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⌂ ∞ ▽ Elite on the 6502

Keyboard: TT217

[Acorn Electron version]

       Name: TT217                                                   [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard until a key is pressed

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * gnum calls TT217
             * qv calls TT217
             * TT214 calls TT217
             * mes9 calls via out
             * OUCH 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

   Y                    Y is preserved



 Other entry points:

   out                  Contains an RTS



.TT217

 STY YSAV               \ Store Y in temporary storage, so we can restore it
                        \ later

.t

 DEC KEYB               \ Decrement KEYB, so it is now &FF, to indicate that we
                        \ should process all interrupts, including keyboard
                        \ interrupts, so that we can read the keyboard

 JSR OSRDCH             \ Call OSRDCH to read a character from the keyboard

 INC KEYB               \ Increment KEYB back to 0 to indicate that we are done
                        \ reading the keyboard, so we can ignore interrupts

 TAX                    \ Copy A into X

.out

 RTS                    \ Return from the subroutine