Atari 8-bit

The Atari 8-bit family (400, 800, XL, XE — 1979 onwards) is built around a 1.79 MHz MOS 6502B CPU, but its real strength lies in the custom chips alongside it: ANTIC (graphics), GTIA (display), POKEY (sound + keyboard), and PIA (I/O). Together they give the Atari 8-bit hardware capabilities that rival the C64.

The Atari 8-bit machines are well-documented, have an active community, and offer some genuinely unique features — particularly ANTIC's Display List, which allows multiple graphics modes to be mixed on the same screen.

Languages in the IDE

Language	Use for
CC65 C	Structured programs targeting the Atari 8-bit
6502 assembly	Low-level hardware access and maximum performance

Quick start

Select Atari 8-bit (or the specific model) as the platform.
Choose C / cc65 or Assembly.
The IDE loads an Atari emulator (Altirra-based or similar) with your program.

Hardware overview

The custom chips

Unlike the C64, the Atari 8-bit doesn't map everything through one or two chips. Four custom chips each handle different roles:

ANTIC ($D400–$D41F) — the display processor. It reads a Display List from RAM and drives the electron beam independently of the 6502. The CPU can set up the Display List and then let ANTIC handle the video with minimal intervention.

GTIA ($C000–$C0FF) — the colour/priority chip. It translates ANTIC's output into colours, handles player/missile (sprite) priorities, and supports several colour interpretation modes.

POKEY ($D200–$D21F) — handles sound (4 channels), the keyboard, serial I/O (SIO bus), and timer interrupts. The 4-channel sound hardware is very capable.

PIA ($D300–$D3FF) — joystick ports and some I/O.

The Display List

ANTIC's Display List is a table in RAM that tells ANTIC what to draw on each line of the screen. Each entry is one byte (a mode command) or three bytes (a mode command + address of the data to display). ANTIC reads this table at 1.79 MHz while the CPU is doing something else.

This is powerful: you can mix text and graphics modes on the same screen, change the colour of different parts of the screen by inserting wait-for-line-blank (DLI) instructions, or do raster effects without tying up the CPU.

A minimal Display List:

; Display List for a 40-column text screen
display_list:
    .byte $70, $70, $70     ; 3 × 8 blank lines
    .byte $42               ; Mode 2 text + LMS (Load Memory Scan)
    .word screen_ram        ; Address of first text line
    .byte $02, $02, $02     ; Remaining text lines (24 total)
    .byte $02, $02, $02
    .byte $02, $02, $02
    .byte $02, $02, $02
    .byte $02, $02, $02
    .byte $02, $02, $02
    .byte $02, $02, $02
    .byte $41               ; Jump and wait for VBlank
    .word display_list      ; Loop back to start

Graphics modes

ANTIC supports multiple text and graphics modes, each with different resolutions and colour depths. The OS adds extra modes on top.

From BASIC, modes are selected with GRAPHICS n:

GRAPHICS #	ANTIC mode	Pixels	Colours
0	2	40×24 text	2+bg
8	F	320×192	2
7	D	160×96	4
15	F	160×192	4 (MultiColor)

Graphics modes 1–6 and 9–15 offer various resolutions and colour depths. The GTIA can also apply three additional colour interpretation modes to ANTIC's output, creating the "GTIA modes" with unusual colour/resolution tradeoffs.

Memory map

Range	Contents
$0000–$00FF	Zero page (OS uses some — check the OS manual)
$0100–$01FF	Stack
$0200–$02FF	OS page 2
$0300–$03FF	OS page 3
$0400–$04FF	OS page 4 (floating point)
$0600–$BFFF	Free RAM (varies by model and cart)
$C000–$CFFF	GTIA registers
$D000–$D0FF	(Reserved)
$D200–$D2FF	POKEY registers
$D300–$D3FF	PIA registers
$D400–$D4FF	ANTIC registers
$D800–$DFFF	Floating point ROM
$E000–$FFFF	OS ROM

Key hardware registers

ANTIC:

Address	Register	Purpose
$D400	DMACTL	DMA enable, display width
$D402/$D403	DLISTL/H	Display List address
$D407	PMBASE	Player/Missile base address
$D40A	WSYNC	Write to halt CPU until end of scanline
$D40B	VCOUNT	Current scanline counter
$D40C	PENH	Light pen horizontal
$D40E	NMIEN	NMI enable (VBlank and DLI)

