developing a device driver.htm

What is this ``device driver stuff anyway? Here s a very short introduction to the concept.

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<H3>2.1.3.3&nbsp;&nbsp;&nbsp;&nbsp;Specifying Whether the Device Supports Byte 
Stream Access </H3>
<P><A name=nx_id_53></A>Most devices support byte stream access. You can view 
this access as sequentially accessing data through the device. For example, a 
sequence of characters typed at a terminal constitutes a byte stream. Most block 
devices can also be accessed in this manner. When a block device is accessed as 
a stream of bytes, the access is typically called ``raw'' access. When accessed 
this way, the data on the block device is accessed sequentially without any 
underlying structure being placed on the data (for example, disk sectors). 
<P>For the <TT>NONE</TT> device, the Yes box on the worksheet shown in <A 
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C-5</A> is marked. <A name=SpecActionsonInterrupts></A>
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<H3>2.1.3.4&nbsp;&nbsp;&nbsp;&nbsp;Specifying Actions to Take on Interrupts 
</H3>
<P><A name=nx_id_54></A>Use this space on the worksheet in <A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devcharac_wkshtfordevnone">Figure 
C-5</A> to summarize what the driver interrupt handlers will do when the device 
generates an interrupt. For example, a terminal-type character driver's 
interrupt handler can receive a character that was typed on a user's keyboard. 
Typically, the interrupt handler must determine the source of the interrupt, 
respond to the interrupt (for example, by reading in the data), and perform the 
appropriate actions to cause the interrupt to be dismissed. 
<P>Some other issues concerning interrupts are: 
<UL>
  <P>
  <LI>Locking out interrupts when performing vulnerable operations 
  <P></P>
  <LI>Not locking out interrupts for an extended period of time 
  <P></P>
  <LI>Queueing the data so that the data-handling operation can be interrupted 
  </LI></UL>
<P>
<P>Because the <TT>NONE</TT> device has no underlying physical hardware, it 
cannot generate interrupts. Therefore, this part of the worksheet shown in <A 
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C-5</A> is left blank. <A name=SpecHowDevReset></A>
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<H3>2.1.3.5&nbsp;&nbsp;&nbsp;&nbsp;Specifying How to Reset the Device </H3>
<P><A name=nx_id_55></A><A name=nx_id_56></A>If the bus that the device 
controller is connected to supports the reset function, the device driver must 
be able to stop all current work and place the device connected to the 
controller in a known, quiescent state. 
<P>For example purposes, the <TT>NONE</TT> device can be connected to the 
TURBOchannel bus. To keep the example driver simple, the reset function will not 
be implemented as part of the <TT>/dev/none</TT> device driver. Thus, the 
worksheet shown in <A 
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<TT>/dev/none</TT> device driver. <A name=OtherDevChar></A>
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<H3>2.1.3.6&nbsp;&nbsp;&nbsp;&nbsp;Specifying Other Device Characteristics </H3>
<P>Use this space to identify other characteristics of the device that might 
influence how you design your device driver. The worksheet shown in <A 
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C-6</A> is left blank to indicate that there are no other device characteristics 
associated with the <TT>NONE</TT> device. <A name=descruse_ofdev></A>
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<H3>2.1.4&nbsp;&nbsp;&nbsp;&nbsp;Describing Device Usage </H3>
<P><A 
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C-7</A> shows the device usage information for the <TT>NONE</TT> device 
associated with the <TT>/dev/none</TT> driver. <A name=nx_id_57></A>As the 
worksheet shows, you gather the following information about device usage: 
<UL>
  <P>
  <LI>The documentation you have on the device 
  <P></P>
  <LI>The number of instances of this device type that can reside on the system 
  <P></P>
  <LI>The purpose of the device </LI></UL>
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