Name: SUN (Part 2 of 4) Type: Subroutine Category: Drawing suns Summary: Draw the sun: Start from bottom of screen and erase the old sun 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.
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 version to expand it, and tap it again to show to all variations.
Cassette, Flight, Docked, 6502SP
MasterLDY #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 sunLDY YMAX \ 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