GTIA:

Address	Register	Purpose
$C000–$C003	M0PF–M3PF	Missile/player collisions
$C010–$C017	COLPM0–COLPF3	Player and playfield colours
$C018	COLBK	Background colour
$C01F	CONSOL	Console keys (Start/Select/Option)

POKEY:

Address	Register	Purpose
$D200/$D201	AUDF1/AUDC1	Channel 1 frequency/control
$D202/$D203	AUDF2/AUDC2	Channel 2
$D204/$D205	AUDF3/AUDC3	Channel 3
$D206/$D207	AUDF4/AUDC4	Channel 4
$D208	AUDCTL	Audio control (frequency base, poly clocks)
$D209	STIMER	Start timers
$D20E	IRQEN	IRQ enable
$D20F	SKCTL	Serial/keyboard control

Players and Missiles (sprites)

Atari 8-bit "hardware sprites" are called Players (wide) and Missiles (narrow):

4 Players: each 8 pixels wide, 128/192/256 scanlines tall (selectable height), one colour per player
4 Missiles: each 2 pixels wide, associated with a player's colour, or combined into a 5th player

Players and Missiles are defined as vertical byte strips in RAM, starting at the address specified by PMBASE. The horizontal position is set in GTIA registers.

; Position Player 0 at X=100
LDA #100
STA $D000       ; HPOSP0 — Player 0 horizontal position

; Enable Player 0 DMA
LDA #$0E        ; DMACTL: enable PM DMA, wide display
STA $D400

Programming with cc65 C

cc65 includes an atari platform target with headers for all the hardware chips:

#include <atari.h>
#include <stdio.h>
#include <conio.h>

int main(void) {
    /* Change background colour */
    GTIA_WRITE.colbk = 0x94;    /* Blue-ish */

    /* Enable graphics mode 0 (text) */
    /* In cc65, use the OS screen() function or direct GTIA */

    printf("Hello, Atari!\n");

    /* Read joystick 0 (PORTA on PIA) */
    unsigned char joy = PIA.porta;
    /* Bits 0-3: Up/Down/Left/Right (active low) */
    if (!(joy & 0x01)) {
        /* Joystick up */
    }

    return 0;
}

BASIC quick reference

In Atari BASIC (accessed via the OS ROM):

10 GRAPHICS 7+16     : REM Mode 7, no text window (+16)
20 SETCOLOR 2,4,6    : REM Set playfield colour 2: hue 4, lum 6
30 COLOR 1           : REM Set drawing colour register 1
40 PLOT 80,48        : REM Plot pixel at (80, 48)
50 DRAWTO 100,80     : REM Draw line to (100, 80)
60 STICK(0)          : REM Read joystick 0 (15=centre, bits for directions)
70 STRIG(0)          : REM Read fire button 0 (1=not pressed, 0=pressed)

Display List Interrupts (DLI)

DLIs are a powerful feature — the ANTIC chip can trigger an NMI on specific scanlines, allowing you to change colours, scroll positions, or switch modes mid-screen.

; Simple DLI — change background colour on a specific scanline
dli_handler:
    PHA                 ; Save A
    LDA #$28            ; New background colour (orange)
    STA $C018           ; COLBK (WSYNC isn't needed here but helps timing)
    PLA
    RTI

; Set DLI handler address
    LDA #<dli_handler
    STA $0200           ; VDSLST (Display List IRQ vector, low byte)
    LDA #>dli_handler
    STA $0201           ; VDSLST high byte
    LDA #$80            ; Enable DLI NMI
    STA $D40E           ; NMIEN

Add $80 to a Display List entry to trigger the DLI on that line.

Tips

Use WSYNC for timing — writing any value to $D40A (WSYNC) halts the CPU until the end of the current scanline. This is the easiest way to time hardware accesses precisely.

Use the OS SIO for storage — the Atari's Serial I/O (SIO) bus handles disk drives, cassette, and other peripherals through a clean OS interface. Don't bit-bang the hardware if you don't have to.

Collision detection is hardware — GTIA tracks collisions between players, missiles, and playfields automatically. Read the collision registers ($C000–$C00F) for zero-cost collision data.

POKEY has built-in serial — the SIO bus uses POKEY for clocking. If you're writing custom SIO-compatible hardware, POKEY handles the timing.