Input on Programming the Atari VCS in Assembly

The Joystick

Mon, 01 Jan 0001 00:00:00 +0000

The Joystick#

The standard CX40 joystick is the simplest input on the system: five switches — four directions and a fire button — and nothing analog. The four directions of both joysticks are read from a single RIOT port, SWCHA; the fire button comes in elsewhere (see Buttons & Console Switches).

One byte, two joysticks#

SWCHA packs both controllers into its eight bits — player 0 in the high nibble, player 1 in the low nibble — each nibble holding right/left/down/up:

Buttons & Console Switches

Mon, 01 Jan 0001 00:00:00 +0000

Buttons & Console Switches#

The joystick directions come from SWCHA, but the fire buttons and the console switches live elsewhere — and they introduce the one idea that separates “input that works” from “input that feels right”: telling a held button from a just-pressed one.

Fire buttons (INPT4 / INPT5)#

A joystick’s fire button is read not from the RIOT but from two TIA registers: INPT4 for player 0, INPT5 for player 1. The state is in bit 7, and like everything else it’s active-low — pressed reads 0. Because the flag is in bit 7, BIT is the natural test:

Paddles

Mon, 01 Jan 0001 00:00:00 +0000

Paddles#

Everything so far has been a switch: on or off, one bit. Paddles are different — they’re analog, a knob that turns through a continuous range — and the VCS has no analog-to-digital converter. Reading one is a small, clever piece of analog timing, and it’s the most unusual input on the machine.

Reading time, not voltage#

A paddle is a potentiometer: turning the knob changes a resistance. The TIA reads it by charging a capacitor through that resistance and timing how long the charge takes — a low resistance charges fast, a high resistance slowly. The charge time is the knob position.

Driving & Keyboard Controllers

Mon, 01 Jan 0001 00:00:00 +0000

Driving & Keyboard Controllers#

The joystick, paddle, and console switches are the common cases — but the same controller ports accept stranger devices, and two are worth knowing because they show how flexible the VCS’s I/O really is. The driving controller is a paddle look-alike that isn’t analog at all, and the keyboard controller is a twelve-key matrix. Both are built entirely from the SWCHA and INPT registers you’ve already met.