$6003/24579/WD+3:       Data Register

ROM-Reference:

 CMP $6003  : $CA81
 CPY $6003  : $C35F
 LDA $6003  : $C84F $C977 $CD21
 STA $6003  : $C3FA $C40E $C422 $C435 $C446 $C455 $C465 $C475 $C485 $C496
              $C4AA $C4BE $C4D1 $C4F3 $C50B $C51D $C53A $C677 $C751 $CE7E
 STY $6003  : $C34F