Master Boot Record
The Master Boot Record is the same for pretty much all Operating Systems. It is located on the first Sector of the Hard Drive, at Cylinder 0, Head 0, Sector 1. It is the first piece of code that your computer runs after it has checked all of your hardware (POST) and turned control of loading software over the hard drive. It also contains the partition table, which defines the different sections of your hard drive. Basically if anything happens to this little 512 byte section, your hard drive is brain dead. Kinda scary, eh? :)
Offset
Description
Size
000h
Executable Code (Boots Computer)
446 Bytes
1BEh
1st Partition Entry (See Next Table)
16 Bytes
1CEh
2nd Partition Entry
16 Bytes
1DEh
3rd Partition Entry
16 Bytes
1EEh
4th Partition Entry
16 Bytes
1FEh
Boot Record Signature (55h AAh)
2 Bytes
Partition Entry (Part of MBR)
Offset
Description
Size
00h
Current State of Partition (00h=Inactive, 80h=Active)
1 Byte
01h
Beginning of Partition - Head
1 Byte
02h
Beginning of Partition - Cylinder/Sector (See Below)
1 Word
04h
Type of Partition (See List Below)
1 Byte
05h
End of Partition - Head
1 Byte
06h
End of Partition - Cylinder/Sector
1 Word
08h
Number of Sectors Between the MBR and the First Sector in the Partition
1 Double Word
0Ch
Number of Sectors in the Partition
1 Double Word
Cylinder/Sector Encoding
I guess back in the days of 10MB hard drives and 8086's, code was at a premium. So they did everything they could to preserve space. Unfortunately now we have to live with it, but luckily they created new ways of translating the system so the 1024 Cylinder Limit (2^10) isn't too big of a problem, for newer computers, at least. Older ones usually need some sort of Disk Overlay program to make them see the whole hard drive.
Anyway, to get the Sector out of this, you need to apply an AND mask ($3F) to it. To get the Cylinder, you take the high byte and OR it with the low byte that has been AND masked with ($C0) and then Shifted Left Two. It's not very easy to explain, so I'll just show you how I did it with two routines I made (In Pascal) for Encoding and Decoding the Cylinder/Sector. Hopefully even if you don't know Pascal you'll be able to read it.
Function CylSecEncode(Cylinder, Sector : Word) : Word;
Begin
CylSecEncode := (Lo(Cylinder) shl 8) or (Hi(Cylinder) shl 6) or Sector;
End;
Procedure CylSecDecode(Var Cylinder, Sector : Word; CylSec : Word);
Begin
Cylinder := Hi(CylSec) or ((Lo(CylSec) and $C0) shl 2);
Sector := (CylSec and $3F);
End;
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Cylinder Bits 7 to 0
Cylinder Bits 9+8
Sector Bits 5 to 0
Partition Type Listing
There are more than just these shown, but I've only included that ones relevant to MS Operating Systems.
Value
Description
00h
Unknown or Nothing
01h
12-bit FAT
04h
16-bit FAT (Partition Smaller than 32MB)
05h
Extended MS-DOS Partition
06h
16-bit FAT (Partition Larger than 32MB)
0Bh
32-bit FAT (Partition Up to 2048GB)
0Ch
Same as 0BH, but uses LBA1 13h Extensions
0Eh
Same as 06H, but uses LBA1 13h Extensions
0Fh
Same as 05H, but uses LBA1 13h Extensions
Reading Multiple Partitions
Although having multiple partitions in FAT32 isn't as likely as in FAT16, it still works the same way. The first partition is the Primary Partition, and everything else is stored in the Extended Partition. It's a little tricky when it comes to reading those extra partitions though (not a lot, just a little). The first record in the partition table shows where the Primary partition is (how big it is, where it starts, and where it ends). The second entry in the partition table shows where the Entire Extended Partition is (which may include more than just one partition). To read any more partitions, you go to the where it says the Extended Partition starts, and read the first sector. It acts just like the MBR. It'll have blank where the code is supposed to be, and in the partition table it will have for it's first entry the next Partition in the Drive, and if there are anymore, there will be another Extended partition, just like before. However, all references to Sector Numbers are made using the that new MBR point as the reference, making it a virtual drive. Just incase this doesn't make much sense (and by the way I explain things I can understand if it doesn't), let me show you how a drive with three partitions is setup.
MBR of Whole Drive
Entry #1 - Points to Partition #1
Entry #2 - Points to the Entire Extended Partition
You would read the first sector of that Extended Partition, and see another MBR Structure.
MBR of Extended Partition
Entry #1 - Points to Partition #2
Entry #2 - Points to Rest of Extended Partition after Partition #2
Now, all references to Sector Numbers (most specifically the entry at Offset 08h) in those Entries wouldn't be referenced from the start of the drive, but from the start of the Extended Partition. However, the CHS (Cylinder, Head, Sector) numbers would still be right.
Once again, you would read the first sector of that Extended Partition, and see the next MBR.
MBR of Rest of Extended Partition
Entry #1 - Points to Partition #3
No Entry #2, since this was the Last Partition
If there were another partition, the pattern would continue just like before, until the last one was reached.
FAT32 Boot Record
This information is located in the first sector of every partition.
