Description: Apple II Machine Language: Accessing the keyboard
Header: Apple II Machine Language: Accessing the keyboard

 When writing a machine language routine to check the keyboard for a single
 character, make sure that the routine checks for the high bit at the keyboard,
 signifing valid data, before the routine attempts to read data there.

 Here is an example of such a routine:

          ORG  $300
 KBD       EQU  $C000      ; location of keyboard data
 KBDSTRB   EQU  $C010      ; location of keyboard strobe latch

 ReadKey   BIT  KBD        ; test high bit of data reg.
          BPL  ReadKey    ; If not set retest till it is set
          LDA  KBD        ; Data at KBD is Valid (High Bit set)
          BIT  KBDSTRB    ; now clear the keyboard for the next char
          RTS             ; and we are done.

 The act of striking a key will set the strobe bit automatically, so this
 routine clears it before returning.

 If you are testing for a particular keystroke, put the test in a seperate
 routine. Here is an example of how that would be done:

 ...
          BIT  KBDSTRB     ; Clear out any data that is already at KBD
 WaitForA  JSR  ReadKey     ; get a key from the keyboard
          CMP  #$C1        ; is it an A????
          BNE  WaitForA    ; Nope go get another
          RTS
 ...

 These methods apply to all Apple II's: II, II Plus, IIe, IIe enhanced,
 and IIc.
 Apple Technical Communications


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