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