Skip to navigation

⌂ ∞ Elite on the BBC Micro and NES

Controllers: DecreaseJoystick

[NES version, Bank 7]

       Name: DecreaseJoystick                                        [Show more]
       Type: Subroutine
   Category: Controllers
    Summary: Decrease a joystick value by a specific amount, jumping straight
             to the indicator centre if we decrease from the right-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 DecreaseJoystick
             * IncreaseJoystick calls via decj2




 Arguments:

   X                    The value (pitch or roll rate) to decrease

   joystickDelta        The amount to decrease the value in X by



 Other entry points:

   decj2                Return a value of X = 128, for the centre of the
                        indicator



.DecreaseJoystick

 TXA                    ; Set X = X - joystickDelta
 SEC
 SBC joystickDelta
 TAX

 BCS decj1              ; If the subtraction didn't underflow, jump to decj1 to
                        ; skip the following instruction

 LDX #1                 ; Set X = 1 so X doesn't get smaller than 1 (so we can't
                        ; go past the left end of the indicator)

.decj1

 BPL decj3              ; If X < 127 then the decreased value is in the
                        ; left-hand side of the indicator, so jump to decj3 to
                        ; return from the subroutine

                        ; If we get here then the decreased value is still in
                        ; the right-hand side of the indicator, so we return a
                        ; value of 128, for the centre of the indicator

.decj2

 LDX #128               ; Set X = 128 to jump to indicator to the centre of the
                        ; indicator, so increasing or decreasing a value towards
                        ; the centre of the indicator immediately jumps to the
                        ; middle point of the indicator

.decj3

 RTS                    ; Return from the subroutine