53248-53505



($D300 to $D3FF). ANTIC uses 54272 to 54783 ($D400 to $D5FF).
ANTIC, POKEY and G/CTIA are Large Scale Integration (LSI) circuit
chips. Don't confuse this chip ROM with the OS ROM which is be
found in higher memory. For the most extensive description of these
chips, see the Atari Hardware Manual.

There are two blocks of unused, unavailable memory in the I/O areas:
53504 to 53759 ($D100 to $D1FF) and 54784 to 55295 ($D600 to
$D7FF).

Many of the following registers can't be read directly, since they are
hardware registers. Writing to them can often be difficult because in
most cases the registers change every 30th second (stage two
VBLANK) or even every 60th second (stage one VBLANK)! That's
where the shadow registers mentioned earlier come in. The values
written into these ROM locations are drawn from the shadow registers;
to effect any "permanent" change in BASIC (i.e., while your program
is running), you have to write to these shadow registers (in direct mode
or while your program is running; these values will all be reset to their
initialization state on RESET or powerup).

Shadow register locations are enclosed in parentheses; see these
locations for further descriptions. If no shadow register is mentioned,
you may be able to write to the location directly in BASIC. Machine
language is fast enough to write to the ROM locations and may be able
to bypass the shadow registers entirely.

Another feature of many of these registers is their dual nature. They
are read for one value and written to for another. The differences
between these functions are noted by the (R) for read and (W) for write
functions. You will notice that many of these dual-purpose registers
also have two labels.

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CTIA or GTIA

53248-53505    D000-D0FF
     GTIA (or CTIA) is a special television interface chip designed
     exclusively for the Atari to process the video signal. ANTIC
     controls most of the C/GTIA chip functions. The GTIA shifts the
     display by one-half color clock off what the CTIA displays, so it
     may display a different color than the CTIA in the same piece of
     software. However, this shift allows players and playfields to
     overlap perfectly.

     There is no text window available in GTIA modes, but you can
     create a defined area on your screen with either a DLI (see
     COMPUTE!, September 1982) or by POKEing the GTIA mode
     number into location 87 ($57), POKEing 703 with four and then
     setting the proper bits in location 623 ($26F) for that mode. Only in