Offset
Description
Size
00h
Jump Code + NOP
3 Bytes
03h
OEM Name (Probably MSWIN4.1)
8 Bytes
0Bh
Bytes Per Sector
1 Word
0Dh
Sectors Per Cluster
1 Byte
0Eh
Reserved Sectors
1 Word
10h
Number of Copies of FAT
1 Byte
11h
Maximum Root Directory Entries (N/A for FAT32)
1 Word
13h
Number of Sectors in Partition Smaller than 32MB (N/A for FAT32)
1 Word
15h
Media Descriptor (F8h for Hard Disks)
1 Byte
16h
Sectors Per FAT in Older FAT Systems (N/A for FAT32)
1 Word
18h
Sectors Per Track
1 Word
1Ah
Number of Heads
1 Word
1Ch
Number of Hidden Sectors in Partition
1 Double Word
20h
Number of Sectors in Partition
1 Double Word
24h
Number of Sectors Per FAT
1 Double Word
28h
Flags (Bits 0-4 Indicate Active FAT Copy) (Bit 7 Indicates whether FAT Mirroring is Enabled or Disabled <Clear is Enabled>) (If FAT Mirroring is Disabled, the FAT Information is only written to the copy indicated by bits 0-4)
1 Word
2Ah
Version of FAT32 Drive (High Byte = Major Version, Low Byte = Minor Version)
1 Word
2Ch
Cluster Number of the Start of the Root Directory
1 Double Word
30h
Sector Number of the File System Information Sector (See Structure Below) (Referenced from the Start of the Partition)
1 Word
32h
Sector Number of the Backup Boot Sector (Referenced from the Start of the Partition)
1 Word
34h
Reserved
12 Bytes
40h
Logical Drive Number of Partition
1 Byte
41h
Unused (Could be High Byte of Previous Entry)
1 Byte
42h
Extended Signature (29h)
1 Byte
43h
Serial Number of Partition
1 Double Word
47h
Volume Name of Partition
11 Bytes
52h
FAT Name (FAT32)
8 Bytes
5Ah
Executable Code
420 Bytes
1FEh
Boot Record Signature (55h AAh)
2 Bytes
File System Information Sector
Usually this is the Second Sector of the partition, although since there is a reference in the Boot Sector to it, I'm assuming it can be moved around. I never got a complete picture of this one. Although I do know where the important fields are at.
Offset
Description
Size
00h
First Signature (52h 52h 61h 41h)
1 Double Word
04h
Unknown, Currently (Might just be Null)
480 Bytes
1E4h
Signature of FSInfo Sector (72h 72h 41h 61h)
1 Double Word
1E8h
Number of Free Clusters (Set to -1 if Unknown)
1 Double Word
1ECh
Cluster Number of Cluster that was Most Recently Allocated.
1 Double Word
1F0h
Reserved
12 Bytes
1FCh
Unknown or Null
2 Bytes
1FEh
Boot Record Signature (55h AAh)
2 Bytes
FAT32 Drive Layout
Offset
Description
Start of Partition
Boot Sector
Start + # of Reserved Sectors
Fat Tables
Start + # of Reserved + (# of Sectors Per FAT * 2) <Assuming that FAT Mirroring is Enabled, I personally haven't seen a case where it wasn't, but I guess there is always the possibility>
Data Area (Starts with Cluster #2)
Cluster Meaning
A Cluster is a Group of Sectors on the Hard Drive that have information in them. A 4K Cluster has 8 Sectors in it (512*8=4096). Each Cluster is given a spot in the FAT Table. When you look at an Entry in the FAT, the number there tells you whether or not that cluster has data in it, and if so, if it is the end of the data or there is another cluster after it. All Data on a Partition starts with Cluster #2. If the FAT Entry is 0, then there is no data in that cluster. If the FAT Entry is 0FFFFFFFh, then it is the last entry in the chain.
This is one of my biggest holes in my information. I am unable to find anyplace that shows what numbers mean what when it comes to the FAT table. I was able to tell the end of the chain just by looking at a FAT32 Drive, but I don't know what stands for a BAD Cluster or what the maximum valid number for showing data is.
For now, you can calculate the maximum valid cluster in a partition with this formula:
( (# of Sectors in Partition) - (# of Sectors per Fat * 2) - (# of Reserved Sectors) ) / (# of Sectors per Cluster)
If there is any remainder in the answer to that formula, it just means that there were a few extra clusters at the end of the partition (probably not enough to make another cluster), so you can just get rid of anything after the decimal point.
Thanks to Andrew Clausen for pointing this formula out to me.
Directory Table
Another aspect when looking at a File System at Low Level is the Directory Table. The Directory Table is what stores all of the File and Directory Entries. Basically there is only one difference between the Directory Table of FAT16 and FAT32, so go here to look at FAT16's Structure. The Difference is : the Reserved OS/2 Byte (Offset 20 [14h]) in the Short Filename Structure is replaced with the High Word of the Cluster Number (since it's now 4 bytes instead of 2).
Footnotes
1 - LBA = Logical Block Addressing - Uses the Int 13h Extensions built into newer BIOS's to access data above the 8GB barrier, or to access strickly in LBA mode, instead of CHS (Cylinder, Head, Sector).
