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| <h4 class="subsection">9.1.5 Alpha Assembler Directives</h4> |
| |
| <p><samp><span class="command">as</span></samp> for the Alpha supports many additional directives for |
| compatibility with the native assembler. This section describes them only |
| briefly. |
| |
| <p><a name="index-Alpha_002donly-directives-534"></a>These are the additional directives in <code>as</code> for the Alpha: |
| |
| <dl> |
| <dt><code>.arch </code><var>cpu</var><dd>Specifies the target processor. This is equivalent to the |
| <samp><span class="option">-m</span><var>cpu</var></samp> command-line option. See <a href="Alpha-Options.html#Alpha-Options">Options</a>, |
| for a list of values for <var>cpu</var>. |
| |
| <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 |
| compatibility with the OSF/1 assembler, but is ignored. When generating |
| <code>.mdebug</code> information, this will create a procedure descriptor for |
| the function. In ELF, it will mark the symbol as a function a-la the |
| generic <code>.type</code> directive. |
| |
| <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 |
| a-la the generic <code>.size</code> directive. |
| |
| <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 |
| function's stack frame. <var>mask</var> is interpreted a bit mask in which |
| bit <var>n</var> set indicates that register <var>n</var> is saved. The registers |
| are saved in a block located <var>offset</var> bytes from the <dfn>canonical |
| frame address</dfn> (CFA) which is the value of the stack pointer on entry to |
| the function. The registers are saved sequentially, except that the |
| return address register (normally <code>$26</code>) is saved first. |
| |
| <p>This and the other directives that describe the stack frame are |
| currently only used when generating <code>.mdebug</code> information. They |
| may in the future be used to generate DWARF2 <code>.debug_frame</code> unwind |
| information for hand written assembly. |
| |
| <br><dt><code>.fmask </code><var>mask</var><code>, </code><var>offset</var><dd>Indicate which of the floating-point registers are saved in the current |
| stack frame. The <var>mask</var> and <var>offset</var> parameters are interpreted |
| as with <code>.mask</code>. |
| |
| <br><dt><code>.frame </code><var>framereg</var><code>, </code><var>frameoffset</var><code>, </code><var>retreg</var><code>[, </code><var>argoffset</var><code>]</code><dd>Describes the shape of the stack frame. The frame pointer in use is |
| <var>framereg</var>; normally this is either <code>$fp</code> or <code>$sp</code>. The |
| frame pointer is <var>frameoffset</var> bytes below the CFA. The return |
| address is initially located in <var>retreg</var> until it is saved as |
| indicated in <code>.mask</code>. For compatibility with OSF/1 an optional |
| <var>argoffset</var> parameter is accepted and ignored. It is believed to |
| indicate the offset from the CFA to the saved argument registers. |
| |
| <br><dt><code>.prologue </code><var>n</var><dd>Indicate that the stack frame is set up and all registers have been |
| spilled. The argument <var>n</var> indicates whether and how the function |
| uses the incoming <dfn>procedure vector</dfn> (the address of the called |
| function) in <code>$27</code>. 0 indicates that <code>$27</code> is not used; 1 |
| indicates that the first two instructions of the function use <code>$27</code> |
| to perform a load of the GP register; 2 indicates that <code>$27</code> is |
| used in some non-standard way and so the linker cannot elide the load of |
| the procedure vector during relaxation. |
| |
| <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 |
| <code>.prologue</code>, but without the other semantics of needing to |
| be inside an open <code>.ent</code>/<code>.end</code> block. |
| |
| <p>The <var>which</var> argument should be either <code>no</code>, indicating that |
| <code>$27</code> is not used, or <code>std</code>, indicating that the first two |
| instructions of the function perform a GP load. |
| |
| <p>One might use this directive instead of <code>.prologue</code> if you are |
| also using dwarf2 CFI directives. |
| |
| <br><dt><code>.gprel32 </code><var>expression</var><dd>Computes the difference between the address in <var>expression</var> and the |
| GP for the current object file, and stores it in 4 bytes. In addition |
| to being smaller than a full 8 byte address, this also does not require |
| a dynamic relocation when used in a shared library. |
| |
| <br><dt><code>.t_floating </code><var>expression</var><dd>Stores <var>expression</var> as an <span class="sc">ieee</span> double precision value. |
| |
| <br><dt><code>.s_floating </code><var>expression</var><dd>Stores <var>expression</var> as an <span class="sc">ieee</span> single precision value. |
| |
| <br><dt><code>.f_floating </code><var>expression</var><dd>Stores <var>expression</var> as a VAX F format value. |
| |
| <br><dt><code>.g_floating </code><var>expression</var><dd>Stores <var>expression</var> as a VAX G format value. |
| |
| <br><dt><code>.d_floating </code><var>expression</var><dd>Stores <var>expression</var> as a VAX D format value. |
| |
| <br><dt><code>.set </code><var>feature</var><dd>Enables or disables various assembler features. Using the positive |
| name of the feature enables while using ‘<samp><span class="samp">no</span><var>feature</var></samp>’ disables. |
| |
| <dl> |
| <dt><code>at</code><dd>Indicates that macro expansions may clobber the <dfn>assembler |
| temporary</dfn> (<code>$at</code> or <code>$28</code>) register. Some macros may not be |
| expanded without this and will generate an error message if <code>noat</code> |
| is in effect. When <code>at</code> is in effect, a warning will be generated |
| if <code>$at</code> is used by the programmer. |
| |
| <br><dt><code>macro</code><dd>Enables the expansion of macro instructions. Note that variants of real |
| instructions, such as <code>br label</code> vs <code>br $31,label</code> are |
| considered alternate forms and not macros. |
| |
| <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. |
| Accepted for compatibility with the OSF/1 assembler, but <samp><span class="command">as</span></samp> |
| does not do instruction scheduling, so these features are ignored. |
| </dl> |
| </dl> |
| |
| <p>The following directives are recognized for compatibility with the OSF/1 |
| assembler but are ignored. |
| |
| <pre class="example"> .proc .aproc |
| .reguse .livereg |
| .option .aent |
| .ugen .eflag |
| .alias .noalias |
| </pre> |
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