📄 introduction to device drivers.mht
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<P>
<H3>1.6.5 Device Controller </H3>
<P><A name=3Dnx_id_25></A>A device controller is the hardware interface =
between=20
the computer and a peripheral device. Sometimes a controller handles =
several=20
devices. In other cases, a controller is integral to the device. <A=20
name=3Dnx_id_26></A><A name=3Dperiph_devs></A>
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<H3>1.6.6 Peripheral Device </H3>
<P><A name=3Dnx_id_27></A>A peripheral device is hardware, such as a =
disk=20
controller, that connects to a computer system. It can be controlled by =
commands=20
from the computer and can send data to the computer and receive data =
from it. <A=20
name=3Dnx_id_28></A>Examples of peripheral devices include:=20
<UL>
<P>
<LI>A data acquisition device, like a digitizer=20
<P></P>
<LI>A line printer=20
<P></P>
<LI>A disk or tape drive </LI></UL>
<P><A name=3Dexofread_char></A>
<P>
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<H2>1.7 Example of Reading a Character </H2>
<P><A name=3Dnx_id_29></A>This section provides an example of how =
Digital UNIX=20
processes a read request of a single character in raw mode from a =
terminal. (Raw=20
mode returns single characters.) Although the example takes a simplified =
view of=20
character processing, it does show how control can pass from a user =
program to=20
the kernel to the device driver. It also shows that interrupt processing =
occurs=20
asynchronously from other device driver activity. <A =
name=3Dnx_id_30></A>
<P><A=20
href=3D"http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUB=
VD-TE_html/drivertut3.html#chardrv_interex">Figure=20
1-3</A> summarizes the flow of control between a user program, the =
kernel, the=20
device driver, and the hardware. The figure shows the following sequence =
of=20
events:=20
<UL>
<P>
<LI>A read request is made to the device driver (C-1 to C-3).=20
<P></P>
<LI>The character is captured by the hardware (I-4 and I-5).=20
<P></P>
<LI>The interrupt is generated (I-6).=20
<P></P>
<LI>The interrupt handler services the interrupt (I-7 to I-9).=20
<P></P>
<LI>The character is returned (C-10 to C-13). </LI></UL>
<P><A=20
href=3D"http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUB=
VD-TE_html/drivertut3.html#chardrv_interex">Figure=20
1-3</A> provides a snapshot of the processing that occurs in the reading =
of a=20
single character. The following sections elaborate on this sequence. <A=20
name=3Dchardrv_interex></A>
<H3>Figure 1-3: Simple Character Driver Interrupt Example</H3>
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<P>
<H3>1.7.1 A Read Request Is Made to the Device =
Driver=20
</H3>
<P>A user program issues a <TT>read</TT> system call (C-1). <A=20
name=3Dnx_id_32></A>The figure shows that the <TT>read</TT> system call =
passes=20
three arguments: a file descriptor (the <TT>fd</TT> argument), the =
character=20
pointer to where the information is stored (the <TT>buf</TT> argument), =
and an=20
integer (the value 1) that tells the driver's <TT>read</TT> interface =
how many=20
bytes to read. The calling sequence is blocked inside the device =
driver's=20
<TT>read</TT> interface because the buffer where the data is stored is =
empty,=20
indicating that there are currently no characters available to satisfy =
the read.=20
The kernel's <TT>read</TT> interface makes a request of the device =
driver's=20
<TT>read</TT> interface to perform a read of the character based on the=20
arguments passed by the <TT>read</TT> system call (C-2). Essentially, =
the driver=20
<TT>read</TT> interface is waiting for a character to be typed at the =
terminal's=20
keyboard. The currently blocked process that caused the kernel to call =
the=20
driver's <TT>read</TT> interface is not running in the CPU (C-3). <A=20
name=3Dacharcap_byhard></A>
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