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⌂ ∞ Elite on the BBC Micro and NES

Drawing pixels: PIXEL

[Elite-A, Parasite]

       Name: PIXEL                                                   [Show more]
       Type: Subroutine
   Category: Drawing pixels
    Summary: Draw a 1-pixel dot, 2-pixel dash or 4-pixel square by sending a
             draw_pixel command to the I/O processor

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DOEXP calls PIXEL
             * PIXEL2 calls PIXEL
             * TT22 calls PIXEL




 Draw a point at screen coordinate (X, A) with the point size determined by the
 distance in ZZ. This applies to the top part of the screen (the monochrome
 mode 4 portion).



 Arguments:

   X                    The screen x-coordinate of the point to draw

   A                    The screen y-coordinate of the point to draw

   ZZ                   The distance of the point (further away = smaller point)



.PIXEL

 PHA                    \ Store the y-coordinate on the stack

 LDA #&82               \ Send command &82 to the I/O processor:
 JSR tube_write         \
                        \   draw_pixel(x, y, distance)
                        \
                        \ which will draw a pixel at (x, y) with the size
                        \ determined by the distance

 TXA                    \ Send the first parameter to the I/O processor:
 JSR tube_write         \
                        \   * x = X

 PLA                    \ Send the second parameter to the I/O processor:
 JSR tube_write         \
                        \   * y = A

 LDA ZZ                 \ Send the third parameter to the I/O processor:
 JMP tube_write         \
                        \   * distance = ZZ
                        \
                        \ and return from the subroutine using a tail call