PC Hardware in a Nutshell, Second Edition

Gameport Interface

A standard PC gameport is actually a simple general-purpose analog data-acquisition port that supports four simple switched inputs and four variable-voltage inputs. A game controller connected to this interface uses one or more of the switched inputs to support buttons and one or more of the variable inputs to support a joystick, steering wheel, or similar device.

A paddle (originally used for Pong) is the simplest game controller. It uses only one switched input for a button and one variable input for a knob that controls movement in one direction. Accordingly, a gameport can support up to four paddles. A joystick uses one or two switched inputs for buttons and two variable inputs for a central stick that controls movement in both the horizontal (x-axis) and vertical (y-axis) directions. Accordingly, a gameport can support one or two joysticks. More complex game controllers use more of the switched inputs for additional buttons, and more of the variable inputs to control a third (z-axis) direction or for other purposes. This means that a gameport can support only one such controller.

Gameports do not require an IRQ, a DMA channel, or mapped memory, and occupy only one I/O base address. The downside of this small resource footprint is that gameports are not interrupt-driven, so the CPU must constantly poll the gameport to detect when a button is pressed or a stick is moved. Polling can require as much as 10% of the CPU, which degrades performance when you least ...