6502: programs and shared memory
The processor executes program and operating system instructions. It has an 8-bit data bus and 16-bit addressing, seeing 64 KiB at a time. RAM, ROM and device registers share that space, so 64 KiB of addresses does not mean 64 KiB free for a program. Clock speed is approximately 1.77 MHz for PAL and 1.79 MHz for NTSC. ANTIC takes some cycles. The 6502C variant, called SALLY, is distinct from the CMOS 65C02.
ANTIC: video defined by an instruction list
ANTIC reads a display list from memory. Entries define the mode of successive screen sections and the location of display data. A screen can therefore mix text with graphics at different resolutions. ANTIC fetches data through DMA rather than requiring the CPU to copy each byte. It also handles scrolling and DLI interrupts. The programmer prepares the display list and image data; ANTIC reads them while drawing each frame. ANTIC mode numbers differ from BASIC GRAPHICS mode numbers.
GTIA: colours and Player/Missile graphics
GTIA combines ANTIC's playfield with Player/Missile graphics. Four player objects and four narrower missiles have separate positions, widths and colours. Registers determine overlap priority and report collisions. The number of simultaneous colours depends on the display mode.
Early 400/800 machines have CTIA. It handles the basic graphics but lacks GTIA's additional modes.
POKEY: sound and communication
POKEY has four sound channels. Frequency dividers and noise generators create tones and effects; selected channels can be paired for finer frequency control. It also handles keyboard scanning, paddle measurement, interrupts and serial data transfer. SIO thus uses part of the same chip as music. PIA additionally handles joystick directions and memory banking, while GTIA reads fire buttons. POKEY does considerably more than generate sound.