.CHKON LDA K3 ; Set A = K3 + K CLC ADC K LDA K3+1 ; Set A = K3+1 + 0 + any carry from above, so this ADC #0 ; effectively sets A to the high byte of K3(1 0) + K: ; ; (A ?) = K3(1 0) + K ; ; so A is the high byte of the x-coordinate of the right ; edge of the circle BMI PL21 ; If A is negative then the right edge of the circle is ; to the left of the screen, so jump to PL21 to set the ; C flag and return from the subroutine, as the whole ; circle is off-screen to the left LDA K3 ; Set A = K3 - K SEC SBC K LDA K3+1 ; Set A = K3+1 - 0 - any carry from above, so this SBC #0 ; effectively sets A to the high byte of K3(1 0) - K: ; ; (A ?) = K3(1 0) - K ; ; so A is the high byte of the x-coordinate of the left ; edge of the circle BMI PL31 ; If A is negative then the left edge of the circle is ; to the left of the screen, and we already know the ; right edge is either on-screen or off-screen to the ; right, so skip to PL31 to move on to the y-coordinate ; checks, as at least part of the circle is on-screen in ; terms of the x-axis BNE PL21 ; If A is non-zero, then the left edge of the circle is ; to the right of the screen, so jump to PL21 to set the ; C flag and return from the subroutine, as the whole ; circle is off-screen to the right .PL31 LDA K4 ; Set P+1 = K4 + K CLC ADC K STA P+1 LDA K4+1 ; Set A = K4+1 + 0 + any carry from above, so this ADC #0 ; does the following: ; ; (A P+1) = K4(1 0) + K ; ; so A is the high byte of the y-coordinate of the ; bottom edge of the circle BMI PL21 ; If A is negative then the bottom edge of the circle is ; above the top of the screen, so jump to PL21 to set ; the C flag and return from the subroutine, as the ; whole circle is off-screen to the top STA P+2 ; Store the high byte in P+2, so now we have: ; ; P(2 1) = K4(1 0) + K ; ; i.e. the maximum y-coordinate of the circle on-screen ; (which we return) LDA K4 ; Set X = K4 - K SEC SBC K TAX LDA K4+1 ; Set A = K4+1 - 0 - any carry from above, so this SBC #0 ; does the following: ; ; (A X) = K4(1 0) - K ; ; so A is the high byte of the y-coordinate of the top ; edge of the circle BMI PL44 ; If A is negative then the top edge of the circle is ; above the top of the screen, and we already know the ; bottom edge is either on-screen or below the bottom ; of the screen, so skip to PL44 to clear the C flag and ; return from the subroutine using a tail call, as part ; of the circle definitely appears on-screen BNE PL21 ; If A is non-zero, then the top edge of the circle is ; below the bottom of the screen, so jump to PL21 to set ; the C flag and return from the subroutine, as the ; whole circle is off-screen to the bottom CPX Yx2M1 ; If we get here then A is zero, which means the top ; edge of the circle is within the screen boundary, so ; now we need to check whether it is in the space view ; (in which case it is on-screen) or the dashboard (in ; which case the top of the circle is hidden by the ; dashboard, so the circle isn't on-screen). We do this ; by checking the low byte of the result in X against ; Yx2M1, and returning the C flag from this comparison. ; The value in Yx2M1 is the y-coordinate of the bottom ; pixel row of the space view, so this does the ; following: ; ; * The C flag is set if coordinate (A X) is below the ; bottom row of the space view, i.e. the top edge of ; the circle is hidden by the dashboard ; ; * The C flag is clear if coordinate (A X) is above ; the bottom row of the space view, i.e. the top ; edge of the circle is on-screen RTS ; Return from the subroutineName: CHKON [Show more] Type: Subroutine Category: Drawing circles Summary: Check whether any part of a circle appears on the extended screenContext: See this subroutine in context in the source code References: This subroutine is called as follows: * CIRCLE calls CHKON * SUN (Part 1 of 2) calls CHKON
Arguments: K The circle's radius K3(1 0) Pixel x-coordinate of the centre of the circle K4(1 0) Pixel y-coordinate of the centre of the circle
Returns: C flag Clear if any part of the circle appears on-screen, set if none of the circle appears on-screen (A X) Minimum y-coordinate of the circle on-screen (i.e. the y-coordinate of the top edge of the circle) P(2 1) Maximum y-coordinate of the circle on-screen (i.e. the y-coordinate of the bottom edge of the circle)
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Subroutine PL21 (category: Drawing planets)
Return from a planet/sun-drawing routine with a failure flag
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Label PL31 is local to this routine
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Subroutine PL44 (category: Drawing planets)
Return from a planet/sun-drawing routine with a success flag