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Lecture # 41
#include <stdio.h> #include <dos.h> #include <bios.h> #include <alloc.h> typedef struct tagfcb { unsigned char filename [8]; unsigned char ext[3]; unsigned char attrib; unsigned char reserved [10]; unsigned int time,date; unsigned int firstcluster; unsigned long int size; }FCB; typedef struct tagBPB { unsigned int byte spersec; unsigned char secperclust; unsigned int reservedsecs; unsigned char fats; unsigned int rootdirents; unsigned int smallsecs; unsigned char media;
unsigned int fatsecs; unsigned int secspertrack; unsigned int heads; unsigned long int hiddensecs; unsigned long int hugesecs; unsigned char driveno; unsigned char reserved; unsigned char bootsignature; unsigned long int volumeid; unsigned char volumelabel[11]; unsigned char filesystem[8]; }BPB;
struct bootblock { unsigned char jumpinst[3]; unsigned char osname[8]; BPB bpb; unsigned char code[448]; };
DPB far * getdpb(int drive) { DPB far *dpb=(DPB far *)0; _asm push ds; _asm mov ah,0x _asm mov dl,byte ptr drive; _asm mov dx,ds; _asm int 0x21; _asm pop ds _asm cmp al,0xff _asm je finish _asm mov word ptr dpb+2,dx _asm mov word ptr dpb,bx return dpb; finish: return ((DPB far *)(0)); }
void main (void) { unsigned char filename[9]; struct bootblock bb; unsigned char ext[4]; FCB * dirbuffer; unsigned int * FAT; DPB d; DPB far * dpbptr; int i,flag; unsigned int cluster; puts("Enter filename:"); gets (filename); puts("Enter Extension"); gets(ext); if ((absread(0x05,1,0, &bb))==0) puts ("Success"); else{ puts("Failure"); exit(0); }
• Quick Format
-- initializes the data structures for file
management.
-- initializes and sets the size of FAT, root
directory etc, according to the drive size.
-- initializes the data in boot block and
places appropriate boot strap code for
the boot block.
Disk Partitioning Software
• Write the code part of partition table to
appropriately load the Boot Block of active
partition in primary partition table.
• Places data in the partition table regarding
primary and extended partitions.
• As per specification of the user assigns a
appropriate size to primary and extended
partition by modifying their data part.
Scan Disk
Surface Scan for Bad Sectors
• It attempts to write a block.
• After write it reads back the block contents.
• Performs the CRC test on data read back.
• If there is an error then the data on that block
is not stable the cluster of that block should be
marked bad.
• The cluster is marked bad by placing the
appropriate code for bad cluster so that they
may not be allocated to any file.
Lost Chains
• The disk scanning software may also look
for lost chains.
• Lost chains are chains in FAT which
apparently don’t belong to any file.
• They may occur due to some error in the
system like power failure during deletion
process.
Defragmenter
- Disk fragmentation is unwanted.
- Fragmentation means that clusters of a same file are not
contiguously placed, rather they are far apart, increasing seek
time hence access time.
- So its desirable that files clusters may be placed contiguously,
this can be done by compaction or defragme ntation.
- Defragmentation Software reserves space for each file in
contiguous block by moving the data in clusters and
readjusting.
- As a result of defragmentation the FAT entries will change
and data will move from one cluster to other localized cluster to
reduce seek time.
- Defragmentation has high computation cost and thus cannot
be performe d frequently.
File Restoration
- FAT structure provides the possibility of recovering a file
after deletion, if its clusters were contiguous and have not been
over-written.
- DOS perform file deletion by placing 0xE5 at the first byte of
it FCB entry and placing 0’s (meaning available) in the entries
for the file clusters in the FAT.
- Two task should be performed successfully to undelete a file
-- Replacing the 0xE5 entry in FCB by a valid file name
character.
-- placing the appropriate values in FAT for
representation of file cluster chain.
- If any one of the above cannot be done then the file cannot be
fully recovered.
