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Lecture # 33
The Root Directory
root
The DOS directory structure is a Tree like structure. The top most level of the tree being
the root directory. The root directory contains files and folders. Each folder can contains
more files and folders and so on it continues recursively as shown by the slide.
File
• Is logically viewed as an organization of Data.
• Physically it can be a collection of clusters or
blocks.
• O.S. needs to maintain information about the
cluster numbers of which a file may be comprised
of.
Control information about files are maintained in a data structure called the File control
block (FCB). The FCB for each file is created and stored in the disk.
File Control Block (FCB)
0B BYTE Attribute
1Ah WORD First Cluster
1Ch DWORD size
18h WORD Date
16h WORD Time
0C BYTE [10] Reserved + used by OS /
08 BYTE [3] File extension
00 BYTE [8] File Name in 8 bytes
Offset TYPE Description
The root directory consists of FCBs for all the files and folders stored on the root
directory. To obtains these FCBs, the portion on disk reserved for root directory can be
read.
User
Data
Root
Dir
FAT FAT
Boot Block
Location of Root Directory
The user data area is divided into clusters. The first cluster in user data area is numbered
2 in a FAT based systems. A cluster is not the same as block and also there are no system
calls available which use the cluster number. All the system calls use the LSN address. If
the cluster number is known it should be converted into LSN to access the blocks within
the cluster. Moreover all the information about file management uses the cluster number
rather than the LSN for simplicity and for the purpose of managing large disk space. So
here we devise a formula to convert the cluster number into LSN.
Root User Data
Directory
FAT FAT
Re served Block Clust 2
Blocks for FAT
Converting a Cluster # into Sector
First Block of User Data
No. of entries*32/byte s per block
using the information the above slide the following formula can be devised as shown in
the slide below to convert the cluster number into LSN.
No. of System Area Blocks =
Reserved Block + Sector per FAT * No. of
FAT’s + No. of entries * 32 / Bytes per Block
First User Block No. =
No. of System Area Blocks
Sector No. =
(Clust_no – 2)* Blocks per Clust + First User
Block #
The following memory dump extracts the starting cluster number from the FCB of a file
and then converts the cluster number in sector to get its starting block.
Directory Dump (Again)
-l 1000 0 13 e
-d 1000 2c
13AD:1000 E5 5F 41 4E 53 20 20 20-54 58 54 20 10 BB 19 70 ._ANS TXT ...p 13AD:1010 3C 33 3C 33 00 00 60 05-72 28 02 00 4F 8F 00 00 <3<3...r(..O... 13AD:1020 41 43 00 70 00 61 00 70-00 65 00 0F 00 54 72 00 AC.p.a.p.e...Tr. 13AD:1030 2E 00 74 00 78 00 74 00-00 00 00 00 FF FF FF FF ..t.x.t......... 13AD:1040 43 50 41 50 45 52 20 20-54 58 54 20 00 BB 19 70 CPAPER TXT ...p 13AD:1050 3C 33 3C 33 00 00 60 05-72 28 02 00 4F 8F 00 00 <3<3...r(..O... 13AD:1060 54 45 53 54 20 20 20 20-54 58 54 20 18 AB 29 70 TEST TXT ..)p 13AD:1070 3C 33 3C 33 00 00 AE 70-3C 33 4A 00 31 00 00 00 <3<3...p<3J.1... 13AD:1080 E5 4F 4F 54 20 20 20 20-54 58 54 20 18 4D 4E 70 .OOT TXT. MNp 13AD:1090 3C 33 3C 33 00 00 D5 70-3C 33 4B 00 B3 8A 00 00 <3<3...p<3K..... 13AD:10A0 E5 4F 4F 54 20 20 20 20-54 58 54 20 18 4D 4E 70 .OOT TXT. MNp 13AD:10B0 3C 33 3C 33 00 00 22 71-3C 33 4B 00 26 00 00 00 <3<3.."q<3K.&... 13AD:10C0 52 4F 4F 54 20 20 20 20-54 58 54 20 18 25 55 71 ROOT TXT .%Uq 13AD:10D0 3C 33 3C 33 00 00 56 71-3C 33 4B 00 0D 00 00 00 <3<3..Vq<3K.....
File Contents =
File Cluster # = 4a File Start Sector # = (4a – 2 )*1 + 21H = 69H
The contents of the blocks/cluster at the start of file are then examined by loading the
sectors within the first cluster to that file in the following slide. Here the contents of the
file can be seen on the right side column.
