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77 lines
3.8 KiB
XML
77 lines
3.8 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Copyright (c) 2016 OpenPOWER Foundation
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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version="5.0"
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xml:id="dbdoclet.50569332_13537">
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<title>Introduction</title>
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<para>The Run-Time Abstraction Service (RTAS) functions are provided by
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LoPAR platforms to insulate the OS from having to know about and
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manipulate a number of key platform hardware functions which ordinarily
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require platform-dependent code. The OS calls RTAS functions rather than
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manipulating hardware registers directly, reducing the need for platform
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tailoring by the OS. This method of abstracting access to these platform
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functions also permits hardware designers considerable flexibility in
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hardware implementation. Since RTAS is provided by the platform developer,
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this approach places the responsibility for supporting the platform
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hardware design with the platform developer, not the OS developer. This
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permits a degree of independence between the schedules of hardware and
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software and reduces the release and test requirements for the OS, since it
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can be tested to conform to the RTAS interface and not to every specific
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hardware implementation. See
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<xref linkend="dbdoclet.50569332_20008"/>
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for a list of all RTAS calls, and
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which ones are required based on which LoPAR options that are implemented
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in the platform.</para>
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<para>In order for platforms to achieve this separation of OS code from
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hardware implementation dependencies, RTAS defines an interface between the
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platform and the OS that provides control of some of the common devices
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found on all platforms. RTAS is a system programming interface that is
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realized, on a specific platform, by an RTAS implementation. The RTAS
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implementation provides the platform specific processing of the common
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components. RTAS limits itself to the run-time control of non-I/O,
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typically system board-resident, hardware features. Traditionally, features
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such as these have been implemented differently on different platforms. The
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different implementations have required much effort and platform-dependent
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code in the OS. RTAS permits the OS to operate over a much wider range of
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platforms without specialized code for each platform.</para>
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<para>In general, the OS should not access RTAS resources directly. It
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should call RTAS to control the resource.</para>
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<para>OS drivers are necessary to provide device specific processing for
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IOAs.</para>
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<para>The role of RTAS versus OF is very important to understand. OF and
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RTAS are both platform-specific software, and both are tailored by the
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platform developer to manipulate the specific platform hardware. However,
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RTAS is intended to be present during the execution of the OS, and to be
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called by the OS to access platform hardware features on behalf of the OS,
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whereas OF need not be present when the OS is running. This frees
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OF’s memory to be used by applications. RTAS is small enough to
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painlessly coexist with the OS and applications.</para>
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<para>This document uses the term RTAS to refer both to the architected RTAS
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interface and to an RTAS implementation.</para>
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</section>
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