Input#

Player input arrives through two chips. The joystick directions and the console switches come from the RIOT’s I/O ports SWCHA and SWCHB; the fire buttons and analog paddles come through the TIA’s INPT0–INPT5 registers. Either way, reading input is a memory-mapped load and a bit test — cheap. The craft is in interpreting those bits: active-low logic, telling a held button from a fresh press, and turning a paddle’s analog knob into a number at all.

Paired with Collisions, this is what closes the loop from “a picture on screen” to “a game the player controls.”

  • The Joystick — SWCHA’s two-players-in-one-byte layout, and the active-low rule that trips up everyone.
  • Buttons & Console Switches — the fire buttons (INPT4/INPT5), the SWCHB console switches, and held-vs-just-pressed edge detection.
  • Paddles — reading an analog knob with no ADC, by timing a charging capacitor across the frame.
  • Driving & Keyboard Controllers — the Indy 500 spinner (a digital encoder, not a paddle) and the keypad matrix scanned by turning the port into outputs.

Joystick and switch bits are active-low: a pressed direction reads as 0, not 1. Isolate the bit with AND/BIT and branch on zero, rather than comparing to a positive value.

Backward Reading & Clearing The Joystick Forward