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 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.
POKEY (POt KEYboard integrated circuit) is a digital Input/Output chip. It handles such disparate tasks as the 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.