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⌂ ∞ Elite on the BBC Micro and NES

Keyboard: TT217

[Elite-A, Parasite]

       Name: TT217                                                   [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard until a key is pressed by sending a get_key
             command to the I/O processor

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * gnum calls TT217
             * qv calls TT217
             * DK4 calls via out




 Scan the keyboard until a key is pressed, and return the key's ASCII code.
 If, on entry, a key is already being held down, then wait until that key is
 released first (so this routine detects the first key down event following
 the subroutine call).



 Returns:

   X                    The ASCII code of the key that was pressed

   A                    Contains the same as X



 Other entry points:

   out                  Contains an RTS



.TT217

.t

 LDA #&8D               \ Send command &8D to the I/O processor:
 JSR tube_write         \
                        \   =get_key()
                        \
                        \ which waits for any current key presses to be released
                        \ (if any), and then waits for a key to be pressed
                        \ before returning the result as an ASCII value

 JSR tube_read          \ Set A to the response from the I/O processor, which
                        \ will contain the ASCII value of the key press

 TAX                    \ Copy the response into X

.out

 RTS                    \ Return from the subroutine