c64-game-ports

name

c64-game-ports

description

The two Commodore 64 control (game) ports: reading joysticks, paddles, a light pen, and mouse, and how they map to hardware registers (CIA1 $DC00/$DC01, SID POT $D419/$D41A, VIC light-pen $D013/$D014). Use this skill WHEN a user asks "which port is player 1", "how do I read the joystick", "what bits are up/down/ left/right/fire", "how do I read paddles", "why are paddles unreliable in BASIC", "how does the light pen work", or mentions control port 1/2, $DC00, $DC01, fire button, or POTX/POTY. Pairs with c64-cia, c64-sid, and c64-vic-ii.

C64 Control (Game) Ports

The C64 has two DE-9 control ports on the right side. Each takes one joystick or one paddle pair; a light pen goes in control port 1 only. The ports are wired to CIA1 (the joystick/fire-button/keyboard-scan chip) and, for paddles, to the SID POT lines.

Hardware mapping — which register is which port

Read at	CIA1 register	Reads control port
56321 / $DC01	Port B (PRB)	Control port 1
56320 / $DC00	Port A (PRA)	Control port 2

Note the crossover: port 1 is at the higher address ($DC01), port 2 at the lower ($DC00). Each port's joystick uses the low 5 bits:

bit 4 3 2 1 0
    │ │ │ │ └─ 0 = UP     (switch 0)
    │ │ │ └─── 1 = DOWN   (switch 1)
    │ │ └───── 2 = LEFT   (switch 2)
    │ └─────── 3 = RIGHT  (switch 3)
    └───────── 4 = FIRE   (switch 4)

Active LOW: a bit reads 1 when the switch is open (not pressed) and 0 when closed (pressed/pushed). So "no input" reads %11111 = 31 on the low 5 bits; pushing up pulls bit 0 to 0, etc. Bits 5–7 carry other CIA1 data (keyboard matrix, paddle select).

Reading a joystick

BASIC (port 2 here; use $DC01 / PEEK(56321) for port 1):

100 JV=PEEK(56320)        : REM raw port-2 value
110 FR=(JV AND 16)        : REM 0 = fire pressed, 16 = not pressed
120 JV=15-(JV AND 15)     : REM 0=none,1=up,2=down,4=left,8=right (combine for diagonals)

JV (after line 120)	Direction
0	none
1	up
2	down
4	left
5	up & left
6	down & left
8	right
9	up & right
10	down & right

Assembly (reads port 2 at $DC00; use $DC01 for port 1; carry = fire state):

djrr    lda $dc00     ; read port (use $dc01 for the other port)
        ldy #0
        ldx #0
        lsr a         ; shift each direction bit into carry
        bcs djr0      ; bit 0 (up):   carry set = switch open
        dey           ;               switch closed -> dy = -1 (move up)
djr0    lsr a         ; bit 1 (down)
        bcs djr1
        iny
djr1    lsr a         ; bit 2 (left)
        bcs djr2
        dex
djr2    lsr a         ; bit 3 (right)
        bcs djr3
        inx
djr3    lsr a         ; bit 4 -> carry = FIRE (C=1 not pressed, C=0 pressed)
        stx dx        ; dx/dy are 2's-complement -1/0/+1 deltas
        sty dy
        rts

Keyboard-scan conflict on port 1

CIA1 also scans the keyboard, and the keyboard matrix shares lines with the control ports — especially port 1 (PRB/$DC01), which is on the same side the keyboard rows/columns use. Reading a joystick in port 1 can register phantom keypresses and vice versa; pressing certain keys can look like joystick motion. This is why single-joystick software overwhelmingly prefers port 2. The full CIA1 keyboard-scan register behavior is in c64-cia.

Which port is player 1?

Hardware facts: control port 1 reads at $DC01, control port 2 reads at $DC00. There is no hardware notion of "player 1" — both ports are equal silicon. By long-standing software convention, single-player and one-joystick games most often use control port 2 (it dodges the keyboard-scan interference on port 1). Two-player games then use port 1 for player 2. But this is convention, not a hardware law — always check the specific game; some titles use port 1.

Paddles

A paddle pair connects through a game port to both CIA1 and SID. The analog position is read from the SID POT registers:

Register	Addr
POTX	54297 / $D419	paddle X position (0–255)
POTY	54298 / $D41A	paddle Y position (0–255)

Each port has two pots (X and Y), so two paddles per port = up to four paddles total. But SID has only one POTX/POTY pair, so you select which port's pots are connected via the top two bits of CIA1 Port A ($DC00): write $40/$80 to PRA to switch the multiplexer between the two paddle pairs, wait briefly for the RC reading to settle, then read $D419/$D41A. Paddle fire buttons appear on the joystick bits (bit 2 = pot-X button, bit 3 = pot-Y button).

PADDLES ARE NOT RELIABLE READ FROM BASIC ALONE — the timing/multiplexing is too tight, so even from BASIC you SYS a short ML routine (the manual's pdlrd routine, also given as DATA statements) and PEEK the results. Full routine in references/game-ports.md. The SID POT register details live in c64-sid.

Light pen

A light pen plugs into control port 1 only. On a low-going edge (the pen sensing the beam) the VIC-II latches the current beam position:

Register	Addr	Contents
LPX	19 / $D013	X latch — high 8 bits of a 9-bit (512-state) X counter → 2-dot resolution
LPY	20 / $D014	Y latch — 8 bits, single-raster resolution in the visible area

The latch fires at most once per frame (later triggers in the same frame are ignored), and reads are noisy, so average several samples (3+). Light-pen register/IRQ detail is in c64-vic-ii.

Mouse

The Commodore 1351 mouse plugs into a control port and, in its proportional mode, reports movement through the same SID POT registers ($D419/$D41A) the paddles use (its buttons appear on the joystick fire/direction bits). The C64 manuals predate the 1351 and don't document it; treat it as a paddle-style POT device on the control port.

Quick answers

• "Which port is player 1?" Hardware: port 1 = $DC01, port 2 = $DC00, no built-in "player" meaning. By convention single-joystick games use port 2.
• "How do I read the joystick?" JV=PEEK(56320) (port 2) or PEEK(56321) (port 1); bits 0–4 = up/down/left/right/fire, active LOW.
• "How do I tell if fire is pressed?" (PEEK(56320) AND 16)=0 → pressed.
• "Why are paddles garbage in BASIC?" Timing — SYS the ML read routine and PEEK the values from $D419/$D41A.

How to read the references

• references/game-ports.md — verbatim PRG Ch6 "The Game Ports", "Paddles", and "Light Pen": the joystick switch diagram, the BASIC + assembly joystick read routines, the JV→direction table, the full four-paddle ML read routine (with DATA-statement BASIC loader), and the light-pen latch description. Read this for the exact code and the paddle multiplexing/select sequence.

Cross-links

• CIA1 registers, data-direction, and the keyboard-scan that conflicts on port 1 → c64-cia
• The SID POT (paddle) registers $D419/$D41A → c64-sid
• The VIC-II light-pen latch $D013/$D014 and its IRQ → c64-vic-ii