Atari hardware

The screen, colour, graphics and sound hardware of each machine that runs Atari BASIC, and where machine-code and data blocks live in its memory.

The two machines are one design fitted with different amounts of RAM. The display, colour, sound and controller hardware below is the same on both — the same 6502 at 1.79 MHz, the same ANTIC, GTIA, POKEY and PIA — so only the Atari 400's memory differs, and it differs in one number.

Atari 800

Screen modes

GRAPHICS <mode> selects a mode, clears the screen and rebuilds the display list. The picture and its display list are laid out downwards from the top of fitted RAM every time, so the screen has no fixed address: PEEK(88) + 256 * PEEK(89) is where it currently starts.

Mode	Kind	Cells / pixels	Colours
GRAPHICS 0	text	40 × 24	1, in two luminances
GRAPHICS 1	text	20 × 24	5
GRAPHICS 2	text	20 × 12	5
GRAPHICS 3	graphics	40 × 24	4
GRAPHICS 4	graphics	80 × 48	2
GRAPHICS 5	graphics	80 × 48	4
GRAPHICS 6	graphics	160 × 96	2
GRAPHICS 7	graphics	160 × 96	4
GRAPHICS 8	graphics	320 × 192	1, in two luminances
GRAPHICS 9	graphics	80 × 192	16 luminances of one hue
GRAPHICS 10	graphics	80 × 192	9, from the whole palette
GRAPHICS 11	graphics	80 × 192	16 hues at one luminance

Modes 1 to 8 keep a four-line text window at the foot of the screen, where PRINT and INPUT go while the picture stays untouched. Adding 16 to the mode number drops it — GRAPHICS 8+16 is the full 320 × 192 screen — and adding 32 keeps what is already on the screen instead of clearing it.

The window costs display, not coordinates. The figures above are the whole picture, and they stay the whole picture with the window present: in GRAPHICS 8 the machine still accepts PLOT 0, 191, it simply shows the top 160 rows. Anything drawn below that is in memory and off the screen until the mode is re-selected with 16 added. Modes 9 to 11 have no text window at all, so the 16 makes no difference to them.

Modes 9, 10 and 11 are GTIA modes: the same 320 × 192 bitmap the display processor draws for GRAPHICS 8, read four bits at a time by the colour chip instead of one. That is where the wide pixel and the deep palette both come from.

GRAPHICS 0 is the 40 × 24 text screen BASIC sits in at its prompt. Never print into column 39: the screen editor reads a character written there as the end of a logical line and pushes the rest of the screen down a row — which tears a drawn picture several moves after the fact. Stop at column 38.

Colour

SETCOLOR <register>, <hue>, <luminance> says what colour one of the five colour registers holds, as a hue from 0 to 15 and a brightness from 0 to 14. Only the even luminances are distinct — the chip ignores the bottom bit — so the palette a program can name is 16 hues × 8 brightnesses, 128 colours in all, plus the greys of hue 0.

COLOR <colour> is the other half of the pair, and it chooses the register rather than the colour: it says which register PLOT and DRAWTO will draw with next. In a text mode COLOR is instead the ATASCII code of the character to draw.

In GRAPHICS 0 the whole screen is one colour: register 2 is the background, register 1 supplies the characters' luminance against it, and register 4 is the border. A program that wants the text screen to change colour animates the background rather than colouring one word. Modes 1 and 2 are the exception — there a character's own code chooses one of four colour registers, so those two text modes carry five colours at once.

Setting a colour through SETCOLOR writes a shadow location in low memory that the vertical-blank routine copies to the chip fifty times a second. POKEing the hardware register directly is undone at the next frame; POKE the shadow instead.

Graphics

PLOT <x>, <y> draws one point in the register COLOR selected and leaves the graphics cursor there; DRAWTO <x>, <y> draws a straight line on from it. The origin is the top-left corner, x runs right and y runs down, and the ranges are the ones in the mode table above. LOCATE <x>, <y>, <numvar> reads a point back — the register number in a graphics mode, the character's code in a text one — which is how a game finds out what it is about to move into. XIO 18 fills an area outlined by plotted lines.

Text modes have block and line graphics of their own: ATASCII 0 to 26 are the shapes printed on the fronts of the keycaps, typed with CTRL and the key they are drawn on, and every character has an inverse-video twin 128 codes higher. An inverse space is a solid block, which is how a text-mode game draws bricks, walls and a paddle. The escape codes page lists what a program can hold.

The machine's players and missiles — hardware sprites — are not reachable from BASIC: there is no keyword for them, and driving them means POKEing GTIA and handing it a page of RAM. That is machine-code work, and the memory section below is where a routine to do it goes. The hardware is here: ANTIC fetches player and missile graphics each scanline and GTIA draws them, so a routine that sets PMBASE and enables the DMA gets sprites on screen.

Sound

SOUND <voice>, <pitch>, <distortion>, <volume> plays a tone on one of POKEY's four voices and keeps playing it until the voice is changed or the program ends. The voice is 0 to 3, and all four can sound at once.

