Mice are conceptually one of the simplest device drivers in the Linux operating system. Not all mice are handled by the kernel; rather, there is a two-layer abstraction. The kernel provides services for mice that cannot be driven directly by the user libraries and applications. That is, mice other than serial mice. On top of this library and application, programs (selection or gpm) provide a single common interface to all mice whether supported directly by the kernel or via a serial line.
Mice are conceptually one of the simplest device drivers in the Linux operating system. Not all mice are handled by the kernel; rather, there is a two-layer abstraction. The kernel provides services for mice that cannot be driven directly by the user libraries and applications. That is, mice other than serial mice. On top of this library and application, programs (selection or gpm) provide a single common interface to all mice whether supported directly by the kernel or via a serial line.
gpm — the general purpose mouse driver handles cutting and pasting on the text consoles, and provides a general library for mouse-aware applications. It also handles the sharing of mouse services with the X Windows user interface.
Sometimes a mouse speaks a sufficiently convoluted protocol that the protocol is handled by gpm itself. However, most mice use a common protocol called the bus mouse protocol.
Each read from a bus mouse interface device returns a block of data. The first three bytes of each block are defined as follows:
Byte 0:0×80 + the buttons currently down
Byte 1:A signed value for the shift in X position
Byte 2:A signed value for the shift in Y position
An application can choose to read more than 3 bytes. Some mice send device-specific information in the rest of the block, while in others any additional bytes read will be zero.
The position values are truncated if they exceed the 8-bit range (that is, -127 <= delta <= 127)….
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