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⌂ ∞ Elite on the BBC Micro and NES

Version analysis of SUN (Part 2 of 4)

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: SUN (Part 2 of 4)
       Type: Subroutine
   Category: Drawing suns
    Summary: Draw the sun: Start from the bottom of the screen and erase the
             old sun line by line
  Deep dive: Drawing the sun



 This part erases the old sun, starting at the bottom of the screen and working
 upwards until we reach the bottom of the new sun.

Code variation 1 of 1Related to the Master version

In the Master version, the screen size is not hard-coded, but is stored in a dedicated location, an approach that was presumably inherited from the non-BBC versions of the game.

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, 6502SP
Master
 LDY #2*Y-1             \ Set Y = y-coordinate of the bottom of the screen,
                        \ which we use as a counter in the following routine to
                        \ redraw the old sun

 LDY Yx2M1              \ Set Y = y-coordinate of the bottom of the screen,
                        \ which we use as a counter in the following routine to
                        \ redraw the old sun

 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

.PLFL2

 CPY TGT                \ If Y = TGT, we have reached the line where we will
 BEQ PLFL               \ start drawing the new sun, so there is no need to
                        \ keep erasing the old one, so jump down to PLFL

 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 PLF13              \ 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

.PLF13

 DEY                    \ Decrement the loop counter

 BNE PLFL2              \ Loop back for the next line in the line heap until
                        \ we have either gone through the entire heap, or
                        \ reached the bottom row of the new sun