Hong Kong Polytechnic University Department of Electronic and Information Engineering. Experiment On DOS File system

Transcription

1 DOSFS/CC/v1 Hong Kong Polytechnic University Department of Electronic and Information Engineering Experiment On DOS File system Objectives : To study how an OS manages its file system in a floppy disk. After completing this experiment, you should know the organization of a floppy disk, and know how DOS uses directories, boot and File Allocation Table (FAT) to manage its disk files. Software : Appratus : Reference : The DEBUG monitor program (to be run under MSDOS). A formatted Floppy disk with data files H-P. Messmer, "The indispensable PC hardware book," 3rd Ed, Addison-Wesley, 1997 Chapter 30 Section 1-4. Concept In order to store data on the disk, both sides of a floppy disk (platter in hard disk) are coated with magnetic materials. Each side of a double-sided diskette is organized into tracks and s. Figure A1 shows an example of the organization. Tracks are organized as concentric circles and their number per disk varies from disk to disk, depending on the size and the technology. Each track is divided into a number of s, and again the number of s per track varies, depending on the density of the disk and the operating system version used to format the disk. Table A1 shows the parameters of some common floppy disks. The s are grouped into clusters. Cluster size varies among formats, but a common size for floppy disks is 1 per cluster. The file allocation table (FAT) keeps track of what clusters are used to store which files. When a disk is formatted, the first is used for the boot record. It is from the boot record that the computer will be able to ascertain the disk type, density, total number of s in the disk, and other essential information needed by BIOS and the operating system. Table A2 describes the details of the boot record. CSF-LAB/File System/CC/EIE PolyU/p.1

2 After the operating system allocates at least one for the boot record and several s for the FAT, it allocates some s for the directory. Which and how many s are allocated for the directory vary among the different disk types. The function of the FAT is to provide a road map for the operating system to find where each file is located. In a floppy (hard) disk, cluster is the basic unit used to store a file. The size of a file could be too large to fit into a cluster. In that case, more than one clusters are used to store the file. FAT is used to tell which clusters are used and how they are linked together to store the file. In fact, the FAT is so critical to the operating system's ability to locate files that two copies of the FAT are kept on the disk, one for use and another one for backup in case something happens to the first one. Their consistency is handled by the OS automatically. If both are damaged, the OS cannot find any file on that disk. The 2 FATs are always located in the s immediately following the boot record. Three FAT formats, namely FAT12, FAT16 and FAT32, are commonly used nowadays. They use 12 bits, 16 bits and 32 bits respectively to record an entry in a FAT. The number of s used by a FAT varies depending on the size and density of the disk. The first 2 entries of a FAT contain the media descriptor byte, followed by F's to pad the remaining space. There is a one-to-one correspondence between each of the remaining entries in the FAT and each cluster on the disk. In other words, if there are 355 clusters on the disk, there will be 355 FAT entries for these clusters. Each FAT entry indicates the status of that cluster as shown in Table 1. Table 1: FAT entry codes and their meanings Code FAT12 FAT16 Meaning 000H 0000H Unused cluster: cluster has never been used 001H 0001H Free cluster: cluster was used previously but is now free 002-FEFH 0002-FFEFH Cluster is used by a file FF0-FF6H FFF0-FFF6H Reserved FF7H FFF7H Bad cluster--cannot be used FF8-FFFH FFF8-FFFFH Last cluster of a file In order to understand the boot record, disk directory, and FAT's functions, we will use the DEBUG program to access the disk s and dump the information into memory, analyzing it byte by byte. First a couple of items about DEBUG: The -L command can be used to load the specific s of a given disk into a specific area of RAM. The -L command is followed by the address that data from the disk should be loaded into, followed by the drive number, then the starting number, and finally the number of s to be loaded. The drive number is 0 for drive A, 1 for drive B, 2 for drive C, and so on. Sectors are stored starting with 0. All numbers for DEBUG are given in hex. For example, C> DEBUG -L CS: CSF-LAB/File System/CC/EIE PolyU/p.2

3 This will load the boot record ( 0) from drive A (drive number 0), and will load only one. After the load, the -D command is used to dump memory onto the screen. The -D command needs to have an address to display from (usually this is the same address you used to load), and a number of bytes to display (in hex again). For example, C> DEBUG -L CS: D CS:100 17F These commands will load 0 from drive A into memory, then dump the first 17F bytes of that memory to the display. Note that, in Intel machines, the least significant byte of data is always stored in the lower memory location. To store 35F6 16, in memory locations 1300 and 1301, for example, will make memory location 1300 contain F6, and 1301 contain 35. Method Using a diskette and the DEBUG program, analyze the boot record, FAT table, and directory structure of a floppy diskette. CSF-LAB/File System/CC/EIE PolyU/p.3

