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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-F75F975B-A490-4EB5-96CD-B2F8DFC63C7C" xml:lang="en"><title>Asynchronous

    13 Requests</title><shortdesc>This document describes how device drivers use asynchronous requests.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <section id="GUID-834B2C34-5C8B-4D6D-85F3-FADD357AD330">             <p>An

    15 asynchronous request is one which is used, typically, to communicate with

    16 the hardware itself. The time taken for such a request to complete depends

    17 on the hardware, and the client user-side thread may want to continue with

    18 some other processing.  The user-side thread is not blocked and can continue

    19 with other operations, including issuing other requests (synchronous or asynchronous).</p> <p>An

    20 asynchronous request is started by a call to <xref href="GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066.dita#GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066/GUID-FCC4A3A9-69F5-3947-8D0F-2AD12086F1E2"><apiname>RBusLogicalChannel::DoRequest()</apiname></xref>.

    21 Control normally returns as soon as the request has been issued, successfully

    22 or otherwise. Typically, the hardware indicates completion of an operation

    23 by generating an interrupt. This is handled by an Interrupt Service Routine

    24 (ISR) provided by the driver. This, in turn, schedules a DFC, which at some

    25 later time, runs in the context of a kernel-side thread, and signals the client-user

    26 side thread, marking the asynchronous request as complete. </p> <p>More than

    27 one asynchronous request can be outstanding at the same time, each one associated

    28 with its own <xref href="GUID-E0B34F3E-D4C4-3232-B8B1-7DB35B454646.dita"><apiname>TRequestStatus</apiname></xref> object and each identified

    29 by a specific request number. The device driver framework puts no explicit

    30 limit on the number of concurrent outstanding asynchronous requests; any limit

    31 must be enforced by the driver itself. However, the API to cancel a request

    32 implicitly prevents you from uniquely identifying more than 32 concurrent

    33 requests. </p> <p>An outstanding asynchronous request can be cancelled by

    34 a call to <xref href="GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066.dita#GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066/GUID-16E139F5-4473-33C0-BCDD-A92152D4E8E6"><apiname>RBusLogicalChannel::DoCancel()</apiname></xref>. </p>     </section>

    35 </conbody></concept>