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⌂ ∞ Elite on the BBC Micro and NES

Drawing suns: DrawSunEdgeRight

[NES version, Bank 1]

       Name: DrawSunEdgeRight                                        [Show more]
       Type: Subroutine
   Category: Drawing suns
    Summary: Draw a sun line in the tile on the right end of a sun row

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SUN (Part 2 of 2) calls DrawSunEdgeRight




 Arguments:

   A                    The half-width of the sun line

   Y                    The number of the pixel row of the sun line within the
                        tile row (0-7)

   X1                   The pixel x-coordinate of the rightmost tile on the sun
                        line

   YY(1 0)              The centre x-coordinate of the sun



.DrawSunEdgeRight

 CLC                    ; Set X1 = YY(1 0) + A
 ADC YY                 ;
 STA X2                 ; So X2 contains the x-coordinate of the right end of
 LDA YY+1               ; the sun line
 ADC #0

                        ; X1 is already set to the x-coordinate of the rightmost
                        ; tile, so the line we need to draw is from (X1, Y) to
                        ; (X2, Y)

 BEQ DrawSunEdge        ; If the high byte of the result is zero, then the right
                        ; end of the line is on-screen, so jump to DrawSunEdge
                        ; to draw the sun line from (X1, Y) to (X2, Y)

 BMI RTS7               ; If the high byte of the result is negative, then the
                        ; right end of the line is off the left edge of the
                        ; screen, so the line is not on-screen and we jump to
                        ; RTS7 to return from the subroutine (as RTS7 contains
                        ; an RTS)

                        ; If we get here then the right end of the line is past
                        ; the right edge of the screen, so we need to clip the
                        ; right end of the line to fit on-screen

 LDA #253               ; Set X2 = 253 so the line is clipped to the right edge
 STA X2                 ; of the screen

 CMP X1                 ; If X2 <= X1 then the right end of the line is to the
 BEQ RTS7               ; left of the left end of the line, so these are not
 BCC RTS7               ; valid line coordinates and we jump to RTS7 to return
                        ; from the subroutine without drawing anything

 JMP HLOIN              ; Otherwise draw the sun line from (X1, Y) to (X2, Y)
                        ; and return from the subroutine using a tail call