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⌂ ∞ ▽ Elite on the 6502

Drawing lines: LL30

[Elite-A, Parasite]

       Name: LL30                                                    [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a one-segment line by sending a draw_line command to the I/O
             processor

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * LL9 (Part 12 of 12) calls LL30
             * TT15 calls LL30




 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



.LOIN
.LL30

 LDA #&80               \ Send command &80 to the I/O processor:
 JSR tube_write         \
                        \   draw_line(x1, y1, x2, y2)
                        \
                        \ which will draw a line from (x1, y1) to (x2, y2)

 LDA X1                 \ Send the first parameter to the I/O processor:
 JSR tube_write         \
                        \   * x1 = X1

 LDA Y1                 \ Send the second parameter to the I/O processor:
 JSR tube_write         \
                        \   * y1 = Y1

 LDA X2                 \ Send the first parameter to the I/O processor:
 JSR tube_write         \
                        \   * x2 = X2

 LDA Y2                 \ Send the first parameter to the I/O processor:
 JMP tube_write         \
                        \   * y2 = Y2
                        \
                        \ and return from the subroutine using a tail call