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Elite on the BBC Micro

Keyboard: TT17 [Disc version, Docked]

Name: TT17 [Show more] Type: Subroutine Category: Keyboard Summary: Scan the keyboard for cursor key or joystick movement
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: * Main game loop (Part 5 of 6) calls TT17

Scan the keyboard and joystick for cursor key or stick movement, and return the result as deltas (changes) in x- and y-coordinates as follows: * For joystick, X and Y are integers between -2 and +2 depending on how far the stick has moved * For keyboard, X and Y are integers between -1 and +1 depending on which keys are pressed Returns: A The key pressed, if the arrow keys were used X Change in the x-coordinate according to the cursor keys being pressed or joystick movement, as an integer (see above) Y Change in the y-coordinate according to the cursor keys being pressed or joystick movement, as an integer (see above)
.TT17 JSR DOKEY \ Scan the keyboard for flight controls and pause keys, \ (or the equivalent on joystick) and update the key \ logger, setting KL to the key pressed LDA JSTK \ If the joystick is not configured, jump down to TJ1, BEQ TJ1 \ otherwise we move the cursor with the joystick LDA JSTX \ Fetch the joystick roll, ranging from 1 to 255 with \ 128 as the centre point EOR #&FF \ Flip the sign so A = -JSTX, because the joystick roll \ works in the opposite way to moving a cursor on-screen \ in terms of left and right JSR TJS1 \ Call TJS1 just below to set A to a value between -2 \ and +2 depending on the joystick roll value (moving \ the stick sideways) TYA \ Copy Y to A TAX \ Copy A to X, so X contains the joystick roll value LDA JSTY \ Fetch the joystick pitch, ranging from 1 to 255 with \ 128 as the centre point, and fall through into TJS1 to \ set Y to the joystick pitch value (moving the stick up \ and down) .TJS1 TAY \ Store A in Y LDA #0 \ Set the result, A = 0 CPY #16 \ If Y >= 16 set the C flag, so A = A - 1 SBC #0 CPY #64 \ If Y >= 64 set the C flag, so A = A - 1 SBC #0 CPY #192 \ If Y >= 192 set the C flag, so A = A + 1 ADC #0 CPY #224 \ If Y >= 224 set the C flag, so A = A + 1 ADC #0 TAY \ Copy the value of A into Y LDA KL \ Set A to the value of KL (the key pressed) RTS \ Return from the subroutine .TJ1 LDA KL \ Set A to the value of KL (the key pressed) LDX #0 \ Set the initial values for the results, X = Y = 0, LDY #0 \ which we now increase or decrease appropriately CMP #&19 \ If left arrow was pressed, set X = X - 1 BNE P%+3 DEX CMP #&79 \ If right arrow was pressed, set X = X + 1 BNE P%+3 INX CMP #&39 \ If up arrow was pressed, set Y = Y + 1 BNE P%+3 INY CMP #&29 \ If down arrow was pressed, set Y = Y - 1 BNE P%+3 DEY STX T \ Set T to the value of X, which contains the joystick \ roll value LDX #0 \ Scan the keyboard to see if the SHIFT key is currently JSR DKS4 \ being pressed, returning the result in A and X BPL TJe \ If SHIFT is not being pressed, skip to TJe ASL T \ SHIFT is being held down, so quadruple the value of T ASL T \ (i.e. SHIFT moves the cursor at four times the speed \ when using the joystick) TYA \ Fetch the joystick pitch value from Y into A ASL A \ SHIFT is being held down, so quadruple the value of A ASL A \ (i.e. SHIFT moves the cursor at four times the speed \ when using the joystick) TAY \ Transfer the amended value of A back into Y .TJe LDX T \ Fetch the amended value of T back into X LDA KL \ Set A to the value of KL (the key pressed) RTS \ Return from the subroutine