Skip to navigation

⌂ ∞ ▽ Elite on the 6502

Stardust: FLIP

[NES version, Bank 0]

       Name: FLIP                                                    [Show more]
       Type: Subroutine
   Category: Stardust
    Summary: Reflect the stardust particles in the screen diagonal

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * LOOK1 calls FLIP




 Swap the x- and y-coordinates of all the stardust particles. Called by LOOK1
 when we switch views.

 This is a quick way of making the stardust field in the new view feel
 different without having to generate a whole new field. If you look carefully
 at the stardust field when you switch views, you can just about see that the
 new field is a reflection of the previous field in the screen diagonal, i.e.
 in the line from bottom left to top right. This is the line where x = y when
 the origin is in the middle of the screen, and positive x and y are right and
 up, which is the coordinate system we use for stardust).



.FLIP

 LDY NOSTM              ; Set Y to the current number of stardust particles, so
                        ; we can use it as a counter through all the stardust

.FLL1

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDX SY,Y               ; Copy the Y-th particle's y-coordinate from SY+Y into X

 LDA SX,Y               ; Copy the Y-th particle's x-coordinate from SX+Y into
 STA SY,Y               ; the particle's y-coordinate

 TXA                    ; Copy the Y-th particle's original y-coordinate into
 STA SX,Y               ; the particle's x-coordinate, so the x- and
                        ; y-coordinates are now swapped

 LDA SZ,Y               ; Fetch the Y-th particle's distance from SZ+Y into ZZ
 STA ZZ

 DEY                    ; Decrement the counter to point to the next particle of
                        ; stardust

 BNE FLL1               ; Loop back to FLL1 until we have moved all the stardust
                        ; particles

 RTS                    ; Return from the subroutine