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ZL1ABS > TECH 11.08.08 01:48l 157 Lines 5624 Bytes #999 (0) @ WW
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The root sector
---------------
The root sector is the first sector on every hard disk. It contains the
program that loads the boot sector of the bootable partition and the
partition table. The last two bytes of the root sector must be 55 aa
(hex).
The partition table begins at 1be. It contains 4 * 16 Bytes for the four
possible partitions.
All numbers are zero based except the start/end-sector number (may be
1-63).
One partition entry contains the following:
1 Byte - Bootable Flag. Must be 0 (not bootable) or 80h (bootable).
At most one Partition may be bootable at a time.
(somewhere I read the bootable flag may also be 81h for the
second drive - does anybody know anything about that?)
1 Byte - Start Head. The number of the head of the first sector of the
partition.
2 Bytes - Start Sector + Cylinder. The Bits are as follows:
CCSSSSSS CCCCCCCC
where the first byte contains the sector number (1 - 63), and
the high two bits of the cylinder number. The second byte con-
tains the low eight bits of the cylinder number.
1 Byte - System Indicator. For DOS this may be:
1 - 12-bit FAT, 16-bit sector number
4 - 16-bit FAT, 16-bit sector number
5 - Extended Partition
6 - 16-bit FAT, 32-bit sector number
1 Byte - End Head. Head Number of the last sector of the partition
2 Bytes - End Sector + Cylinder. Same format as Start Sector + Cylinder
4 Bytes - First Sector. Number of the first sector of the partition. This
corresponds to the Start Head, Sector + Cylinder. High Byte
comes first.
4 Bytes - Total number of Sectors.
The function check_rootsector_validity checks the following:
- Signature Bytes (55 aa) in the last two bytes of the sector
- not more than one bootable partition
- Bootable flag is 0 or 80h
- Start/End sector of a partition is not 0
- Start/End sector & head are not greater than drive geometry allows
- Start cylinder * sectors * heads + start head * sectors + start sector -
1
= first sector (where sectors is no. of sectors per track, heads is
no. of heads of the drive)
- End cylinder * sectors * heads + end head * sector + end sector = first
sector + number of sectors
- if System Indicator is 0, all other bytes of partition entry are 0
- all partitions except the first begin on cylinder boundaries (head = 0,
sectors = 1)
- all partition end on cylinder boundaries
- partitions don't overlap
- no free space between partitions
The boot sector
---------------
The boot sector is the first sector of every partition. It contains the
program that boots the operating system and the bios parameter block.
The last two bytes must again contain 55 aa. The information in the
boot sector is the following:
00 3 bytes jump instruction ('eb xx 90' or 'e9 xx xx')
03 8 bytes OEM name and version - e.g. MSDOS5.0
0b 2 bytes bytes per sector - should be 512
0d 1 byte sectors per cluster - power of two
0e 2 bytes reserved sectors - typically 1 (boot sector)
10 1 byte number of FATs - must be 2
11 2 bytes number of rootdirectory entries - typically 512
13 2 bytes number of sectors (short) - 0, if BIGDOS partition
15 1 byte media descriptor - typically f8h
16 2 bytes sectors per FAT - varies
18 2 bytes sectors per track
1a 2 bytes number of heads
1c 2 bytes number of hidden sectors (low)
- extended BPB since DOS 4.0 -
1e 2 bytes number of hidden sectors (high)
20 4 bytes number of sectors (long)
24 1 byte physical drive number - 80h or 81h
25 1 byte reserved
26 1 byte signature - 29h
The function check_bootsector_validity checks the following:
- correct jump instruction
- signature bytes 55 aa in the last two bytes of the sector
- bytes per sector = 512
- sectors per cluster is power of two
- reserved sectors = 1
- number of FATs = 2
- number of rootdirectory entries is multiple of 16
- media descriptor = f8h
- sectors per fat <= 256
- sectors per fat big enough to hold complete FAT
- sectors per track matches BIOS info
- number of heads matches BIOS info
- hidden sectors = start sector
- signature = 29h, if BIGDOS
- physical drive number = actual drive number
- number of sectors matches partition info
- system indicator byte in root sector matches partition type
The FAT
-------
The File Allocation Table contains the information how the clusters of the
disk are linked to files. Every directory entry contains a pointer to the
first cluster of the file. The corresponding cluster entry in the FAT con-
tains a pointer to the next cluster, or an EOF marker (FFFF for 16-bit
FATs,
FFF for 12-bit FATs) if the cluster is the last one of the file.
Bad clusters are marked with FFF7 or FF7. Empty clusters are marked with
0.
The first cluster on the disk is cluster number 2, it begins at the first
sector after the root directory. The FAT entries for the clusters 0 and 1
contain the media descriptor byte (usually F8h for harddisk) and two or
three FFh bytes.
There exist two copies of the FAT on a normal DOS partition, these two
copies must be identical. FAT 2 is the primary FAT.
The function check_fat_validity checks if the two FATs are identical and
if
the entries 0 and 1 contain what they are supposed to.
The function check_fat_empty checks if the cluster entries that cover the
new partition contain either 0 (empty) or FFF7 (Bad cluster).
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I hope you find this information useful. If you found anything not to be
exact or if you have additions, please let me know asap.
Arno Schaefer
schaefer@rbg.informatik.th-darmstadt.de
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