12.24 Mice, Trackpads, and Other Pointing Devices

12.24 Mice, Trackpads , and Other Pointing Devices

Along with disk drives , keyboards, and display devices, pointing devices are probably the most common peripherals you'll find on modern personal computers. Pointing devices are actually among the more simple peripheral devices, providing a very simple data stream to the computer. Pointing devices generally come in two categories: those that return the relative position of the pointer and those that return the absolute position of the pointing device. A relative position is simply the change in position since the last time the system read the device; an absolute position is some set of coordinate values within a fixed coordinate system. Mice, trackpads, and trackballs are examples of devices that return relative coordinates; touch screens, light pens, pressure-sensitive tablets, and joysticks are examples of devices that return absolute coordinates. Generally , it's easy to translate an absolute coordinate system to a relative one, but a bit more problematic to convert a relative coordinate system to an absolute one. This latter conversion requires a constant reference point that may become meaningless if, for example, someone lifts a mouse off the surface and sets it down elsewhere. Fortunately, most windowing systems work with relative coordinate values from pointing devices, so the limitations of pointing devices that return relative coordinates are not a problem.

Early mice were typically opto-mechanical devices that rotated two encoding wheels that were oriented along the X- and Y-axes of the mouse body. Usually, both of these wheels were encoded to send 2-bit pulses whenever they would move a certain distance. One bit told the system that the wheel had moved a certain distance, and the other bit told the system which direction the wheel had moved. ^[6] By constantly tracking the four bits (two bits for each axis) from the mouse, the computer system could determine the mouse's distance and direction traveled, and keep a very accurate calculation of the mouse's position in between application requests for that position.

One problem with having the CPU track each mouse movement is that when moved quickly, mice can generate a constant and high-speed stream of data. If the system is busy with other computations , it might miss some of the incoming mouse data and would therefore lose track of the mouse's position. Furthermore, using the host CPU to keep track of the mouse position consumes CPU time that could be put to better use doing application computations.

As a result, mouse manufacturers decided early on to incorporate a simple microcontroller in the mouse package. This simple microcontroller keeps track of the physical mouse movements and responds to system requests for mouse coordinate updates, or at the very least generates interrupts on a periodic basis when the mouse position changes. Most modern mice connect to the system via the USB and respond with positional updates to system requests that occur about every eight milliseconds .

Because of the wide acceptance of the mouse as a GUI pointing device, computer manufacturers have created many other devices that serve the same purpose. The motivation behind developing most of these devices has been to increase portability - mice aren't the most convenient pointing devices to attach to a laptop computer system on the road. Trackballs, strain gauges (the little 'stick' you'll find between the G and H keys on many laptops), trackpads, trackpoints, and touch screens are all examples of devices that manufacturers have attached to portable computers and PDAs to create more portable pointing devices. Though these devices vary with respect to their convenience to the end user , to the OS they can all look like a mouse. So, from a software perspective, there is little difference between these devices.

In modern OSes, the application rarely interfaces with a pointing device directly. Instead, the OS is responsible for tracking the mouse position and updating cursors and other mouse effects in the system. The OS typically notifies an application when some sort of pointing device event occurs that the application should consider. Though applications may query the pointing device's status, as a normal state of affairs they don't manage the pointing device's position. In response to a query from an application, the OS will return the position of the system cursor and the state of the buttons on the pointing device. The OS may also notify the application whenever a pointer device event, such as a button press, occurs.

^[6] Actually, this is a bit of a simplification, but we will ignore that fact here.