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Elite on the BBC Micro

Drawing suns: SUN (Part 4 of 4) [6502SP 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 old sun 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 entry point 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\ If we get here, we have successfully made it from the \ bottom line of the screen to the top, and the old sun \ has been replaced by the new one CLC \ Clear the C flag to indicate success in drawing the \ sun LDA K3 \ Set SUNX(1 0) = K3(1 0) STA SUNX LDA K3+1 STA SUNX+1 JSR HBFL \ Call HBFL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