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Drawing suns: WPLS

[BBC Micro disc version, Docked]

Name: WPLS [Show more] Type: Subroutine Category: Drawing suns Summary: Remove the sun from the screen 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: No direct references to this subroutine in this source file

We do this by redrawing it using the lines stored in the sun line heap when the sun was originally drawn by the SUN routine.
Arguments: SUNX(1 0) The x-coordinate of the vertical centre axis of the sun
.WPLS LDA LSX \ If LSX < 0, the sun line heap is empty, so return from BMI PL44 \ the subroutine (as PL44 contains a CLC and RTS) LDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the sun that's currently on LDA SUNX+1 \ screen STA YY+1 LDY #2*Y-1 \ #Y is the y-coordinate of the centre of the space \ view, so this sets Y as a counter for the number of \ lines in the space view (i.e. 191), which is also the \ number of lines in the LSO block .WPL2 LDA LSO,Y \ Fetch the Y-th point from the sun line heap, which \ gives us the half-width of the sun's line on this line \ of the screen BEQ P%+5 \ 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 DEY \ Decrement the loop counter BNE WPL2 \ Loop back for the next line in the line heap until \ we have gone through the entire heap DEY \ This sets Y to &FF, as we end the loop with Y = 0 STY LSX \ Set LSX to &FF to indicate the sun line heap is empty RTS \ Return from the subroutine