Disk Utilities-System Programming-Lecture Notes, Study notes of System Programming

Download Disk Utilities-System Programming-Lecture Notes and more Study notes System Programming in PDF only on Docsity! 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 bytespersec; 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]; }; docsity.com DPB far * getdpb(int drive) { DPB far *dpb=(DPB far *)0; _asm push ds; _asm mov ah,0x32 _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); } docsity.com 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. docsity.com Looking for Lost Chains • For each file entry in the directory structure its chain in FAT is traversed. • All the cluster in the file are marked. • When done with all the files and folders, if some non-zero and non-reserved clusters are left then they belong to some lost chains. • The lost chains are firstly discretely identified and then each chain can either be restored to newly named files or can be deleted. Cross References • If a cluster lie in more than one file chain, then its said to be Cross Referenced. • Cross references can pose great problems. • Cross references can be detected easily by traversing through the chain of all files and marking the cluster # during traversal. • If a cluster is referenced more than once then it indicates a cross reference. • To solve the problem only one reference should be maintained. 7 14 EOF 9 7 53 5 7 11 13 14 docsity.com 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 defragmentation. • 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 performed 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. docsity.com