Drawing pixels: PIXEL

       Name: PIXEL                                                   [Show more]
       Type: Subroutine
   Category: Drawing pixels
    Summary: Implement the draw_pixel command (draw space view pixels)
  Deep dive: Drawing monochrome pixels in mode 4
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * tube_table calls PIXEL


 This routine is run when the parasite sends a draw_pixel command. It draws a
 dot in the space view.


 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)


 Returns:

   Y                    Y is preserved


.PIXEL

 JSR tube_get           \ Get the parameters from the parasite for the command:
 TAX                    \
 JSR tube_get           \   draw_pixel(x, y, distance)
 TAY                    \
 JSR tube_get           \ and store them as follows:
 STA ZZ                 \
                        \   * X = the pixel's x-coordinate
                        \
                        \   * Y = the pixel's y-coordinate
                        \
                        \   * ZZ = the pixel's distance

 TYA                    \ Copy the pixel's y-coordinate from Y into A

 LSR A                  \ Set SCH = &60 + A >> 3
 LSR A
 LSR A
 ORA #&60
 STA SCH

 TXA                    \ Set SC = (X >> 3) * 8
 AND #%11111000
 STA SC

 TYA                    \ Set Y = Y AND %111
 AND #%00000111
 TAY

 TXA                    \ Set X = X AND %111
 AND #%00000111
 TAX

 LDA ZZ                 \ If distance in ZZ >= 144, then this point is a very
 CMP #144               \ long way away, so jump to PX3 to fetch a 1-pixel point
 BCS PX3                \ from TWOS and EOR it into SC+Y

 LDA TWOS2,X            \ Otherwise fetch a 2-pixel dash from TWOS2 and EOR it
 EOR (SC),Y             \ into SC+Y
 STA (SC),Y

 LDA ZZ                 \ If distance in ZZ >= 80, then this point is a medium
 CMP #80                \ distance away, so jump to PX13 to stop drawing, as a
 BCS PX13               \ 2-pixel dash is enough

                        \ Otherwise we keep going to draw another 2 pixel point
                        \ either above or below the one we just drew, to make a
                        \ 4-pixel square

 DEY                    \ Reduce Y by 1 to point to the pixel row above the one
 BPL PX14               \ we just plotted, and if it is still positive, jump to
                        \ PX14 to draw our second 2-pixel dash

 LDY #1                 \ Reducing Y by 1 made it negative, which means Y was
                        \ 0 before we did the DEY above, so set Y to 1 to point
                        \ to the pixel row after the one we just plotted

.PX14

 LDA TWOS2,X            \ Fetch a 2-pixel dash from TWOS2 and EOR it into this
 EOR (SC),Y             \ second row to make a 4-pixel square
 STA (SC),Y

.PX13

 RTS                    \ Return from the subroutine