2.11BSD#431 - /usr/src/local/mtools/fat_read.c

   1: #include <stdio.h>
   2: #include "msdos.h"
   3: 
   4: int fat_len;                /* length of FAT table (in sectors) */
   5: unsigned int end_fat;           /* the end-of-chain marker */
   6: unsigned int last_fat;          /* the last in a chain marker */
   7: unsigned char *fat_buf;         /* the File Allocation Table */
   8: 
   9: extern int fat_bits;
  10: 
  11: /*
  12:  * Get and decode a FAT (file allocation table) entry.  Returns the cluster
  13:  * number on success or 1 on failure.
  14:  */
  15: 
  16: unsigned int
  17: fat_decode(num)
  18: unsigned int num;
  19: {
  20:     unsigned int fat, fat_hi, fat_low, byte_1, byte_2;
  21:     int start;
  22: 
  23:     if (fat_bits == 12) {
  24:         /*
  25: 		 *	|    byte n     |   byte n+1    |   byte n+2    |
  26: 		 *	|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|
  27: 		 *	| | | | | | | | | | | | | | | | | | | | | | | | |
  28: 		 *	| n+0.0 | n+0.5 | n+1.0 | n+1.5 | n+2.0 | n+2.5 |
  29: 		 *	    \_____  \____   \______/________/_____   /
  30: 		 *	      ____\______\________/   _____/  ____\_/
  31: 		 *	     /     \      \          /       /     \
  32: 		 *	| n+1.5 | n+0.0 | n+0.5 | n+2.0 | n+2.5 | n+1.0 |
  33: 		 *	|      FAT entry k      |    FAT entry k+1      |
  34: 		 */
  35:                     /* which bytes contain the entry */
  36:         start = num * 3 / 2;
  37:         if (start <= 2 || start + 1 > (fat_len * MSECTOR_SIZE))
  38:             return(1);
  39: 
  40:         byte_1 = *(fat_buf + start);
  41:         byte_2 = *(fat_buf + start + 1);
  42:                     /* (odd) not on byte boundary */
  43:         if (num % 2) {
  44:             fat_hi = (byte_2 & 0xff) << 4;
  45:             fat_low = (byte_1 & 0xf0) >> 4;
  46:         }
  47:                     /* (even) on byte boundary */
  48:         else {
  49:             fat_hi = (byte_2 & 0xf) << 8;
  50:             fat_low = byte_1 & 0xff;
  51:         }
  52:         fat = (fat_hi + fat_low) & 0xfff;
  53:     }
  54:     else {
  55:         /*
  56: 		 *	|    byte n     |   byte n+1    |
  57: 		 *	|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|
  58: 		 *	| | | | | | | | | | | | | | | | |
  59: 		 *	|         FAT entry k           |
  60: 		 */
  61:                     /* which bytes contain the entry */
  62:         start = num * 2;
  63:         if (start <= 3 || start + 1 > (fat_len * MSECTOR_SIZE))
  64:             return(1);
  65: 
  66:         fat = (*(fat_buf + start + 1) * 0x100) + *(fat_buf + start);
  67:     }
  68:     return(fat);
  69: }
  70: 
  71: /*
  72:  * Read the entire FAT table into memory.
  73:  */
  74: 
  75: void
  76: fat_read(start)
  77: int start;
  78: {
  79:     int buflen;
  80:     char *malloc();
  81:     void perror(), exit(), disk_read();
  82:                     /* only the first copy of the FAT */
  83:     buflen = fat_len * MSECTOR_SIZE;
  84:     fat_buf = (unsigned char *) malloc((unsigned int) buflen);
  85:     if (fat_buf == NULL) {
  86:         perror("fat_read: malloc");
  87:         exit(1);
  88:     }
  89:                     /* read the FAT sectors */
  90:     disk_read((long) start, fat_buf, buflen);
  91: 
  92:                     /* the encoding scheme */
  93:     if (fat_bits == 12) {
  94:         end_fat = 0xfff;
  95:         last_fat = 0xff8;
  96:     }
  97:     else {
  98:         end_fat = 0xffff;
  99:         last_fat = 0xfff8;
 100:     }
 101:     return;
 102: }

Defined functions

Defined variables