSTER * $BPB_SectorsPerCluster * $BPB_BytesPerSector ))
49152
brock@pinky-pie:~$ dd if=boot.img of=my.cmdline.txt skip=49152 count=142
142+0 records in
142+0 records out
72704 bytes (73 kB) copied, 0.0142756 s, 5.1 MB/s
brock@pinky-pie:~ dd if=boot.img of=my.cmdline.txt skip=49152 count=142 bs=1
142+0 records in
142+0 records out
142 bytes (142 B) copied, 0.00969174 s, 14.7 kB/s
brock@pinky-pie:~$ dc
80 512 * /
dc: stack empty
f
40960
49152 + f
90112
brock@pinky-pie:~$ BPB_FirstDataSector=$(( $BPB_NumSectorsPerFat * $BPB_NumFATs + $BPB_ReservedSectorCount ))
brock@pinky-pie:~$ echo $BPB_FirstDataSector
80
brock@pinky-pie:~$ echo $(( $BPB_CMDLINE_LUSTER * $BPB_SectorsPerCluster ))
96
brock@pinky-pie:~$ BPB_CmdlineSector=$(($BPB_FirstDataSector + $BPB_CMDLINE_LUSTER * $BPB_SectorsPerCluster ))
brock@pinky-pie:~$ echo $BPB_CmdlineSector
176
brock@pinky-pie:~$ brock@pinky-pie:~$ dd if=boot.img of=cmd.txt.sector bs=512 skip=$BPB_CmdlineSector count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.00166996 s, 307 kB/s
brock@pinky-pie:~$ dd if=cmd.txt.sector of=cmdline.txt bs=1 count=142
142+0 records in
142+0 records out
142 bytes (142 B) copied, 0.0117197 s, 12.1 kB/s
brock@pinky-pie:~$ cat cmdline.txt
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
brock@pinky-pie:~$