54019



                     register; zero equals direction control register.
     1               Set to zero.
     0 (write)       Peripheral A interrupt (IRQ) enable. One equals
                     enable. Set by the OS but available to the user;
                     reset on powerup.

54019          D303           PBCTL
     (W/R) Port B controller. Initialized to 60 ($3C) by the OS IRQ
     code. PBCTL is the same as PACTL, above, with the following
     exception (this may actually perform the same function as in
     PACTL, but I am not sure of the distinction between descriptions):

     Bit             Function
     3               Peripheral command identification (serial bus
                     command), initialized to 60 ($3C).

     Ports can be used for external control applications by the
     technically minded reader who is willing to do some soldering to
     develop cables and connectors. A good example can be found in
     COMPUTE!, February 1981, where the author gives directions for
     using jacks three and four as a printer port. The Macrotronic
     printer cables use just this method, bypassing the 830 interface
     entirely (one way of reducing your hardware costs). Theoretically,
     the entire Atari can be controlled through the ports!

---------------------------------------------------------------------------
Locations 54020 to 54271 ($D304 to $D3FF) are repeats of locations
54016 to 54019 ($D300 to $D303).

---------------------------------------------------------------------------

ANTIC

54272-54783    D400-D5FF
     ANTIC is a special, separate microprocessor used in your Atari
     to control C/GTIA, the screen display, and other screen-related
     functions including processing the NMI interrupts. It uses its own
     instruction set, called the display list, which tells ANTIC where to
     find the screen data in RAM and how to display it. ANTIC also
     uses an internal four bit counter called the Delta Counter (DCTR)
     to control the vertical dimension of each block.

54272          D400           DMACTL
     (W) Direct Memory Access (DMA) control. It is also used to
     define one- or two-line resolution for players and to turn on
     players and missiles. Values are POKEd into the shadow register,
     559 ($22F), and are also described there. You POKE the shadow
     register with the following numbers in order to:

     Turn off the playfield                           0
     Use narrow playfield                             1