==Hardware==
The 400/800 originally used the standard 6502B CPU. Later 400/800 CPU board was released that used the Sally 6502C (the C is for custom, not CMOS) chip. The Sally chip became equipped all new Atari 400 & 800, and their successors.
In addition to its 1.79 Mhz [[MOS 6502|6502]] CPU, the Atari computer offloads some of the work to its specialized chips: ANTIC, CTIA & POKEY.
*CTIA is a television interface chip. ANTIC directly controls most of CTIA's operations, but the 6502 CPU can be programmed to intercede and control some or all of CTIA's functions. CTIA converts the digital commands from ANTIC (or the 6502) into a video signal that goes to the TV set or monitor. CTIA also adds some factors of its own, such as colour values, sprites graphics, and collision detection.
When launched in the US, the Atari 400 & 800 came with a CTIA (Color Television Interface Adapter) chip, which offered 9 graphic and text modes (GR.0-8) in Atari BASIC and a 128-colour palette. In early 1982, CTIA was replaced by GTIA (Graphic Television Interface Adapter), which provided 3 additional graphics modes (GR.9-10-11) in Atari BASIC, with a 256-colour palette. The GTIA chip became standard and equipped all new Atari 400 & 800, as well as all computers that succeeded them: the 1200XL / 600XL / 800XL / 65XE / 130XE / XEgs / 800XEand their successors.
POKEY (POt KEYboard integrated circuit) is a digital Input/Output chip. It handles such disparate tasks as the serial I/O (SIO) bus and its peripherals, audio generation, keyboard scan, and random number generation. It also digitizes the resistive paddle inputs and controls maskable interrupt (IRQ) requests from peripherals.