CHAPTER 14
THE HARDWARE CHIPS

The previous chapters described the operating system of the computer. 
The following chapters will examine the hardware which supports the
6502 and the hardware's associated software.

THE GTIA CHIP

The GTIA (George's Television Interface Adapter) is the main video
circuit in the computer.  It controls the following functions.

                           GTIA functions

Priority of overlapping objects

Color and brightness, including information from the antic chip.

Player/missile control.

console switches and game control triggers.

THE ANTIC CHIP

The main job of the ANTIC chip is interpreting the display buffer for
the GTIA chip.  The ANTIC chip is somewhat of a processor in it's own
right.  The program which runs it is called the display list and
usually resides just before the display buffer in memory.

The ANTIC chip operates independent of the 6502.  It operates by
direct memory access (DMA).  The ANTIC chip gives a HALT signal the
6502, causing the 6502 to give up control of the address bus.  The
ANTIC chip can then read any data it needs to from memory.

                        ANTIC chip functions

DMA (Direct Memory Access) control.

NMI (Non-Maskable Interrupt) control.

LIGHT PEN READING

WSYNC (wait for horizontal sync)

THE POKEY CHIP

The most important jobs of the POKEY chip are reading the keyboard and
operating the serial port.  It also has the following functions.

                        POKEY chip functions

Keyboard reading.

Serial port.

Pot (game paddles) reading.

Sound generation.

System timers.

IRQ (maskable interrupt) control.

Random number generator.


THE PIA CHIP

The PIA (Parallel Interface Adapter) is a commonly used I/O chip.  It
consists of two 8 bit parallel ports with hand shaking lines.  In the
Atari, it has the following functions.


Game controller port control (bi-directional).

Peripheral control and interrupt lines.



Registers in the hardware chips are treated as memory addresses.  Many
of the registers are write only.  These registers cannot be read from
after they are written to.  Other registers control one function when
written to and give the status of an entirely different function when
read from.  Still other registers are strobes.  Any command which
causes the address of one of these registers to appear on the address
bus will cause their functions to be performed.

The write only registers have shadow registers in RAM.  Data to be put
in the registers is usually put into the shadow registers.  The data
in the shadow registers is automatically moved to the operating
registers during vertical blank.

For register use and address, see the previous chaptes on the
associated functions.