Pitch is a divider from 0 to 255, so a lower number is a higher note. Distortion is an even number from 0 to 14: 10 is a pure tone, 14 a lower-pitched pure tone, and the values below them are the noise settings a program uses for explosions and engines. Volume runs 0 to 15, and the four voices are summed before they leave the chip, so four voices at full volume distort — keep the total at or below 32.

SOUND <voice>, 0, 0, 0 silences one voice; END silences all four. STOP and the BREAK key do not, so a program halted mid-note keeps sounding it.

The console buzzer is separate from POKEY: printing the {bell} escape clicks the speaker directly, through whatever the four voices are doing.

Timing

The machine here is a PAL one: a 1.7734 MHz 6502 over 312 scanlines of 114 cycles each, PEEK(53268) answering 1. A program written for a 60 Hz NTSC Atari — 262 lines, and a different colour clock — runs, but anything timed against the frame runs slow, and its colours come out of a different palette.

ANTIC and the processor share one bus, so every byte the chip fetches is a cycle the program does not get: the display list, the playfield, the players and missiles, and nine cycles of memory refresh whatever else is happening. The playfield is the large term, and the text modes are the expensive ones — a character mode fetches a row of the font on every scanline. GRAPHICS 0 costs roughly a third of the frame, which is why a delay loop is worth timing rather than calculating.

Those stolen cycles are taken off the front of each scanline here rather than spread through it as the chip really takes them. The count per line is right, so anything measured in whole lines or frames matches the machine; a routine timed to a handful of cycles within one scanline does not.

Memory

The whole of the machine's address space, region by region. Zoom in to open a band into the parts it groups, and select a region for its addresses and what sits there.

Forty-eight kilobytes are fitted, but a BASIC program has nothing like 48K. The cartridge covers everything from 40960 (0xA000) and holds the RAM behind it off the bus, the operating system keeps the first two kilobytes, and the screen and its display list come off the top — so the program area runs from 2048 (0x0800) up to about 39968, and FRE(0) reports what is left of it. Selecting a graphics mode takes more off the top than the text screen does.

An Atari program can carry fixed-address machine code or data — memory blocks — that load into RAM alongside the BASIC program before it runs. A block may sit anywhere from 1024 (0x0400) to the top of fitted RAM, and new blocks are offered 1536 (0x0600) — page 6, the one page of this map that neither the operating system nor BASIC ever writes to, and where an Atari machine-code routine has always gone. Addresses are shown in decimal here, because Atari BASIC has no hexadecimal and a POKE has to be written that way; the block editor accepts either notation.

Two bands inside that window are reserved with a warning rather than refused. 1024–1535 (0x0400–0x05FF) is the operating system's buffers — the cassette record, the spare page a disk system would take, and the line buffer every PRINT passes through — so a block there survives only while the program leaves the devices alone. The top 992 bytes are the screen and its display list, which is the least a mode will claim: a graphics mode takes more, so a block near the ceiling is certain to be overwritten.

No Atari file export carries blocks. Page 6 is outside what SAVE and CSAVE write — the machine's own save routines write BASIC's program area and nothing else — so blocks travel with the document through the project bundle and through share links instead. On Run the IDE refuses to start if a block would overlap the BASIC program.

X = USR(<addr>) calls a routine. USR pushes the arguments it was given and then a byte counting them, so the routine must PLA that count before anything else or its RTS returns into nothing.

See the machine code guide and the cross-dialect Machine code & data blocks overview. Every mnemonic, directive and operand form the assembly editor accepts is in the 6502 assembly reference.

Atari 400

Screen modes

Identical to the Atari 800 — the same ANTIC and the same operating system, so the same GRAPHICS modes and the same text window. The 400 simply has less room to hold a screen in: GRAPHICS 8 needs about 8K of the 16K fitted, which is half the machine, and a program that wants the high-resolution modes has little space left for itself.

Colour

Identical to the 800 — the same GTIA, the same five colour registers and the same SETCOLOR / COLOR pair described above.

Graphics

Identical to the 800. The 400's keyboard is a flat membrane rather than moving keys, but the graphics characters it types are the same ATASCII codes.

Sound

Identical to the 800 — the same POKEY and the same four-voice SOUND.

Timing

Identical to the 800 — the same PAL ANTIC and the same cycle-stealing.

Memory

The whole of the machine's address space, region by region. Zoom in to open a band into the parts it groups, and select a region for its addresses and what sits there.

As on the 800, but RAM ends at 16384 (0x4000), and everything between there and the cartridge is empty sockets: the operating system sizes the memory at power-on by writing to it and reading it back, and stops where the writes stop sticking. The screen and its display list come off the top of the 16K rather than the 48K, so the program area runs from 2048 up to about 15392.

The same block window, the same default of 1536, the same two warned bands and the same run-time checks described above apply — the top of the window moves down with the RAM, and nothing else changes.