amiga-bootcamp/10_devices/gameport.md

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gameport.device — Joystick and Mouse Ports

Overview

gameport.device provides access to the Amiga's two controller ports (mouse/joystick). The hardware reads controller state through custom chip registers JOY0DAT/JOY1DAT and the CIA-A port for fire buttons. Understanding the hardware protocol is critical for FPGA cores.

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Hardware Registers

Register	Address	Controller	Function
JOY0DAT	$DFF00A	Port 1 (mouse)	Quadrature-encoded position data
JOY1DAT	$DFF00C	Port 2 (joystick)	Direction bits or quadrature data
JOYTEST	$DFF036	Both	Write to set counter test value
POTGO	$DFF034	Both	Proportional/paddle port control
POTGOR	$DFF016	Both	Read paddle/proportional values
CIA-A PRA	$BFE001	Both	Fire buttons (active low)

JOYxDAT Bit Layout

Bits 15–8: Y counter (or vertical quadrature)
Bits 7–0:  X counter (or horizontal quadrature)

Mouse Quadrature Decoding

For mouse input, the X/Y values are quadrature counters — they increment/decrement as the mouse moves:

/* Read mouse delta: */
UWORD joy = custom->joy0dat;  /* current counter */
WORD dx = (WORD)(joy & 0xFF) - (WORD)(prev_joy & 0xFF);
WORD dy = (WORD)(joy >> 8) - (WORD)(prev_joy >> 8);
/* dx/dy = movement since last read (signed, wraps at 255→0) */
prev_joy = joy;

Joystick Digital Decoding

For digital joysticks, the direction bits are encoded:

UWORD joy = custom->joy1dat;
BOOL right = joy & 0x0002;
BOOL left  = (joy >> 1) ^ (joy & 0x0001);  /* XOR of bit 1 and bit 0 */
BOOL down  = joy & 0x0200;
BOOL up    = (joy >> 9) ^ ((joy >> 8) & 0x0001);

/* Fire button — from CIA-A: */
BOOL fire  = !(ciaa->ciapra & 0x80);  /* port 2 button, active low */
/* Port 1 fire = bit 6 of CIAA PRA */

Important

The joystick directional decoding uses XOR between adjacent bits — not direct reads. This is because the hardware shares the same quadrature interface used for mice. This is a common source of bugs in FPGA implementations.

Fire Buttons

Button	Register	Bit	Port
Port 1 fire (left mouse)	CIA-A PRA $BFE001	6	Mouse port
Port 2 fire (joystick)	CIA-A PRA $BFE001	7	Joystick port
Port 1 middle/right	POTGOR $DFF016	8, 10	Mouse port
Port 2 second button	POTGOR $DFF016	12, 14	Joystick port

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Using gameport.device (OS Level)

struct MsgPort *gpPort = CreateMsgPort();
struct IOStdReq *gpReq = (struct IOStdReq *)
    CreateIORequest(gpPort, sizeof(struct IOStdReq));

/* Open port 1 (joystick port): */
OpenDevice("gameport.device", 1, (struct IORequest *)gpReq, 0);

/* Set controller type: */
UBYTE type = GPCT_ABSJOYSTICK;
gpReq->io_Command = GPD_SETCTYPE;
gpReq->io_Data    = (APTR)&type;
gpReq->io_Length  = 1;
DoIO((struct IORequest *)gpReq);

/* Set trigger conditions: */
struct GamePortTrigger trigger;
trigger.gpt_Keys   = GPTF_UPKEYS | GPTF_DOWNKEYS;
trigger.gpt_Timeout = 0;   /* no timeout */
trigger.gpt_XDelta  = 1;   /* report on any movement */
trigger.gpt_YDelta  = 1;
gpReq->io_Command = GPD_SETTRIGGER;
gpReq->io_Data    = (APTR)&trigger;
gpReq->io_Length  = sizeof(trigger);
DoIO((struct IORequest *)gpReq);

/* Read events: */
struct InputEvent ie;
gpReq->io_Command = GPD_READEVENT;
gpReq->io_Data    = (APTR)&ie;
gpReq->io_Length  = sizeof(ie);
SendIO((struct IORequest *)gpReq);

/* Wait for joystick event: */
Wait(1L << gpPort->mp_SigBit);
WaitIO((struct IORequest *)gpReq);
/* ie now contains joystick movement/button data */

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Controller Types

Constant	Value	Device
GPCT_MOUSE	1	Standard Amiga mouse
GPCT_RELJOYSTICK	2	Relative joystick (proportional)
GPCT_ABSJOYSTICK	3	Absolute/digital joystick

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References

• NDK39: devices/gameport.h, hardware/custom.h, hardware/cia.h
• HRM: Amiga Hardware Reference Manual — Controller Ports chapter
• See also: input.md — input handler chain that receives gameport events
• See also: keyboard.md — keyboard shares CIA-A interrupt infrastructure