BTR4 ...[   
 BTY= ...[   
 BW5G ...[??? MASK SECT FILE FGN 24 DOC  
 BWK6    #MAC           MASK                [F   
 BWSY ...[     BITS 0-4 = NO OF BITS - 1, 5-18 = DISPLACEMENT,19-23 = LS BIT 
 BX4Q    [     SET UP 21? WITH A MASK OF THE FIELD   
 BXJB    #DEF           21?=-1@L((%A@L-19)-23)@L(23-(%A&#37))
 BY42    #NOR