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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