559



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559            22F            SDMCTL
     Direct Memory Access (DMA) enable. POKEing with zero allows
     you to turn off ANTIC and speed up processing by 30%. Of
     course, it also means the screen goes blank when ANTIC is
     turned off! This is useful to speed things up when you are doing a
     calculation that would take a long time. It is also handy to turn off
     the screen when loading a drawing, then turning it on when the
     screen is loaded so that it appears instantly, complete on the
     screen. To use it you must first PEEK(559) and save the result in
     order to return your screen to you. Then POKE 559,0 to turn off
     ANTIC. When you are ready to bring the screen back to life,
     POKE 559 with the number saved earlier.

     This location is the shadow register for 54272 ($D400), and the
     number you PEEKed above defines the playfield size, whether or
     not the missiles and players are enabled, and the player size
     resolution. To enable your options by using POKE 559, simply
     add up the values below to obtain the correct number to POKE
     into SDMCTL. Note that you must choose only one of the four
     playfield options appearing at the beginning of the list:

     Option                          Decimal   Bit
     No playfield                          0   0
     Narrow playfield                      1   0
     Standard playfield                    2   0,1
     Wide playfield                        3   0,1
     Enable missle DMA                     4   2
     Enable player DMA                     8   3
     Enable player and missile
       DMA                                12   2,3
     One line player resolution           16   4
     Enable instructions to fetch
       DMA                                32   5 (see below)

     Note that two-line player resolution is the default and that it is not
     necessary to add a value to 559 to obtain it. I have included the
     appropriate bits affected in the table above. The default is 34
     ($22).

     The playfield is the area of the TV screen you will use for display,
     text, and graphics. Narrow playfield is 128 color clocks (32
     characters wide in GR.0), standard playfield is 160 color clocks
     (40 characters), and wide playfield is 192 color clocks wide (48
     characters). A color clock is a physical measure of horizontal
     distance on the TV screen. There are a total of 228 color clocks on
     a line, but only some of these (usually 176 maximum) will be
     visible due to screen limitations. A pixel, on the other hand, is a