IO (CRU ACCESS)                                      PAGE I10
          -------------------------------------------------------------
 
          CRU ACCESS INSTRUCTION
 
          Format         CALL IO(type,bits,cru-base,variable,variable
                         [,...])
 
           The IO types 2 and 3 can be used to control devices.
          IO always must be the CRU address divided by 2 as any 
          value above 8192 will be out of range. The cru-base must be  
          divided by 2 as the 9901 chip ignores the least significant 
          bits of the base register it uses. See Editor Assembler 
          Manual page 61. The CRU data to be written should be right  
          justified in the byte or word. The least significant bit 
          will output to or input from the CRU address specified by 
          the CRU base address. Subsequent bits will come from or go 
          to sequentially higher CRU addresses. If the CRU input reads 
          less than 8 bits, the unused bits in the byte are reset to 
          zero. If the CRU input reads less than 16 bits but more than 
          8 bits, the unused bits in the full two 8 bit bytes will be 
          reset to zero.
 
          Programs
          Line 100 display what it does | >100 DISPLAY AT(1,1)ERASE ALL
          for you.                      |  :"THIS PROGRAM CHECKS FOR
                                        |    UNUSUAL KEYS BEING PRESSED
                                        |  , EVEN IF MORE THEN FOUR KEY
                                        |  ARE BEING PRESSED AT ONCE"
          Line 110 scans CRU at >0006   | >110 CALL IO(2,16,3,A,B):: IF
          and reports keys pressed.     |   A=18 AND B=255 THEN 110 ELS
                                        |  E CALL HPUT(24,3,RPT$(" ",30
                                        |  ),24,24,STR$(A)&" "&STR$(B))
          Line 120 more reports.        | >120 IF A=146 THEN CALL HPUT(
                                        |  24,3,"FUNCTION KEY")ELSE IF
                                        |  B=191 THEN CALL HPUT(24,3,"C
                                        |  ONTROL KEY")ELSE IF B=223 TH
                                        |  EN CALL HPUT(24,3,"SHIFT KEY
                                        |  ")
          Line 130 still more reports.  | >130 IF B=251 THEN CALL HPUT(
                                        |  24,3,"ENTER KEY")ELSE IF B=2
                                        |  53 THEN CALL HPUT(24,3,"SPAC
                                        |  E BAR")ELSE IF B=254 THEN CA
                                        |  LL HPUT(24,3,"PLUS/EQUAL KEY
                                        |  ")
          Line start over scan of keys. | >140 GOTO 110