TC: int biosdisk( int cmd, int drive, int head, int track,

			    int sector, int nsects, void *buffer )

	- prototype in bios.h

	- issues interrupt 0x13 for BIOS disk operations
	- drive = (0=A:, 1=2nd floppy, 0x80=drive 0, 0x81=drive 1
	- cmd = 0  reset diskette system (hard disk drive reset)
		1  return status of last disk operation, ignore parms
		2  read nsects disk sectors, with starting sector
		   defined by head, track, & sector, reading 512 bytes
		   per sector into buffer
		3  write nsects disk sectors, with starting sector
		   defined by head, track, & sector, writing 512 bytes
		   per sector from buffer
		4  verify nsects disk sectors, with starting sector
		   defined by head, track, & sector
		5  format a track defined by head & track, where buffer
		   points to a table of sector headers to be written

	 For XT or AT only:

	    6 - formats a track & sets bad sector flags
	    7 - formats drive beginning at specific track
	    8 - returns current drive parms in 1rst 4 bytes of buffer
	    9 - initializes drive-pair characteristics
	   10 - long read (4 extra bytes per sector for ECC)
	   11 - long write (4 extra bytes per sector for ECC)
	   12 - disk seek
	   13 - alternate disk reset
	   14 - reads sector buffer
	   15 - writes sector buffer
	   16 - tests if named drive is ready
	   17 - recalibrates drive
	   18 - controller ram diagnostic
	   19 - drive diagnostic
	   20 - controller internal diagnostic

	- drive numbers represent physical drives (0 = A:, 1 = B:,
	  80h = drive 0, 81h = drive 1
	- disk reads should be retried at least 3 times
	- MS C uses _bios_disk()
	- see  INT 13  for more return codes and more information