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⌂ ∞ ▽ Elite on the BBC Micro and NES

Drawing suns: SUN (Part 4 of 4)

[6502 Second Processor version]

       Name: SUN (Part 4 of 4)                                       [Show more]
       Type: Subroutine
   Category: Drawing suns
    Summary: Draw the sun: Continue to the top of the screen, erasing the old
             sun line by line
  Deep dive: Drawing the sun

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * CIRCLE calls via RTS2




 This part erases any remaining traces of the old sun, now that we have drawn
 all the way to the top of the new sun.



 Other entry points:

   RTS2                 Contains an RTS



 LDA SUNX               \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the
 STA YY                 \ vertical centre axis of the old sun that's currently
 LDA SUNX+1             \ on-screen
 STA YY+1

.PLFL3

 LDA LSO,Y              \ Fetch the Y-th point from the sun line heap, which
                        \ gives us the half-width of the old sun's line on this
                        \ line of the screen

 BEQ PLF9               \ If A = 0, skip the following call to HLOIN2 as there
                        \ is no sun line on this line of the screen

 JSR HLOIN2             \ Call HLOIN2 to draw a horizontal line on pixel line Y,
                        \ with centre point YY(1 0) and half-width A, and remove
                        \ the line from the sun line heap once done

.PLF9

 DEY                    \ Decrement the line number in Y to move to the line
                        \ above

 BNE PLFL3              \ Jump up to PLFL3 to redraw the next line up, until we
                        \ have reached the top of the screen

.PLF8

                        \ If we get here, we have successfully made it from the
                        \ bottom line of the screen to the top, and the old sun
                        \ has been replaced by the new one

 CLC                    \ Clear the C flag to indicate success in drawing the
                        \ sun

 LDA K3                 \ Set SUNX(1 0) = K3(1 0)
 STA SUNX
 LDA K3+1
 STA SUNX+1

 JSR HBFL               \ Call HBFL to send the contents of the horizontal line
                        \ buffer to the I/O processor for drawing on-screen

.RTS2

 RTS                    \ Return from the subroutine