#include <stdio.h>#include "msdos.h"#define CHK_FATint fat_len;				/* length of FAT table (in sectors) */unsigned int end_fat;			/* the end-of-chain marker */unsigned int last_fat;			/* the last in a chain marker */unsigned char *fat_buf;			/* the File Allocation Table */extern int fat_bits;extern unsigned num_clus;/* * Get and decode a FAT (file allocation table) entry.  Returns the cluster * number on success or 1 on failure. */unsigned intfat_decode(num)unsigned int num;{	unsigned int fat, fat_hi, fat_low, byte_1, byte_2, start;	if (fat_bits == 12) {		/*		 *	|    byte n     |   byte n+1    |   byte n+2    |		 *	|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|		 *	| | | | | | | | | | | | | | | | | | | | | | | | |		 *	| n+0.0 | n+0.5 | n+1.0 | n+1.5 | n+2.0 | n+2.5 |		 *	    \_____  \____   \______/________/_____   /		 *	      ____\______\________/   _____/  ____\_/		 *	     /     \      \          /       /     \		 *	| n+1.5 | n+0.0 | n+0.5 | n+2.0 | n+2.5 | n+1.0 |		 *	|      FAT entry k      |    FAT entry k+1      |		 */					/* which bytes contain the entry */		start = num * 3 / 2;		if (start <= 2 || start + 1 > (fat_len * MSECTOR_SIZE))			return(1);		byte_1 = *(fat_buf + start);		byte_2 = *(fat_buf + start + 1);					/* (odd) not on byte boundary */		if (num % 2) {			fat_hi = (byte_2 & 0xff) << 4;			fat_low = (byte_1 & 0xf0) >> 4;		}					/* (even) on byte boundary */		else {			fat_hi = (byte_2 & 0xf) << 8;			fat_low = byte_1 & 0xff;		}		fat = (fat_hi + fat_low) & 0xfff;	}	else {		/*		 *	|    byte n     |   byte n+1    |		 *	|7|6|5|4|3|2|1|0|7|6|5|4|3|2|1|0|		 *	| | | | | | | | | | | | | | | | |		 *	|         FAT entry k           |		 */					/* which bytes contain the entry */		start = num * 2;		if (start <= 3 || start + 1 > (fat_len * MSECTOR_SIZE))			return(1);		fat = (*(fat_buf + start + 1) * 0x100) + *(fat_buf + start);	}	return(fat);}/* * Read the entire FAT table into memory.  Crude error detection on wrong * FAT encoding scheme. */voidfat_read(start)int start;{	int i, fat_size;	unsigned int buflen;	char *malloc();	void perror(), exit(), disk_read();#ifdef INT16	long junk;	junk = (long) fat_len * MSECTOR_SIZE;		if (junk > 65535L) {		fprintf(stderr, "fat_read: FAT table is too large\n");		exit(1);	}#endif /* INT16 */	/*	 * Let's see if the length of the FAT table is appropriate for	 * the number of clusters and the encoding scheme	 */#ifdef CHK_FAT	fat_size = (fat_bits == 12) ? (num_clus +2) * 3 / 2 : (num_clus +2) * 2;	fat_size = (fat_size / 512) + ((fat_size % 512) ? 1 : 0);	if (fat_size != fat_len) {		fprintf(stderr, "fat_read: Wrong FAT encoding?\n");		exit(1);	}#endif /* CHK_FAT */					/* only the first copy of the FAT */	buflen = fat_len * MSECTOR_SIZE;	fat_buf = (unsigned char *) malloc(buflen);	if (fat_buf == NULL) {		perror("fat_read: malloc");		exit(1);	}					/* read the FAT sectors */	for (i=start; i<start+fat_len; i++)		disk_read((long) i, &fat_buf[(i-start)*MSECTOR_SIZE], MSECTOR_SIZE);					/* the encoding scheme */	if (fat_bits == 12) {		end_fat = 0xfff;		last_fat = 0xff8;	}	else {		end_fat = 0xffff;		last_fat = 0xfff8;	}	return;}