fatSupport.c

/******************************************************************************
 * Supporting functions for the FAT project:
 *
 *  read_sector
 *  write_sector
 *
 *  get_fat_entry
 *  set_fat_entry
 *  
 * Authors: Andy Kinley, Archana Chidanandan, David Mutchler and others.
 *          March, 2004.
 *****************************************************************************/

#include <stdio.h>

/******************************************************************************
 * FILE_SYSTEM_ID -- the file id for the file system (here, the floppy disk
 *                   filesystem)
 * BYTES_PER_SECTOR -- the number of bytes in each sector of the filesystem
 *****************************************************************************/

extern FILE* FILE_SYSTEM_ID;
extern int BYTES_PER_SECTOR;

/******************************************************************************
 * read_sector
 *
 * Read the specified sector from the file system and store that sector in the
 * given buffer
 *
 * sector_number:  The number of the sector to read (0, 1, 2, ...)
 * buffer:  The array into which to store the contents of the sector that is
 *          read
 *
 * Return: the number of bytes read, or -1 if the read fails.
 *****************************************************************************/

int read_sector(int sector_number, unsigned char* buffer)
{
   int bytes_read;

   if (fseek(FILE_SYSTEM_ID,
             (long) sector_number * (long) BYTES_PER_SECTOR, SEEK_SET) != 0)
   {
	   printf("Error accessing sector %d\n", sector_number);
      return -1;
   }

   bytes_read = fread(buffer, sizeof(char), BYTES_PER_SECTOR, FILE_SYSTEM_ID);

   if (bytes_read != BYTES_PER_SECTOR)
   {
      printf("Error reading sector %d\n", sector_number);
      return -1;
   }

   return bytes_read;
}


/*****************************************************************************
 * write_sector
 *
 * Write the contents of the given buffer to the filesystem at the specified
 * sector
 *
 * sector_number:  The number of the sector to write (0, 1, 2, ...)
 * buffer:  The array whose contents are to be written
 *
 * Return: the number of bytes written, or -1 if the read fails.
 ****************************************************************************/

int write_sector(int sector_number, unsigned char* buffer) 
{
   int bytes_written;

   if (fseek(FILE_SYSTEM_ID,
       (long) sector_number * (long) BYTES_PER_SECTOR, SEEK_SET) != 0) 
   {
      printf("Error accessing sector %d\n", sector_number);
      return -1;
   }

   bytes_written = fwrite(buffer,
                          sizeof(char), BYTES_PER_SECTOR, FILE_SYSTEM_ID);

   if (bytes_written != BYTES_PER_SECTOR) 
   {
      printf("Error reading sector %d\n", sector_number);
      return -1;
   }

   return bytes_written;
}


/*****************************************************************************
 * get_fat_entry
 *
 * Get the specified entry from the given FAT
 *
 * fat_entry_number:  The number of the FAT entry to get (0, 1, 2, ...)
 * fat:  The fat table from which to get the specified entry
 *
 * printf("Directory listing returned %d entries:\n", dnum);
 * Return: the value at the specified entry of the given FAT
 ****************************************************************************/

int get_fat_entry(int fat_entry_number, unsigned char* fat) 
{
   int offset;
   int uv, wx, yz;

   offset = 3 * fat_entry_number / 2;

   // Two FAT12 entries are stored into three bytes;
   // if these bytes are uv,wx,yz then the two FAT12 entries are xuv and yzw

   // odd fat entry number, return yzw
   if (fat_entry_number & 0x0001) 
   {
      wx = (int) fat[offset];
      yz = (int) fat[offset + 1];
      return ( (yz << 4)  |  ( (wx & 0x00f0) >> 4));
   } 
   // even fat entry number, return xuv
   else 
   {
      uv = (int) fat[offset];
      wx = (int) fat[offset + 1];
      return ( ((wx & 0x000f) << 8)  |  uv );
   }
}


/******************************************************************************
 * set_fat_entry
 *
 * Set the specified entry in the given FAT to the given value
 *
 * fat_entry_number:  The number of the FAT entry to set (0, 1, 2, ...)
 * value:  The given value to place in the FAT entry
 * fat:  The fat table in which to set the given value at the specified entry
 *****************************************************************************/

void set_fat_entry(int fat_entry_number, int value, unsigned char* fat) 
{
   int offset;
   int uv, wx, yz, a, b, c;

   offset = 3 * fat_entry_number / 2;

   // Two FAT12 entries are stored into three bytes;
   // if these bytes are uv,wx,yz then the two FAT12 entries are xuv and yzw
   // Let 0a,bc denote the fat_entry_number, written as two bytes (high and
   // low, respectively)

   a = value & 0x0f00;
   b = value & 0x00f0;
   c = value & 0x000f;

   // odd fat entry number, change yzw to abc, i.e.,
   if (fat_entry_number & 0x0001) 
   {
      // wx = cx;
      fat[offset]     = (unsigned char) ((c << 4)  |  (fat[offset] & 0x000f));
      // yz = ab;
      fat[offset + 1] = (unsigned char) ((a >> 4)  |  (b >> 4));
   }
   // even fat entry number, change xuv to abc, i.e.,
   else
   {
      // uv = bc;
      fat[offset]     = (unsigned char) (b | c);
      // wx = wa;
      fat[offset + 1] = (unsigned char) ((fat[offset + 1]  & 
                                          0x00f0)  |  (a >> 8));
   }
}