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Elite on the BBC Micro

Keyboard: TT217 [Elite-A, Docked]

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 References: This subroutine is called as follows: * gnum calls TT217 * qv calls TT217 * DK4 calls entry point out * mes9 calls entry point 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 LDY #2 \ Delay for 2 vertical syncs (2/50 = 0.04 seconds) so we JSR DELAY \ don't take up too much CPU time while looping round JSR RDKEY \ Scan the keyboard for a key press and return the \ internal key number in X (or 0 for no key press) BNE t \ If a key was already being held down when we entered \ this routine, keep looping back up to t, until the \ key is released .t2 JSR RDKEY \ Any pre-existing key press is now gone, so we can \ start scanning the keyboard again, returning the \ internal key number in X (or 0 for no key press) BEQ t2 \ Keep looping up to t2 until a key is pressed TAY \ Copy A to Y, so Y contains the internal key number \ of the key pressed LDA (TRTB%),Y \ The address in TRTB% points to the MOS key \ translation table, which is used to translate \ internal key numbers to ASCII, so this fetches the \ key's ASCII code into A LDY YSAV \ Restore the original value of Y we stored above TAX \ Copy A into X .out RTS \ Return from the subroutine