Joystick Reading

Overview

Read joystick input from the custom chip register JOY1DAT. The Amiga uses quadrature encoding, requiring an XOR operation to decode vertical movement. Horizontal directions are read directly from specific bits. This pattern provides smooth, continuous movement.

Code

; =============================================================================
; JOYSTICK READING - AMIGA
; Read joystick port 2 via JOY1DAT
; Taught: Game 1 (Signal), Unit 2
; CPU: ~20 cycles | Memory: ~30 bytes
; =============================================================================

CUSTOM      equ $dff000
JOY1DAT     equ $00c            ; Joystick port 2 data (offset from CUSTOM)

; Read joystick directions
; Input:  A5 = CUSTOM chip base
; Output: D0 = decoded directions
;         Bit 8 = up, Bit 0 = down
;         Bit 9 = left, Bit 1 = right
read_joystick:
            move.w  JOY1DAT(a5),d0      ; Read joystick data
            move.w  d0,d1
            lsr.w   #1,d1               ; Shift for XOR decode
            eor.w   d1,d0               ; Decode vertical movement
            rts

Usage:

            lea     CUSTOM,a5           ; Set up custom chip base

mainloop:
            bsr     wait_vblank
            bsr     read_joystick       ; D0 = directions

            ; Check directions using BTST
            btst    #8,d0               ; Up?
            beq.s   .no_up
            sub.w   #MOVE_SPEED,player_y
.no_up:
            btst    #0,d0               ; Down?
            beq.s   .no_down
            add.w   #MOVE_SPEED,player_y
.no_down:
            btst    #9,d0               ; Left?
            beq.s   .no_left
            sub.w   #MOVE_SPEED,player_x
.no_left:
            btst    #1,d0               ; Right?
            beq.s   .no_right
            add.w   #MOVE_SPEED,player_x
.no_right:
            bra     mainloop

Trade-offs

Aspect	Cost
CPU	~20 cycles
Memory	~30 bytes
Limitation	No edge detection (continuous input)

When to use: Games with smooth, continuous movement (shooters, racing).

When to avoid: Grid-based games where you want single-step movement per press.

How It Works

The Amiga joystick uses quadrature encoding (like a mouse). The JOY1DAT register returns:

Bits 1 and 9: Directly indicate right and left
Bits 0 and 8: Require XOR decoding for down and up

The XOR decode formula:

decoded = raw XOR (raw >> 1)

This converts the quadrature signals into simple direction flags.

Direction Bit Reference

Bit	Direction	Check
8	Up	`btst #8,d0`
0	Down	`btst #0,d0`
9	Left	`btst #9,d0`
1	Right	`btst #1,d0`

Fire Button

The fire button is read from CIA-A PRA ($BFE001), not the custom chips. The two joystick ports map to different bits:

Bit	Port	Common use
7	Port 0 (left)	Mouse / second joystick
6	Port 1 (right)	Primary joystick

            btst    #6,$bfe001          ; Joystick port 1 fire (active low)
            beq.s   fire_pressed        ; Branch if pressed

Use bit 7 if you're reading port 0 (JOY0DAT) for a mouse or second player.

Port 0 (mouse port) reading

To read the mouse / second joystick on port 0, use JOY0DAT ($DFF00A) instead of JOY1DAT:

JOY0DAT     equ $00a            ; Joystick port 0 / mouse data

read_port0:
            move.w  JOY0DAT(a5),d0
            move.w  d0,d1
            lsr.w   #1,d1
            eor.w   d1,d0               ; Same XOR decode as port 1
            rts

Reading the mouse rather than a joystick yields delta values via the same quadrature encoding — different decode logic if you want pixel-precise mouse movement, but for "two-player digital joystick" the same decode works.

Patterns: VBlank Game Loop

Vault: Commodore Amiga