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196 lines
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196 lines
9.3 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>AVR Function Attributes (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="AVR Function Attributes (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="AVR Function Attributes (Using the GNU Compiler Collection (GCC))">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Function-Attributes.html#Function-Attributes" rel="up" title="Function Attributes">
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<link href="Blackfin-Function-Attributes.html#Blackfin-Function-Attributes" rel="next" title="Blackfin Function Attributes">
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<link href="ARM-Function-Attributes.html#ARM-Function-Attributes" rel="prev" title="ARM Function Attributes">
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</head>
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<body lang="en">
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<a name="AVR-Function-Attributes"></a>
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<div class="header">
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<p>
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Next: <a href="Blackfin-Function-Attributes.html#Blackfin-Function-Attributes" accesskey="n" rel="next">Blackfin Function Attributes</a>, Previous: <a href="ARM-Function-Attributes.html#ARM-Function-Attributes" accesskey="p" rel="prev">ARM Function Attributes</a>, Up: <a href="Function-Attributes.html#Function-Attributes" accesskey="u" rel="up">Function Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="AVR-Function-Attributes-1"></a>
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<h4 class="subsection">6.31.5 AVR Function Attributes</h4>
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<p>These function attributes are supported by the AVR back end:
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</p>
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<dl compact="compact">
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<dt><code>interrupt</code></dt>
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<dd><a name="index-interrupt-function-attribute_002c-AVR"></a>
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<p>Use this attribute to indicate
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that the specified function is an interrupt handler. The compiler generates
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function entry and exit sequences suitable for use in an interrupt handler
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when this attribute is present.
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</p>
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<p>On the AVR, the hardware globally disables interrupts when an
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interrupt is executed. The first instruction of an interrupt handler
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declared with this attribute is a <code>SEI</code> instruction to
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re-enable interrupts. See also the <code>signal</code> function attribute
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that does not insert a <code>SEI</code> instruction. If both <code>signal</code> and
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<code>interrupt</code> are specified for the same function, <code>signal</code>
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is silently ignored.
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</p>
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</dd>
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<dt><code>naked</code></dt>
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<dd><a name="index-naked-function-attribute_002c-AVR"></a>
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<p>This attribute allows the compiler to construct the
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requisite function declaration, while allowing the body of the
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function to be assembly code. The specified function will not have
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prologue/epilogue sequences generated by the compiler. Only basic
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<code>asm</code> statements can safely be included in naked functions
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(see <a href="Basic-Asm.html#Basic-Asm">Basic Asm</a>). While using extended <code>asm</code> or a mixture of
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basic <code>asm</code> and C code may appear to work, they cannot be
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depended upon to work reliably and are not supported.
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</p>
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</dd>
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<dt><code>no_gccisr</code></dt>
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<dd><a name="index-no_005fgccisr-function-attribute_002c-AVR"></a>
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<p>Do not use <code>__gcc_isr</code> pseudo instructions in a function with
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the <code>interrupt</code> or <code>signal</code> attribute aka. interrupt
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service routine (ISR).
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Use this attribute if the preamble of the ISR prologue should always read
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</p><div class="example">
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<pre class="example">push __zero_reg__
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push __tmp_reg__
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in __tmp_reg__, __SREG__
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push __tmp_reg__
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clr __zero_reg__
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</pre></div>
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<p>and accordingly for the postamble of the epilogue — no matter whether
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the mentioned registers are actually used in the ISR or not.
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Situations where you might want to use this attribute include:
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</p><ul>
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<li> Code that (effectively) clobbers bits of <code>SREG</code> other than the
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<code>I</code>-flag by writing to the memory location of <code>SREG</code>.
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</li><li> Code that uses inline assembler to jump to a different function which
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expects (parts of) the prologue code as outlined above to be present.
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</li></ul>
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<p>To disable <code>__gcc_isr</code> generation for the whole compilation unit,
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there is option <samp>-mno-gas-isr-prologues</samp>, see <a href="AVR-Options.html#AVR-Options">AVR Options</a>.
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</p>
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</dd>
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<dt><code>OS_main</code></dt>
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<dt><code>OS_task</code></dt>
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<dd><a name="index-OS_005fmain-function-attribute_002c-AVR"></a>
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<a name="index-OS_005ftask-function-attribute_002c-AVR"></a>
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<p>On AVR, functions with the <code>OS_main</code> or <code>OS_task</code> attribute
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do not save/restore any call-saved register in their prologue/epilogue.
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</p>
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<p>The <code>OS_main</code> attribute can be used when there <em>is
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guarantee</em> that interrupts are disabled at the time when the function
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is entered. This saves resources when the stack pointer has to be
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changed to set up a frame for local variables.
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</p>
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<p>The <code>OS_task</code> attribute can be used when there is <em>no
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guarantee</em> that interrupts are disabled at that time when the function
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is entered like for, e.g. task functions in a multi-threading operating
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system. In that case, changing the stack pointer register is
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guarded by save/clear/restore of the global interrupt enable flag.
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</p>
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<p>The differences to the <code>naked</code> function attribute are:
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</p><ul>
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<li> <code>naked</code> functions do not have a return instruction whereas
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<code>OS_main</code> and <code>OS_task</code> functions have a <code>RET</code> or
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<code>RETI</code> return instruction.
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</li><li> <code>naked</code> functions do not set up a frame for local variables
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or a frame pointer whereas <code>OS_main</code> and <code>OS_task</code> do this
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as needed.
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</li></ul>
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</dd>
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<dt><code>signal</code></dt>
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<dd><a name="index-signal-function-attribute_002c-AVR"></a>
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<p>Use this attribute on the AVR to indicate that the specified
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function is an interrupt handler. The compiler generates function
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entry and exit sequences suitable for use in an interrupt handler when this
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attribute is present.
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</p>
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<p>See also the <code>interrupt</code> function attribute.
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</p>
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<p>The AVR hardware globally disables interrupts when an interrupt is executed.
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Interrupt handler functions defined with the <code>signal</code> attribute
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do not re-enable interrupts. It is save to enable interrupts in a
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<code>signal</code> handler. This “save” only applies to the code
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generated by the compiler and not to the IRQ layout of the
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application which is responsibility of the application.
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</p>
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<p>If both <code>signal</code> and <code>interrupt</code> are specified for the same
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function, <code>signal</code> is silently ignored.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Blackfin-Function-Attributes.html#Blackfin-Function-Attributes" accesskey="n" rel="next">Blackfin Function Attributes</a>, Previous: <a href="ARM-Function-Attributes.html#ARM-Function-Attributes" accesskey="p" rel="prev">ARM Function Attributes</a>, Up: <a href="Function-Attributes.html#Function-Attributes" accesskey="u" rel="up">Function Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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