BOOT SECTOR, RDE AND FAT ANALYSIS AND STUDY

FLOPPY CONSTRUCTION 1.Write - Protect Notch. 2.Hub. 3.Shutter. 4.Outer Jacket. 5.Protective Woolen Film. 6.Magnetic Disk. 7.Sectors.

DATA DISTRIBUTION (PHYSICAL) TRACKS SECTORS 2 sides/disk 80 tracks/side 18 sectors/track 512 bytes/sector Total = bytes = 1440 Kbytes = Approx MB Note: While Calculating disk space 1 KB = 1000 bytes

DATA DISTRIBUTION (LOGICAL) SYSTEM AREADATA AREA / FILE AREA 1.44MB FLOPPY SPACE BOOTSECTORBOOTSECTOR 12 FAT ROOT DIRECTORY ENTRY SECTORS BOOT SECTOR = 1 SECTOR FAT = (9+9) 18 SECTORS RDE = 14 SECTORS TOTAL = 33 SECTORS i.e  TWO FAT RECORDS AND IDENTICAL TO EACH OTHER. 1 SECTOR = 1 CLUSTER.

OUR AIM 1. TO DISPLAY THE BOOT SECTOR OF THE FLOPPY DISK. 2. TO DISPLAY THE ROOT DIRECTORY ENTRY. 3. THE DISPLAY THE FAT INFORMATION AND FILE CONTENTS OF A SPECIFIC FILE.

AIM 1 : BOOT SECTOR.  BOOT SECTOR It gives physical characteristics of the disk. As the name suggests boot sector is of 1 sector i.e. 1 cluster. The 2-byte numbers are stored little endian (low order byte first).

Distribution of BOOT SECTOR. Offset in Boot Sector(HEX) Length in Bytes MnemonicStandard Value 003Jump to bootstrap either EB XX 90 (90 is NOP)or E9 XX XX 038OEM_Identifier"IBM 3.3", "IBM 20.0", "MSDOS5.0", "MSWIN4.0". 0B2Bytes Per Sector0200H 0D1Sectors Per Cluster0001H 0E2Reserved Sectors0000H 101Number Of FATs0002H 112No. of Root Directory Entries224H 132Number Of Sectors2880H 151Media Descriptor TableF0H  1.44MB, F9  1.2 MB 162Sectors Per FAT09 182Sectors Per Track18 1A2No. of Heads2 1C4Hidden Sectors0 204Big Number Of Sectors0 241Physical Drive Number00 251Reserved00 261Extended Boot Signature Record29H bit Binary Volume IDXX 2B11Volume LabelVolume Name 368Reserved00 3E1C2Bootstrap( Signature ) Must END WITH 55 AA.

Ending bit of the boot sector are “55 AA”

THE CODING PART  The interrupt used is 25H : absolute disk read interrupt. Input : AL  00H for drive A: CX  X1h = no. of sectors. DX  X2h = starting sector no. DS:BX  offset of buffer.

Values of X1 and X2 CX(X1)DX(X2) BOOT1H0H RDE0EH13H FAT09H01H Example : MOV AL,00H MOV CX,01H MOV DX,00H MOV BX,OFFSET BUFF INT 25H This will point to the starting address of the boot sector.

Root Directory Entry  Immediately after these FAT sectors, the root directory sectors start.  In a FAT file system, root directories comprise 14 sectors.  You know that the maximum directory entry allowed under the root directory for the FAT file system on a 3.5" floppy disk is 224.  The size for root directory entries is (14 * 512) bytes, which is equivalent to 7168 bytes.  Divide 7168 bytes with the max root entries for the FAT; you will get each entry size for the root directory, which is 32 bytes.  Each 32 bytes holds information about a file or directory.

ROOT DIRECTORY ENTRY Byte RangeDescriptionSize 00HShort name for file8 08Hext3 0BH File attribute (Read only, directory, System file, and so on) 1 0CHReservedA 16HTime2 18HDate2 1AH16-bit cluster number2 1CHFile size in bytes4 Notes on the first byte of the FAT directory entry: 1. If the first byte is equal to 0x00, this entry is available and no entry beyond this one has been used. 2. If the first byte is equal to 0xE5, this entry has been erased and is available for use. 3. If the first byte is equal to 0x05, the actual filename character for this byte is 0xE5.

The Date and Time Format Notes on the Date/Time format of the FAT directory entry:  Date Format: The date field is 16-bit. This is relative to 01/01/1980. Bits 0 - 4: Day of month, range 1-31 Bits 5 - 8: Month of year, range 1-12 Bits : Count of years, range 0 – 127  Time format: This is also 16-bit and its granularity is 2 seconds. Bits 0 - 4: 2-second count, valid value range 0-29 inclusive ( seconds) Bits : Minutes, range 0-59 Bits : Hours, range 0-23

FAT (File Allocation Table)  The traditional DOS file system types are FAT12 and FAT16.  Here FAT stands for File Allocation Table: the disk is divided into clusters, the unit used by the file allocation, and the FAT describes which clusters are used by which files.  The FAT12/16 type is important, not only because of the traditional use, but also because it is useful for data exchange between different operating systems, and because it is the file system type used by all kinds of devices, like digital cameras.

 The first cluster of the data area is cluster #2. That leaves the first two entries of the FAT unused.  In the first byte of the first entry a copy of the media descriptor is stored. The remaining bits of this entry are 1.  In the second entry the end-of-file marker is stored. Which is FFFFH.

F0FF F0FF X 1.5 = 3 Even X 1.5 = 4 Odd X 1.5 = 6 F X 1.5 = 7 FFF

 This concludes the study of the floppy disk but it has much to study in the future.  It the simplest data storage device which can be referred to study the other complex and vast data storage devices like hard disks, Compact discs, flash drives and memory cards which has the most complex physical structure.

ANY QUESTION ? References : 1.The WORLD WIDE WEB. 2.WIKIPEDIA. 3.GOOGLE. And 4.TEACHERS.

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