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<concept id="GUID-204B4155-AE35-5C53-867B-299AF7E5D16A" xml:lang="en"><title>Device

driver services</title><shortdesc>Describes asynchronous services provided by device drivers.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>Some asynchronous services are provided directly by device drivers.</p>

<p>As with services provided by the client-server framework, device driver

provided services are presented to clients through a client API known as the

user-side device driver. This is typically an <codeph>R</codeph> class, derived

from <codeph>RLogicalChannel</codeph>. </p>

<p>When the <codeph>RLogicalChannel</codeph> is opened, the device driver

is initialised and the client’s thread id is noted. Request functions are

converted into a message which encapsulates all the parameters, and sent to

the Kernel-side driver. Completion of a request function usually corresponds

closely with completion of a device operation. On completion, the driver posts

the request status and signals the client thread’s request semaphore.</p>

<p>Device drivers often have limited capability. For instance, an inability

to be opened by multiple client threads simultaneously. Therefore, devices

are often handled by specially tailored server threads. These servers present

a client API with more sophistication, sharing and checking. </p>

<p>Usually, therefore, clients use the server-provided classes rather than

the user-side device driver directly.</p>