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BBC Micro Elite

Drawing lines: LOIN (Part 1 of 7)

Name: LOIN (Part 1 of 7) [View in context] Type: Subroutine Category: Drawing lines Summary: Draw a line: Calculate the line gradient in the form of deltas
This routine 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) HL6 Contains an RTS
.LL30 .LOIN { STY YSAV \ Store Y into YSAV, so we can preserve it across the \ call to this subroutine 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