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