Hardware

Centipede runs on Atari's 1981 Centipede PCB (MAME driver `atari/centiped.cpp`, machine `centiped`). The CPU is a MOS 6502 clocked at 1.512 MHz (12.096 MHz / 8). The 6502 halves to 0.756 MHz whenever it touches playfield RAM (`0x0400`–`0x07BF`) — the video hardware steals the bus there — so the real per-frame instruction count is lower than the nominal 25200 cycles and depends on what the code is doing. The frame rate is a flat 60 Hz. The native raster is 256×256 with a 256×240 visible window; the monitor is mounted rotated 90° counter-clockwise (MAME `ROT270`, a portrait cabinet).

The player controls a shooter along the bottom of the screen with a trackball, not a joystick: the trackball's X and Y are analogue deltas read as up/down counters (a 4-bit magnitude plus a sign bit) at two input ports, not as direction bits.

Two hardware conventions matter when reading the map. (1) A read and a write at one address can be different devices — most visibly at `0x2000`, where a read returns program ROM and a write kicks the watchdog. (2) A bank of single-bit outputs lives in an LS259 addressable latch at `0x1C00`–`0x1C07`: one address per line, and the value written is data bit 7 (`write_d7`) — the coin counters, the two player LEDs (inverted), and the screen-flip bit.

The one interrupt is an IRQ (not an NMI), asserted by the video counter chain a few times per frame; the program acknowledges it by writing `0x1800` (`irq_ack_w`), which clears the 6502 IRQ line. High scores survive power-off in an EAROM (a small serial NVRAM) at `0x1600`–`0x173F`. Sound is a POKEY custom chip at `0x1000`–`0x100F` (also the source of the hardware random number the game reads).

Memory & I/O map

Address	Name	Description
0000:03ff	workRam	Work RAM (see Work RAM)
0400:07bf	videoRam	Video RAM — a 32×30 grid of tile codes (the mushroom field and text)
07c0:07ff	spriteRam	Sprite RAM — 16 objects × 4 fields (the shooter, centipede segments, spider, flea, scorpion, shots)
0800	dsw1	R: DIP switch bank 1 (lives, bonus, difficulty, language)
0801	dsw2	R: DIP switch bank 2 (coinage)
0c00	in0 / trackballX	R: IN0 — trackball X (bits 0-3 count, bit 7 sign), cabinet DIP (bit 4), service (bit 5, active-low), VBLANK (bit 6, active-high)
0c01	in1	R: IN1 (active-low) — start 1P/2P, fire, tilt, coins, service
0c02	in2 / trackballY	R: IN2 — trackball Y (bits 0-3 count, bit 7 sign)
0c03	in3	R: IN3 — cocktail P2 joystick (unused on the upright board)
1000:100f	pokey	R/W: POKEY sound chip; a read returns the hardware random number / pot state
1400:140f	paletteRam	W: colour RAM — the live palette the video hardware reads
1600:163f	earomWrite	W: EAROM (high-score NVRAM) data
1680	earomControl	W: EAROM control
1700:173f	earomRead	R: EAROM data
1800	irqAck	W: acknowledge/clear the 6502 IRQ line
1c00:1c07	outLatch	W (D7): LS259 output latch — one address per line (coin counters, LEDs, screen flip)
2000	rom / watchdog	R: program ROM. W: watchdog reset
2000:3fff	rom	Program ROM, 8192 bytes (`centiped` parts 136001-307.d1 + -308.e1 + -309.fh1 + -310.j1)

IN0 — trackball X, cabinet, service, VBLANK (read at 0x0C00, MIXED polarity)

Bit	Mask	Input
0–3	0x0f	Trackball X magnitude (this frame's horizontal delta)
4	0x10	DIP: cabinet (0 = upright)
5	0x20	Service (active-low)
6	0x40	VBLANK (active-high — the program polls it to sync to the frame)
7	0x80	Trackball X sign (direction)

IN1 — controls + coins (read at 0x0C01, active-low, idle 0xFF)

Bit	Mask	Input
0	0x01	Start 1P
1	0x02	Start 2P
2	0x04	Fire
3	0x08	Fire (cocktail)
4	0x10	Tilt
5	0x20	Coin 1
6	0x40	Coin 2
7	0x80	Service credit

IN2 — trackball Y (read at 0x0C02)

Bit	Mask	Input
0–3	0x0f	Trackball Y magnitude (this frame's vertical delta)
7	0x80	Trackball Y sign (direction)

LS259 output latch (0x1C00–0x1C07, one address per line, value = data bit 7)

Address	Line
1c00	Coin counter (left)
1c01	Coin counter (centre)
1c02	Coin counter (right)
1c03	Player-1 start LED (inverted)
1c04	Player-2 start LED (inverted)
1c07	Screen flip (cocktail)

Addresses `0x1C05`–`0x1C06` are unused.

Video — mushroom-field tilemap + sprites

The playfield is a 32×30 tilemap at `0x0400`–`0x07BF`: one tile code per cell, holding the mushroom field and the on-screen text. Sprite RAM at `0x07C0`–`0x07FF` holds 16 objects of four bytes each — the shooter, the centipede's segments, the spider, the flea, the scorpion, and the shots — each record giving a picture number, a colour, and a screen position. The colours are not fixed in a PROM: the program writes a live palette into colour RAM at `0x1400`–`0x140F`, which the video hardware reads as it draws (this is how the game flashes and recolours the field between waves). The character and sprite pictures come from two `0x800` bitplane ROMs (8×8 characters and 8×16 sprites decode from the same data).