Extra tracks: HookJoystick (Part 2 of 2)

       Name: HookJoystick (Part 2 of 2)                              [Show more]
       Type: Subroutine
   Category: Extra tracks
    Summary: Apply enhanced joystick steering to specific track sections
  Deep dive: Secrets of the extra tracks
             Code hooks in the extra tracks
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.joys4

 STA U                  \ Set U = A
                        \       = high byte of A * x-axis

 JSR Multiply8x8        \ Set (A T) = A * U
                        \           = A * A
                        \           = (A * x-axis) ^ 2

 ASL T                  \ Set (A T) = (A T) * 2
 ROL A                  \           = 2 * (A * x-axis) ^ 2

                        \ So for A = 248 we have:
                        \
                        \   (A T) = 2 * (248/256 * x-axis) ^ 2
                        \         = 2 * (0.969 * x-axis) ^ 2
                        \         = 1.88 * x-axis ^ 2
                        \
                        \ and for A = 190 we have:
                        \
                        \   (A T) = 2 * (190/256 * x-axis) ^ 2
                        \         = 2 * (0.742 * x-axis) ^ 2
                        \         = 1.10 * x-axis ^ 2
                        \
                        \ and for A = 181 we have:
                        \
                        \   (A T) = 2 * (181/256 * x-axis) ^ 2
                        \         = 2 * (0.707 * x-axis) ^ 2
                        \         = 1.00 * x-axis ^ 2

 RTS                    \ Return from the subroutine