Drawing pixels: PX3

       Name: PX3                                                     [Show more]
       Type: Subroutine
   Category: Drawing pixels
    Summary: Plot a single pixel at (X, Y) within a character block
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * PIXEL calls PX3


 This routine is called from PIXEL to set 1 pixel within a character block for
 a distant point (i.e. where the distance ZZ >= &90). See the PIXEL routine for
 details, as this routine is effectively part of PIXEL.


 Arguments:

   X                    The x-coordinate of the pixel within the character block

   Y                    The y-coordinate of the pixel within the character block

   SC(1 0)              The screen address of the character block

   T1                   The value of Y to restore on exit, so Y is preserved by
                        the call to PIXEL


.PX3

 LDA TWOS,X             \ Fetch a 1-pixel byte from TWOS and EOR it into SC+Y
 EOR (SC),Y
 STA (SC),Y

 LDY T1                 \ Restore Y from T1, so Y is preserved by the routine

 RTS                    \ Return from the subroutine