.proc line; X_pos =< X_end skall alltid gälla
    dx = $fe
    dy = $69
    ;;dx
    SEC
    LDA X_end
    SBC X_pos
    STA dx

    SEC
    LDA Y_pos
    SBC Y_end
    STA dy
    BCC down ;normal  Y_pos < Y_end
up:; Y_pos > Y_end
    STA dy
    CMP dx
    BCC shallow; dy < dx
steep:
    jsr line_up_inv
    RTS
shallow: ;dy =< dx
    jsr line_up
    RTS
down:
    EOR #$ff ; Fix bit underflow
    STA dy
    CMP dx
    BCC shallow_; dy < dx
steep_:
    jsr line_down_inv
    RTS
shallow_: ;dy < dx
    jsr line_down
    RTS
.endproc

.include "line_down.s"
.include "line_down_inv.s"
.include "line_up.s"
.include "line_up_inv.s"