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⌂ ∞ Elite on the BBC Micro and NES

Controllers: IncreaseJoystick

[NES version, Bank 7]

       Name: IncreaseJoystick                                        [Show more]
       Type: Subroutine
   Category: Controllers
    Summary: Increase a joystick value by a specific amount, jumping straight
             to the indicator centre if we increase from the left-hand side
  Deep dive: Bolting NES controllers onto the key logger

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * UpdateJoystick calls IncreaseJoystick




 Arguments:

   X                    The value (pitch or roll rate) to decrease

   joystickDelta        The amount to decrease the value in X by



.IncreaseJoystick

 TXA                    ; Set X = X + joystickDelta
 CLC
 ADC joystickDelta
 TAX

 BCC incj1              ; If the addition didn't overflow, jump to incj1 to skip
                        ; the following instruction

 LDX #255               ; Set X = 255 so X doesn't get larger than 255 (so we
                        ; can't go past the right end of the indicator)

.incj1

 BPL decj2              ; If X < 127 then the increased value is still in the
                        ; left-hand side of the indicator, so jump to decj2 to
                        ; return a value of 128, for the centre of the indicator

 RTS                    ; Return from the subroutine