159 lines
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159 lines
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<title>Alpha Directives - Using as</title>
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<link rel="prev" href="Alpha-Floating-Point.html#Alpha-Floating-Point" title="Alpha Floating Point">
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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<p>
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<a name="Alpha-Directives"></a>Next: <a rel="next" accesskey="n" href="Alpha-Opcodes.html#Alpha-Opcodes">Alpha Opcodes</a>,
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Previous: <a rel="previous" accesskey="p" href="Alpha-Floating-Point.html#Alpha-Floating-Point">Alpha Floating Point</a>,
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Up: <a rel="up" accesskey="u" href="Alpha_002dDependent.html#Alpha_002dDependent">Alpha-Dependent</a>
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<h4 class="subsection">9.1.5 Alpha Assembler Directives</h4>
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<p><span class="command">as</span> for the Alpha supports many additional directives for
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compatibility with the native assembler. This section describes them only
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briefly.
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<p><a name="index-Alpha_002donly-directives-531"></a>These are the additional directives in <code>as</code> for the Alpha:
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<dl>
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<dt><code>.arch </code><var>cpu</var><dd>Specifies the target processor. This is equivalent to the
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<span class="option">-m</span><var>cpu</var> command-line option. See <a href="Alpha-Options.html#Alpha-Options">Options</a>,
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for a list of values for <var>cpu</var>.
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<br><dt><code>.ent </code><var>function</var><code>[, </code><var>n</var><code>]</code><dd>Mark the beginning of <var>function</var>. An optional number may follow for
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compatibility with the OSF/1 assembler, but is ignored. When generating
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<code>.mdebug</code> information, this will create a procedure descriptor for
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the function. In ELF, it will mark the symbol as a function a-la the
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generic <code>.type</code> directive.
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<br><dt><code>.end </code><var>function</var><dd>Mark the end of <var>function</var>. In ELF, it will set the size of the symbol
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a-la the generic <code>.size</code> directive.
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<br><dt><code>.mask </code><var>mask</var><code>, </code><var>offset</var><dd>Indicate which of the integer registers are saved in the current
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function's stack frame. <var>mask</var> is interpreted a bit mask in which
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bit <var>n</var> set indicates that register <var>n</var> is saved. The registers
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are saved in a block located <var>offset</var> bytes from the <dfn>canonical
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frame address</dfn> (CFA) which is the value of the stack pointer on entry to
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the function. The registers are saved sequentially, except that the
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return address register (normally <code>$26</code>) is saved first.
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<p>This and the other directives that describe the stack frame are
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currently only used when generating <code>.mdebug</code> information. They
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may in the future be used to generate DWARF2 <code>.debug_frame</code> unwind
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information for hand written assembly.
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<br><dt><code>.fmask </code><var>mask</var><code>, </code><var>offset</var><dd>Indicate which of the floating-point registers are saved in the current
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stack frame. The <var>mask</var> and <var>offset</var> parameters are interpreted
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as with <code>.mask</code>.
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<br><dt><code>.frame </code><var>framereg</var><code>, </code><var>frameoffset</var><code>, </code><var>retreg</var><code>[, </code><var>argoffset</var><code>]</code><dd>Describes the shape of the stack frame. The frame pointer in use is
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<var>framereg</var>; normally this is either <code>$fp</code> or <code>$sp</code>. The
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frame pointer is <var>frameoffset</var> bytes below the CFA. The return
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address is initially located in <var>retreg</var> until it is saved as
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indicated in <code>.mask</code>. For compatibility with OSF/1 an optional
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<var>argoffset</var> parameter is accepted and ignored. It is believed to
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indicate the offset from the CFA to the saved argument registers.
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<br><dt><code>.prologue </code><var>n</var><dd>Indicate that the stack frame is set up and all registers have been
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spilled. The argument <var>n</var> indicates whether and how the function
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uses the incoming <dfn>procedure vector</dfn> (the address of the called
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function) in <code>$27</code>. 0 indicates that <code>$27</code> is not used; 1
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indicates that the first two instructions of the function use <code>$27</code>
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to perform a load of the GP register; 2 indicates that <code>$27</code> is
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used in some non-standard way and so the linker cannot elide the load of
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the procedure vector during relaxation.
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<br><dt><code>.usepv </code><var>function</var><code>, </code><var>which</var><dd>Used to indicate the use of the <code>$27</code> register, similar to
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<code>.prologue</code>, but without the other semantics of needing to
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be inside an open <code>.ent</code>/<code>.end</code> block.
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<p>The <var>which</var> argument should be either <code>no</code>, indicating that
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<code>$27</code> is not used, or <code>std</code>, indicating that the first two
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instructions of the function perform a GP load.
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<p>One might use this directive instead of <code>.prologue</code> if you are
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also using dwarf2 CFI directives.
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<br><dt><code>.gprel32 </code><var>expression</var><dd>Computes the difference between the address in <var>expression</var> and the
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GP for the current object file, and stores it in 4 bytes. In addition
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to being smaller than a full 8 byte address, this also does not require
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a dynamic relocation when used in a shared library.
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<br><dt><code>.t_floating </code><var>expression</var><dd>Stores <var>expression</var> as an <span class="sc">ieee</span> double precision value.
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<br><dt><code>.s_floating </code><var>expression</var><dd>Stores <var>expression</var> as an <span class="sc">ieee</span> single precision value.
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<br><dt><code>.f_floating </code><var>expression</var><dd>Stores <var>expression</var> as a VAX F format value.
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<br><dt><code>.g_floating </code><var>expression</var><dd>Stores <var>expression</var> as a VAX G format value.
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<br><dt><code>.d_floating </code><var>expression</var><dd>Stores <var>expression</var> as a VAX D format value.
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<br><dt><code>.set </code><var>feature</var><dd>Enables or disables various assembler features. Using the positive
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name of the feature enables while using <span class="samp">no</span><var>feature</var> disables.
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<dl>
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<dt><code>at</code><dd>Indicates that macro expansions may clobber the <dfn>assembler
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temporary</dfn> (<code>$at</code> or <code>$28</code>) register. Some macros may not be
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expanded without this and will generate an error message if <code>noat</code>
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is in effect. When <code>at</code> is in effect, a warning will be generated
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if <code>$at</code> is used by the programmer.
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<br><dt><code>macro</code><dd>Enables the expansion of macro instructions. Note that variants of real
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instructions, such as <code>br label</code> vs <code>br $31,label</code> are
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considered alternate forms and not macros.
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<br><dt><code>move</code><dt><code>reorder</code><dt><code>volatile</code><dd>These control whether and how the assembler may re-order instructions.
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Accepted for compatibility with the OSF/1 assembler, but <span class="command">as</span>
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does not do instruction scheduling, so these features are ignored.
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</dl>
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</dl>
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<p>The following directives are recognized for compatibility with the OSF/1
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assembler but are ignored.
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<pre class="example"> .proc .aproc
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.reguse .livereg
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.option .aent
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.ugen .eflag
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.alias .noalias
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</pre>
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