MSC: unsigned _bios_disk(unsigned cmd,struct diskinfo_t *dskinfo)

	- prototype in bios.h

	- cmd	= _DISK_RESET	resets disk system  (INT 13,0)
		= _DISK_STATUS	return disk status  (INT 13,1)
		= _DISK_READ	read disk sectors  (INT 13,2)
		= _DISK_WRITE	write disk sectors  (INT 13,3)
		= _DISK_VERIFY	verify disk sectors  (INT 13,4)
		= _DISK_FORMAT	format disk track  (INT 13,5)

	- dskinfo = pointer to structure containing disk information:

	struct diskinfo_t
	   {
	   unsigned drive;		- physical drive number
	   unsigned head;		- head number
	   unsigned track;		- track number
	   unsigned sector;		- sector number
	   unsigned nsectors;		- sectors to deal with
	   void _far *buffer;		- data buffer
	   };

	- returns status of operation (AX register, AH contains status)
	- drive numbers represent physical drives (0 = A:, 1 = B:,
	  80h = drive 0, 81h = drive 1
	- disk reads should be retried at least 3 times
	- Turbo C uses biosdisk()

	- see  INT 13