Function  24h  Set Relative Record Field                                  (FCB) 
     Set random record field specified by an Struct -FCB
entry   AH      24h 
	DS:DX	address of an opened Struct -FCB
return	Random Record Field of Struct -FCB is set to be same as Current Block
	and Current Record. 
note 1) You must invoke this function before performing random file access. 
     2) The relative record field of FCB (offset 21h) is set to be same as the 
	Current Block (offset 0Ch) and Current Record (offset 20h). 
     3) No error codes are returned. 
     4) The FCB must already be opened.