1: /*
   2:  * Do full cylinder buffered reads from slow devices.  Uses a simple
   3:  * buffered read/delayed write algorithm.
   4:  */
   5: 
   6: #include <stdio.h>
   7: #include "msdos.h"
   8: 
   9: unsigned char *disk_buf;        /* disk read/write buffer */
  10: int disk_size;              /* size of read/write buffer */
  11: long disk_current;          /* first sector in buffer */
  12: int disk_dirty;             /* is the buffer dirty? */
  13: 
  14: extern int fd;
  15: extern long disk_offset;
  16: 
  17: void
  18: disk_read(start, buf, len)
  19: long start;
  20: unsigned char *buf;
  21: int len;
  22: {
  23:     register long i;
  24:     int length;
  25:     unsigned char *buf_ptr, *disk_ptr;
  26:     char *memcpy();
  27:     long where, tail, lseek();
  28:     void perror(), exit(), disk_flush();
  29: 
  30:                     /* don't use cache? */
  31:     if (disk_size == 1) {
  32:         where = (start * MSECTOR_SIZE) + disk_offset;
  33:         if (lseek(fd, where, 0) < 0) {
  34:             perror("disk_read: lseek");
  35:             exit(1);
  36:         }
  37:                     /* read it! */
  38:         if (read(fd, (char *) buf, (unsigned int) len) != len) {
  39:             perror("disk_read: read");
  40:             exit(1);
  41:         }
  42:         return;
  43:     }
  44: 
  45:     tail = start + (len / MSECTOR_SIZE) - 1;
  46:     for (i = start; i <= tail; i++) {
  47:                     /* a "cache" miss */
  48:         if (i < disk_current || i >= disk_current + disk_size) {
  49: 
  50:             if (disk_dirty)
  51:                 disk_flush();
  52: 
  53:             disk_current = (i / disk_size) * disk_size;
  54:             where = (disk_current * MSECTOR_SIZE) + disk_offset;
  55:             length = disk_size * MSECTOR_SIZE;
  56: 
  57:                     /* move to next location */
  58:             if (lseek(fd, where, 0) < 0) {
  59:                 perror("disk_read: lseek");
  60:                 exit(1);
  61:             }
  62:                     /* read it! */
  63:             if (read(fd, (char *) disk_buf, (unsigned int) length) != length) {
  64:                 perror("disk_read: read");
  65:                 exit(1);
  66:             }
  67:         }
  68:                     /* a cache hit... */
  69:         buf_ptr = buf + ((i - start) * MSECTOR_SIZE);
  70:         disk_ptr = disk_buf + ((i - disk_current) * MSECTOR_SIZE);
  71:         memcpy((char *) buf_ptr, (char *) disk_ptr, MSECTOR_SIZE);
  72:     }
  73:     return;
  74: }

Defined functions

Defined variables

disk_buf defined in line 9; used 8 times
disk_current defined in line 11; used 14 times
disk_dirty defined in line 12; used 6 times
disk_size defined in line 10; used 20 times
Last modified: 1992-06-24
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