\ In the cassette and disc versions of Elite, LL30 and \ LOIN are synonyms for the same routine, presumably \ because the two developers each had their own line \ routines to start with, and then chose one of them for \ the final game \ \ In the 6502 Second Processor version, there are three \ different routines. In the parasite, LL30 draws a \ one-segment line, while LOIN draws multi-segment \ lines. Both of these ask the I/O processor to do the \ actual drawing, and it uses a routine called... wait \ for it... LOIN \ \ This, then, is the I/O processor's LOIN routine, which \ is not the same as LL30, or the other LOIN. Got that? .LOIN LDA #128 \ Set S = 128, which is the starting point for the STA S \ slope error (representing half a pixel) ASL A \ Set SWAP = 0, as %10000000 << 1 = 0 STA SWAP LDA X2 \ Set A = X2 - X1 SBC X1 \ = delta_x \ \ This subtraction works as the ASL A above sets the C \ flag BCS LI1 \ If X2 > X1 then A is already positive and we can skip \ the next three instructions EOR #%11111111 \ Negate the result in A by flipping all the bits and ADC #1 \ adding 1, i.e. using two's complement to make it \ positive SEC \ Set the C flag, ready for the subtraction below .LI1 STA P \ Store A in P, so P = |X2 - X1|, or |delta_x| LDA Y2 \ Set A = Y2 - Y1 SBC Y1 \ = delta_y \ \ This subtraction works as we either set the C flag \ above, or we skipped that SEC instruction with a BCS BEQ HLOIN2 \ If A = 0 then Y1 = Y2, which means the line is \ horizontal, so jump to HLOIN2 to draw a horizontal \ line instead of applying Bresenham's line algorithm BCS LI2 \ If Y2 > Y1 then A is already positive and we can skip \ the next two instructions EOR #%11111111 \ Negate the result in A by flipping all the bits and ADC #1 \ adding 1, i.e. using two's complement to make it \ positive .LI2 STA Q \ Store A in Q, so Q = |Y2 - Y1|, or |delta_y| CMP P \ If Q < P, jump to STPX to step along the x-axis, as BCC STPX \ the line is closer to being horizontal than vertical JMP STPY \ Otherwise Q >= P so jump to STPY to step along the \ y-axis, as the line is closer to being vertical than \ horizontalName: LOIN (Part 1 of 7) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a line: Calculate the line gradient in the form of deltas Deep dive: Bresenham's line algorithmContext: 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: * ADDBYT calls LOIN * TTX66 calls LOIN

This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. This stage calculates the line deltas. Arguments: X1 The screen x-coordinate of the start of the line Y1 The screen y-coordinate of the start of the line X2 The screen x-coordinate of the end of the line Y2 The screen y-coordinate of the end of the line

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Subroutine HLOIN2 (category: Drawing lines)

Draw a horizontal line in a specific colour

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Label LI1 is local to this routine

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Label LI2 is local to this routine

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Label STPX in subroutine LOIN (Part 2 of 7)

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Label STPY in subroutine LOIN (Part 5 of 7)