1: #include <stdio.h>
   2: #include "msdos.h"
   3: 
   4: extern int fd, dir_start, num_fat, fat_len, fat_error, fat_bits;
   5: extern unsigned char *fat_buf;
   6: 
   7: /*
   8:  * Puts a code into the FAT table.  Is the opposite of fat_decode().  No
   9:  * sanity checking is done on the code.  Returns a 1 on error.
  10:  */
  11: 
  12: int
  13: fat_encode(num, code)
  14: unsigned int num;
  15: unsigned int code;
  16: {
  17:     int start;
  18: 
  19:     if (fat_bits == 12) {
  20:         /*
  21: 		 *	|    byte n     |   byte n+1    |   byte n+2    |
  22: 		 *	|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|
  23: 		 *	| | | | | | | | | | | | | | | | | | | | | | | | |
  24: 		 *	| n+0.0 | n+0.5 | n+1.0 | n+1.5 | n+2.0 | n+2.5 |
  25: 		 *	    \_____  \____   \______/________/_____   /
  26: 		 *	      ____\______\________/   _____/  ____\_/
  27: 		 *	     /     \      \          /       /     \
  28: 		 *	| n+1.5 | n+0.0 | n+0.5 | n+2.0 | n+2.5 | n+1.0 |
  29: 		 *	|      FAT entry k      |    FAT entry k+1      |
  30: 		 */
  31:                     /* which bytes contain the entry */
  32:         start = num * 3 / 2;
  33:         if (start <= 2 || start + 1 > (fat_len * MSECTOR_SIZE))
  34:             return(1);
  35:                     /* (odd) not on byte boundary */
  36:         if (num % 2) {
  37:             *(fat_buf + start) = (*(fat_buf + start) & 0x0f) + ((code << 4) & 0xf0);
  38:             *(fat_buf + start + 1) = (code >> 4) & 0xff;
  39:         }
  40:                     /* (even) on byte boundary */
  41:         else {
  42:             *(fat_buf + start) = code & 0xff;
  43:             *(fat_buf + start + 1) = (*(fat_buf + start + 1) & 0xf0) + ((code >> 8) & 0x0f);
  44:         }
  45:     }
  46:     else {
  47:         /*
  48: 		 *	|    byte n     |   byte n+1    |
  49: 		 *	|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|
  50: 		 *	| | | | | | | | | | | | | | | | |
  51: 		 *	|         FAT entry k           |
  52: 		 */
  53:                     /* which bytes contain the entry */
  54:         start = num * 2;
  55:         if (start <= 3 || start + 1 > (fat_len * MSECTOR_SIZE))
  56:             return(1);
  57: 
  58:         *(fat_buf + start + 1) =  code / 0x100;
  59:         *(fat_buf + start) =  code % 0x100;
  60:     }
  61:     return(0);
  62: }
  63: 
  64: /*
  65:  * Write the FAT table to the disk.  Up to now the FAT manipulation has
  66:  * been done in memory.  All errors are fatal.  (Might not be too smart
  67:  * to wait till the end of the program to write the table.  Oh well...)
  68:  */
  69: 
  70: void
  71: fat_write()
  72: {
  73:     int fat_start, buflen, dups;
  74:     void disk_write();
  75: 
  76:     if (fd < 0)
  77:         return;
  78: 
  79:     fat_start = dir_start - (fat_len * num_fat);
  80:     buflen = fat_len * MSECTOR_SIZE;
  81: 
  82:     disk_write((long) fat_start, fat_buf, buflen);
  83: 
  84:     /*
  85: 	 * Only duplicate the FAT table if no errors were detected
  86: 	 */
  87:     if (!fat_error) {
  88:         dups = num_fat - 1;
  89:         while (dups--) {
  90:             fat_start += fat_len;
  91:             disk_write((long) fat_start, fat_buf, buflen);
  92:         }
  93:     }
  94:     return;
  95: }
Last modified: 1992-06-24
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