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Elite on the BBC Micro

Drawing suns: SUN (Part 4 of 4) (Disc version)

Name: SUN (Part 4 of 4) [View in context] Type: Subroutine [Compare versions] Category: Drawing suns Summary: Draw the sun: Continue to the top of the screen, erasing old sun Deep dive: Drawing the sun
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.
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\ If we get here, we have successfully made it from the \ bottom line of the screen to the top, and the old sun \ has been replaced by the new one CLC \ Clear the C flag to indicate success in drawing the \ sun LDA K3 \ Set SUNX(1 0) = K3(1 0) STA SUNX LDA K3+1 STA SUNX+1 .RTS2 RTS \ Return from the subroutine