4 Experiment Name Group Date Section A Analysis of Boot Sector Load Sector 0 of the provided floppy disk into CS:100. Fill in the following boot record layout sheet. Item Hex offset As seen Real Hex on disk value Meaning / description 1 0B-0C Bytes per Sector = 2 0D Sector(s) per cluster = 3 0E-0F Reserved (starting at Sector 0) = 4 10 FATs on the diskette = Possible Root Directory entries = Total Sectors on the diskette = Total # of bytes on the diskette = 7 15 Media Descriptor Byte = (read Table A1 for its meaning) Sectors per FAT = Sectors per track = 10 1A-1B Sides (or Heads) = 11 1C-1F Number of hidden Sectors = (Should be 0 for a floppy disk) Total Sectors on the diskette = Used for disk contains more than Sectors. Could be other meaning for different OS versions Reserved, should be all Extended BPB Signature Byte (if present, then the following bytes will also have meaning) A Volume Serial Number Result Dec. value ASCII value 16 2B-35 Volume Label B File System ID From the table above, get or derive the information of the Boot, the first and the second FATs (FAT1 & FAT2), and the root directory to fill in the following table accordingly. Boot FAT1 FAT2 Root directory Size (in terms of number of Sectors) Sectors occupied (from which No. to which No.?) Sector number of the starting In DEC In HEX value value CSF-LAB/File System/CC/EIE PolyU/p.4

5 Section B Analysis of Root Directory From the result you obtained in Section A, you should know which on the diskette is the first of the diskette root directory. Load this to CS:100 and analyze it. You should be able to find some information of 6 files and the disk label in the diskette root directory. Based on the information available in the first of the diskette root directory and Table A3 in the Appendix, determine the files' file attribute, starting cluster, creation date, creation time and file size, and fill in the following table. Attribute Name To be archived (Yes/No) Directory/File (D/F) Volume Label (Yes/No) System file (Yes/No/NA) Hidden File (Yes/No/NA) Read only (Yes/No) Starting cluster Creation time Creation date File size 1 IO.SYS 2 MSDOS.SYS 3 COMMAND.COM 4 DRVSPACE.BIN 5 LAB BDISK 6 LETTER.TXT 7 DATA.GIF You may use Window Explorer to get the corresponding information and compare it with that you derived from the Sector. Section C Analysis of FAT Based on the analysis results obtained in Section A, determine the first s of FAT1 and FAT2. Load the two s to CS:100 and CS:400 respectively, and verify if they are exactly the same. FAT is used to tell which clusters are used and how they are linked together to store the file. You should already get the starting cluster number of file 'DATA.GIF' in the previous Section. Now, starting with this cluster number, determine the cluster chain of file 'DATA.GIF'. The cluster number mentioned so far is actually the sequential order of the corresponding entry in a FAT. To determine the physical location of the corresponding in the diskette, one can use a translation formula given as follows. S = C 2 + Sb + N f S f + Sr Where S = Sector number (which Sector in the diskette) C = Cluster number (which entry in FAT) CSF-LAB/File System/CC/EIE PolyU/p.5

6 S b = Size of the Boot Sector in number of s N = Number of FATs f S = Size of FAT in number of s f S = Size of the root directory in number of s r This formula is valid when a cluster contains 1. Similar formulas can be derived for other cases. Use the above formula to determine the where file 'letter.txt' is in. Load the into cs:100 and verify if the file is really there. - END - CSF-LAB/File System/CC/EIE PolyU/p.6

7 Appendix Byte Medium Double/ Sectors/ Track/ single sided Track side Size DOS version 0F8H Hard disk F0H 3.5" D Mbyte F9H 3.5" D kbyte F9H 5.25" D Mbyte FAH 5.25" S kbyte FAH 3.5" S kbyte FBH 5.25" D kbyte FBH 3.5" D kbyte FCH 5.25" S kbyte FDH 5.25" D kbyte FDH 8" D Mbyte FEH 5.25" S kbyte FEH 8" S kbyte FEH 8" S kbyte FEH 8" S kbyte FFH 5.25" D kbyte 1.10 Table A1. Medium descriptor bytes of different disk formats Item Hex offset Meaning / description 1 0B-0C Bytes per Sector 2 0D Sector(s) per cluster 3 0E-0F Reserved (starting at Sector 0) 4 10 FATs on the diskette Possible Root Directory entries Total Sectors on the diskette 7 15 Media Descriptor Byte (refer to Table A1 for its meaning) Sectors per FAT Sectors per track 10 1A-1B Sides (or Heads) 11 1C-1F Number of hidden Sectors (Should be 0 for a floppy disk) Total Sectors on the diskette Used for disk contains more than Sectors. Could be other meaning for different OS versions Reserved, should be all Extended BPB Signature Byte (if present, then the following bytes will also have meaning) A Volume Serial Number 16 2B-35 Volume Label B File System ID Table A2. Boot record CSF-LAB/File System/CC/EIE PolyU/p.7

8 00h File name 1 (8 bytes, ASCII) 08h 0bh 0ch Extension Attribute 2 Reserved (3 bytes, ASCII) (1 byte) (10 bytes) 32 bytes 16h 18h 1ah 1ch Time of creation/last Change 3 Date of creation/last Change 4 Start cluster File length (2 byte) (2 bytes) (2 bytes, binary) (4 bytes, binary) 1 Values of the 1st character with special meaning: 00h=entry was never in use e5h=entry(file/directory) is deleted 2eh entry is a directory 2 Attribute byte res A D V S H R A- set when file is modified, cleard when it is baked up D- set to indicate that the entry is for a directory V- set to indicate that the entry is for volume label S- set for system files H- set for hidden files R- read-only 3 Time = 2048 x hours + 32 x minutes + seconds/2 4 Date = 512 x (year-1980) + 32 x month+day Table A3. Structure of root directory and subdirectories track 79 track Figure A1. Tracks and s in a floppy disk Note similar arrangement can be found in a hard disk. CSF-LAB/File System/CC/EIE PolyU/p.8