Writing MS-DOS Device Drivers

/*
 * status bits for the return code
 */

#define UNKNOWN_COMMAND 3
#define ERROR       0x8000
#define DONE        0x0100
#define BUSY        0x0200
#define BLK_NOT_FOUND 8

#define MAX_BLK     256 /*  256 blocks       */
#define BLK_SIZE    256 /*  256 bytes/block  */
/*------------- global variables -------------*/

              /* the transfer address specified in
                 the request header */
extern int far *transfer;
              /* the count specified in the request header */
extern int count;
              /* counter */
extern int k1;
              /* counter */
extern int k2;
              /* offset for normalization */
extern unsigned offset;
              /* source pointer */
extern int far *source;
              /* destination pointer */
extern int far *dest;
              /* command specified in request header */
extern char command;
              /* start block specified in request header */
extern int start;

extern  struct parm_block    /* parameter block   */
   {
   unsigned  bps;        /* bytes per block                   */
   char      spau;       /* blocks per allocation unit        */
   unsigned  nrs;        /* number of reserved blocks         */
   char      nfat;       /* number of file allocation tables  */
   unsigned  rent;       /* number of root directory entries  */
   unsigned  tns;        /* total number of blocks    */
   char      mdb;        /* media descriptor byte     */
   unsigned  nsfat;      /* number of blocks per FAT  */
   }   bpb,
       bpb_tab [ ];

/*
 * pointer to the request header
 */

extern struct   request_header
   {
   char         rlength;
   char         unit;
   char         command;
   unsigned     status;
   char         reserved   [ 8 ];
   char         data;
   union
       {
       char far        *transfer;
       char            media_change_code;
       }   b14;
   union
       {
       struct parm_block far   *bpb;
       struct
          {
          unsigned        count;
          unsigned        start;
          } io;
       }   b18;
   } far    *rh;

extern int ram_disk[ ];