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>User-level device drivers</H1
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>Introduction to ppdev</H1
><P
> The printer is accessible through <TT
CLASS="FILENAME"
>/dev/lp0</TT
>;
in the same way, the parallel port itself is accessible through
<TT
CLASS="FILENAME"
>/dev/parport0</TT
>. The difference is in the
level of control that you have over the wires in the parallel port
cable.
</P
><P
> With the printer driver, a user-space program (such as the printer
spooler) can send bytes in <SPAN
CLASS="QUOTE"
>"printer protocol"</SPAN
>.
Briefly, this means that for each byte, the eight data lines are
set up, then a <SPAN
CLASS="QUOTE"
>"strobe"</SPAN
> line tells the printer to
look at the data lines, and the printer sets an
<SPAN
CLASS="QUOTE"
>"acknowledgement"</SPAN
> line to say that it got the byte.
The printer driver also allows the user-space program to read
bytes in <SPAN
CLASS="QUOTE"
>"nibble mode"</SPAN
>, which is a way of
transferring data from the peripheral to the computer half a byte
at a time (and so it's quite slow).
</P
><P
> In contrast, the <TT
CLASS="LITERAL"
>ppdev</TT
> driver (accessed via
<TT
CLASS="FILENAME"
>/dev/parport0</TT
>) allows you to:
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> examine status lines,
</P
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><P
> set control lines,
</P
></LI
><LI
><P
> set/examine data lines (and control the direction of the data
lines),
</P
></LI
><LI
><P
> wait for an interrupt (triggered by one of the status lines),
</P
></LI
><LI
><P
> find out how many new interrupts have occurred,
</P
></LI
><LI
><P
> set up a response to an interrupt,
</P
></LI
><LI
><P
> use IEEE 1284 negotiation (for telling peripheral which transfer
mode, to use)
</P
></LI
><LI
><P
> transfer data using a specified IEEE 1284 mode.
</P
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