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.