introduction to device drivers.htm

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

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<H3>1.7.1&nbsp;&nbsp;&nbsp;&nbsp;A Read Request Is Made to the Device Driver 
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<P>A user program issues a <TT>read</TT> system call (C-1). <A 
name=nx_id_32></A>The figure shows that the <TT>read</TT> system call passes 
three arguments: a file descriptor (the <TT>fd</TT> argument), the character 
pointer to where the information is stored (the <TT>buf</TT> argument), and an 
integer (the value 1) that tells the driver's <TT>read</TT> interface how many 
bytes to read. The calling sequence is blocked inside the device driver's 
<TT>read</TT> interface because the buffer where the data is stored is empty, 
indicating that there are currently no characters available to satisfy the read. 
The kernel's <TT>read</TT> interface makes a request of the device driver's 
<TT>read</TT> interface to perform a read of the character based on the 
arguments passed by the <TT>read</TT> system call (C-2). Essentially, the driver 
<TT>read</TT> interface is waiting for a character to be typed at the terminal's 
keyboard. The currently blocked process that caused the kernel to call the 
driver's <TT>read</TT> interface is not running in the CPU (C-3). <A 
name=acharcap_byhard></A>
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<H3>1.7.2&nbsp;&nbsp;&nbsp;&nbsp;The Character Is Captured by the Hardware </H3>
<P>Later, a user types the letter k on the terminal keyboard (I-4). The letter 
is stored in the device's data register (I-5). <A name=ainterupt_isgen></A>
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<H3>1.7.3&nbsp;&nbsp;&nbsp;&nbsp;The Interrupt Is Generated </H3>
<P>When the user types a key, the console keyboard controller alters some 
signals on the bus. This action notifies the CPU that something has changed 
inside the console keyboard controller. This condition causes the CPU to 
immediately start running the console keyboard controller's interrupt handler 
(I-6). The state of the interrupted process (either some other process or the 
idle loop) is saved so that the process can be returned to its original state as 
though it had never been interrupted in the first place. 
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<H3>1.7.4&nbsp;&nbsp;&nbsp;&nbsp;The Interrupt Handler Services the Interrupt 
</H3>
<P>The console device driver's interrupt handler first checks the state of the 
driver and notices that a pending read operation exists for the original 
process. The console device driver manipulates the controller hardware by way of 
the bus hardware in order to obtain the value of the character that was typed. 
This character value was stored somewhere inside the console controller's 
hardware (I-7). In this case, the value 107 (the ASCII representation for the k 
character) is stored. The interrupt handler stores this character value into a 
buffer that is in a location known to the rest of the console driver interfaces 
(I-8). It then awakens the original, currently sleeping, process so that it is 
ready to run again (I-9). The interrupt handler returns, in effect restoring the 
interrupted process (not the original process yet) so that it may continue where 
it left off. <A name=thechar_isreturned></A>
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<H3>1.7.5&nbsp;&nbsp;&nbsp;&nbsp;The Character Is Returned </H3>
<P>Later, the kernel's process scheduler notices that the original process is 
ready to run, and so allows it to run. After the original process resumes 
running (after the location where it was first blocked), it knows which buffer 
to look at to obtain the typed character (C-10). It removes the character from 
this buffer and puts it into the user's address space (C-11). The device 
driver's <TT>read</TT> interface returns control to the kernel's <TT>read</TT> 
interface (C-12). The kernel <TT>read</TT> interface returns control to the user 
program that previously initiated the read request (C-13). <A 
name=sum_ofexample></A>
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<H3>1.7.6&nbsp;&nbsp;&nbsp;&nbsp;Summary of the Example </H3>
<P>Although this example presents a somewhat simplified view of character 
processing, it does illustrate how control passes from a user program to the 
kernel to the device driver. It also shows clearly that interrupt processing 
occurs asynchronously from other device driver activity. <!--Metrics Tag --><!-- Updated April 22, 2003 --><!-- SiteCatalyst code version: G.0. w/dev site filtering
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