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>Introduction</H1
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>Earlier publication</B
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> </TD
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><P
> Parts of this document first appeared in Linux Magazine under a
ninety day exclusivity.
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><P
> Mice are conceptually one of the simplest device interfaces in the
Linux operating system. Not all mice are handled by the kernel.
Instead there is a two layer abstraction.
</P
><P
> The kernel mouse drivers and userspace drivers for the serial mice are
all managed by a system daemon called <TT
CLASS="APPLICATION"
>gpm</TT
>
- the general purpose mouse driver. <TT
CLASS="APPLICATION"
>gpm</TT
>
handles cutting and pasting on the text consoles. It provides a
general library for mouse-aware applications and it handles the
sharing of mouse services with the
<TT
CLASS="APPLICATION"
>X Window System</TT
> user interface.
</P
><P
> Sometimes a mouse speaks a sufficiently convoluted protocol that the
protocol is handled by <TT
CLASS="APPLICATION"
>Gpm</TT
> itself. Most
of the mouse drivers follow a common interface called the bus mouse
protocol.
</P
><P
> Each read from a bus mouse interface device returns a block of data.
The first three bytes of each read are defined as follows:
<DIV
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><B
>Table 1. Mouse Data Encoding</B
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>Byte 0</TD
><TD
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VALIGN="TOP"
>0x80 + the buttons currently down.</TD
></TR
><TR
><TD
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VALIGN="TOP"
>Byte 1</TD
><TD
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VALIGN="TOP"
>A signed value for the shift in X position</TD
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><TD
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>Byte 2</TD
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>A signed value for the shift in Y position</TD
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>
An application can choose to read more than 3 bytes. The rest of the
bytes will be zero, or may optionally return some additional
device-specific information.
</P
><P
> The position values are truncated if they exceed the 8bit range (that
is -127 <= delta <= 127). While the value -128 does fit into a
byte is not allowed.
</P
><P
> The <SPAN
CLASS="MOUSEBUTTON"
>buttons</SPAN
> are numbered left to right as
0, 1, 2, 3.. and each button sets the relevant bit. So a user pressing
the left and right button of a three button mouse will set bits 0 and 2.
</P
><P
> All mice are required to support the <TT
CLASS="FUNCTION"
>poll</TT
>
operation. Indeed pretty much every user of a mouse device uses
<TT
CLASS="FUNCTION"
>poll</TT
> to wait for mouse events to occur.
</P
><P
> Finally the mice support asynchronous I/O. This is a topic we have not
yet covered but which I will explain after looking at a simple mouse
driver.
</P
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