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


 This part of the routine scales the steering, but if the joystick x-axis high
 byte is zero, then the scaling is applied in a different way. Instead of this:

     2 * (scale * x-axis) ^ 2
   = 2 * scale ^2 * x-axis ^2

 we return this:

   scale ^2 * x-axis

 in other words, we divide the scale factor by 2 * x-axis.


.joys4

                        \ By this point, A contains the scale factor to apply to
                        \ the steering, which is one of the following values:
                        \
                        \   * 181 for a scale factor of 1.00
                        \
                        \   * 202 for a scale factor of 1.25
                        \
                        \   * 205 for a scale factor of 1.28
                        \
                        \   * 212 for a scale factor of 1.37

 STA U                  \ Set U= A
                        \
                        \ So U= 181, 202, 205 or 212

 PLA                    \ Set A= joystick x-axis high byte, which we stored on
                        \ the stack at the start of the routine

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

 PLP                    \ If the joystick x-axis high byte is zero, which we
 BEQ joys5              \ stored on the stack at the start of the routine, jump
                        \ to joys5 to return x-axis * scale ^ 2 instead

 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= 212 we have:
                        \
                        \   (A T)= 2 * (212/256 * x-axis) ^ 2
                        \        = 2 * (0.828 * x-axis) ^ 2
                        \        = 1.37 * x-axis ^ 2
                        \
                        \ and for A= 205 we have:
                        \
                        \   (A T)= 2 * (205/256 * x-axis) ^ 2
                        \        = 2 * (0.801 * x-axis) ^ 2
                        \        = 1.28 * x-axis ^ 2
                        \
                        \ and for A= 202 we have:
                        \
                        \   (A T)= 2 * (202/256 * x-axis) ^ 2
                        \        = 2 * (0.789 * x-axis) ^ 2
                        \        = 1.25 * 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

.joys5

                        \ If we get here then the joystick x-axis high byte is
                        \ zero, U contains the scale factor and A contains the
                        \ top byte of scale * x-axis

 JMP Multiply8x8        \ Calculate:
                        \
                        \   (A T)= A * U
                        \        = scale * scale * x-axis
                        \
                        \ and return the high byte in A, returning from the
                        \ subroutine using a tail call