Function  3Bh   Change Current Directory  (CHDIR) 
entry   AH      3Bh 
	DS:DX   address of ASCIIZ string 
return  flag CF 0       successful 
		1       error 
	AX      error code if any (3) 
note 1) The pathname cannot exceed 64 characters 
     2) The ASCIIZ string may contain drive and subdirectory. 
     3) Drive may be any valid drive (not nescessarily current drive)