.EDGES STA T \ Set T to the line's half-length in argument A CLC \ We now calculate: ADC YY \ STA X2 \ (A X2) = YY(1 0) + A \ \ to set X2 to the x-coordinate of the right end of the \ line, starting with the low bytes LDA YY+1 \ And then adding the high bytes ADC #0 BMI ED1 \ If the addition is negative then the calculation has \ overflowed, so jump to ED1 to return a failure BEQ P%+6 \ If the high byte A from the result is 0, skip the \ next two instructions, as the result already fits on \ the screen LDA #254 \ The high byte is positive and non-zero, so we went STA X2 \ past the right edge of the screen, so clip X2 to the \ x-coordinate of the right edge of the screen LDA YY \ We now calculate: SEC \ SBC T \ (A X1) = YY(1 0) - argument A STA X1 \ \ to set X1 to the x-coordinate of the left end of the \ line, starting with the low bytes LDA YY+1 \ And then subtracting the high bytes SBC #0 BNE ED3 \ If the high byte subtraction is non-zero, then skip \ to ED3 CLC \ Otherwise the high byte of the subtraction was zero, \ so the line fits on-screen and we clear the C flag to \ indicate success RTS \ Return from the subroutine .ED3 BPL ED1 \ If the addition is positive then the calculation has \ underflowed, so jump to ED1 to return a failure LDA #2 \ The high byte is negative and non-zero, so we went STA X1 \ past the left edge of the screen, so clip X1 to the \ y-coordinate of the left edge of the screen .PL44 CLC \ The line does fit on-screen, so clear the C flag to \ indicate success RTS \ Return from the subroutine .ED1 LDA #0 \ Set the Y-th byte of the LSO block to 0 STA LSO,Y \ The line does not fit on the screen, so fall through \ into PL21 to set the C flag to indicate this resultName: EDGES [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a horizontal line given a centre and a half-widthContext: See this subroutine in context in the source code References: This subroutine is called as follows: * HLOIN2 calls EDGES * SUN (Part 3 of 4) calls EDGES * CHKON calls entry point PL44

Set X1 and X2 to the x-coordinates of the ends of the horizontal line with centre x-coordinate YY(1 0), and length A in either direction from the centre (so a total line length of 2 * A). In other words, this line: X1 YY(1 0) X2 +-----------------+-----------------+ <- A -> <- A -> The resulting line gets clipped to the edges of the screen, if needed. If the calculation doesn't overflow, we return with the C flag clear, otherwise the C flag gets set to indicate failure and the Y-th LSO entry gets set to 0. Arguments: A The half-length of the line YY(1 0) The centre x-coordinate Returns: C flag Clear if the line fits on-screen, set if it doesn't X1, X2 The x-coordinates of the clipped line LSO+Y If the line doesn't fit, LSO+Y is set to 0 Y Y is preserved Other entry points: PL44 Clear the C flag and return from the subroutine

[X]

Label ED1 is local to this routine

[X]

Label ED3 is local to this routine