670



670            29E            SUBTMP
     Temporary storage.

671            29F            HOLD2
     Temporary register.

672            2A0            DMASK
     Pixel location mask. DMASK contains zeroes tor all bits which do
     not correspond to the specific pixel to be operated upon, and
     ones for bits which do correspond, according to the GRAPHICS
     mode in use, as follows:

     11111111  Modes 0, 1 and 2:   one pixel per screen display
                                   byte.
     11110000  Modes 9, 10 and 11: two pixels per byte.
     00001111
     11000000  Modes 3, 5 and 7:   four pixels per byte.
     00110000
     00001100
     00000011
     10000000  Modes 4, 6 and 8:   eight pixels per byte.
     01000000

     etc. to:

     00000001

     A pixel (short for picture cell or picture element) is a logical unit
     of video size which depends on the GRAPHICS mode in use for
     its dimensions. The smallest pixel is in GR.8 where it is only .5
     color clock wide and one scan line high. In GR.0 it is also only .5
     color clock wide, but it is eight scan lines high. Here is a chart of
     the pixel sizes for each mode:

                         Text Modes          Graphics modes
     GR. mode        0    1    2    3    4    5    6    7    8
     Scan lines
     per pixel       8    8   16    8    4    4    2    2    1
     Bits
     per pixel       1    1    1    2    1    2    1    2    1
     Color clocks
     per pixel      .5    1    1    4    2    2    1    1   .5
     Characters
     per line       40   20   20   --   --   --   --   --   --
     Pixels
     per width      --   --   --   40   80   80  160  160  320

     The number of pixels per screen width is based on the normal
     playfield screen. See location 559 ($22F) for information on