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Elite on the BBC Micro

Drawing lines: LOIN (Part 1 of 7) [Elite-A, I/O processor]

Name: LOIN (Part 1 of 7) [Show more] Type: Subroutine Category: Drawing lines Summary: Implement the draw_line command (draw a line) Deep dive: Bresenham's line algorithm
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * tube_table calls entry point LL30

This routine is run when the parasite sends a draw_line command. It draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. This stage calculates the line deltas. Returns: Y Y is preserved Other entry points: LL30 LL30 is a synonym for LOIN and draws a line from (X1, Y1) to (X2, Y2)
.LL30 SKIP 0 \ LL30 is a synomym for LOIN \ \ 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 JSR tube_get \ Get the parameters from the parasite for the command: STA X1 \ JSR tube_get \ draw_line(x1, y1, x2, y2) STA Y1 \ JSR tube_get \ and store them as follows: STA X2 \ JSR tube_get \ * X1 = the start point's x-coordinate STA Y2 \ \ * Y1 = the start point's y-coordinate \ \ * X2 = the end point's x-coordinate \ \ * Y2 = the end point's y-coordinate .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 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 \ horizontal