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| <title>Using as: M68K-Branch</title> |
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| Next: <a href="M68K_002dChars.html#M68K_002dChars" accesskey="n" rel="next">M68K-Chars</a>, Up: <a href="M68K_002dopcodes.html#M68K_002dopcodes" accesskey="u" rel="up">M68K-opcodes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Branch-Improvement"></a> |
| <h4 class="subsubsection">9.23.6.1 Branch Improvement</h4> |
| |
| <a name="index-pseudo_002dopcodes_002c-M680x0"></a> |
| <a name="index-M680x0-pseudo_002dopcodes"></a> |
| <a name="index-branch-improvement_002c-M680x0"></a> |
| <a name="index-M680x0-branch-improvement"></a> |
| <p>Certain pseudo opcodes are permitted for branch instructions. |
| They expand to the shortest branch instruction that reach the |
| target. Generally these mnemonics are made by substituting ‘<samp>j</samp>’ for |
| ‘<samp>b</samp>’ at the start of a Motorola mnemonic. |
| </p> |
| <p>The following table summarizes the pseudo-operations. A <code>*</code> flags |
| cases that are more fully described after the table: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> Displacement |
| +------------------------------------------------------------ |
| | 68020 68000/10, not PC-relative OK |
| Pseudo-Op |BYTE WORD LONG ABSOLUTE LONG JUMP ** |
| +------------------------------------------------------------ |
| jbsr |bsrs bsrw bsrl jsr |
| jra |bras braw bral jmp |
| * jXX |bXXs bXXw bXXl bNXs;jmp |
| * dbXX | N/A dbXXw dbXX;bras;bral dbXX;bras;jmp |
| fjXX | N/A fbXXw fbXXl N/A |
| |
| XX: condition |
| NX: negative of condition XX |
| |
| </pre></div> |
| <div align="center"><code>*</code>—see full description below |
| </div><div align="center"><code>**</code>—this expansion mode is disallowed by ‘<samp>--pcrel</samp>’ |
| </div> |
| <dl compact="compact"> |
| <dt><code>jbsr</code></dt> |
| <dt><code>jra</code></dt> |
| <dd><p>These are the simplest jump pseudo-operations; they always map to one |
| particular machine instruction, depending on the displacement to the |
| branch target. This instruction will be a byte or word branch is that |
| is sufficient. Otherwise, a long branch will be emitted if available. |
| If no long branches are available and the ‘<samp>--pcrel</samp>’ option is not |
| given, an absolute long jump will be emitted instead. If no long |
| branches are available, the ‘<samp>--pcrel</samp>’ option is given, and a word |
| branch cannot reach the target, an error message is generated. |
| </p> |
| <p>In addition to standard branch operands, <code>as</code> allows these |
| pseudo-operations to have all operands that are allowed for jsr and jmp, |
| substituting these instructions if the operand given is not valid for a |
| branch instruction. |
| </p> |
| </dd> |
| <dt><code>j<var>XX</var></code></dt> |
| <dd><p>Here, ‘<samp>j<var>XX</var></samp>’ stands for an entire family of pseudo-operations, |
| where <var>XX</var> is a conditional branch or condition-code test. The full |
| list of pseudo-ops in this family is: |
| </p><div class="smallexample"> |
| <pre class="smallexample"> jhi jls jcc jcs jne jeq jvc |
| jvs jpl jmi jge jlt jgt jle |
| </pre></div> |
| |
| <p>Usually, each of these pseudo-operations expands to a single branch |
| instruction. However, if a word branch is not sufficient, no long branches |
| are available, and the ‘<samp>--pcrel</samp>’ option is not given, <code>as</code> |
| issues a longer code fragment in terms of <var>NX</var>, the opposite condition |
| to <var>XX</var>. For example, under these conditions: |
| </p><div class="smallexample"> |
| <pre class="smallexample"> j<var>XX</var> foo |
| </pre></div> |
| <p>gives |
| </p><div class="smallexample"> |
| <pre class="smallexample"> b<var>NX</var>s oof |
| jmp foo |
| oof: |
| </pre></div> |
| |
| </dd> |
| <dt><code>db<var>XX</var></code></dt> |
| <dd><p>The full family of pseudo-operations covered here is |
| </p><div class="smallexample"> |
| <pre class="smallexample"> dbhi dbls dbcc dbcs dbne dbeq dbvc |
| dbvs dbpl dbmi dbge dblt dbgt dble |
| dbf dbra dbt |
| </pre></div> |
| |
| <p>Motorola ‘<samp>db<var>XX</var></samp>’ instructions allow word displacements only. When |
| a word displacement is sufficient, each of these pseudo-operations expands |
| to the corresponding Motorola instruction. When a word displacement is not |
| sufficient and long branches are available, when the source reads |
| ‘<samp>db<var>XX</var> foo</samp>’, <code>as</code> emits |
| </p><div class="smallexample"> |
| <pre class="smallexample"> db<var>XX</var> oo1 |
| bras oo2 |
| oo1:bral foo |
| oo2: |
| </pre></div> |
| |
| <p>If, however, long branches are not available and the ‘<samp>--pcrel</samp>’ option is |
| not given, <code>as</code> emits |
| </p><div class="smallexample"> |
| <pre class="smallexample"> db<var>XX</var> oo1 |
| bras oo2 |
| oo1:jmp foo |
| oo2: |
| </pre></div> |
| |
| </dd> |
| <dt><code>fj<var>XX</var></code></dt> |
| <dd><p>This family includes |
| </p><div class="smallexample"> |
| <pre class="smallexample"> fjne fjeq fjge fjlt fjgt fjle fjf |
| fjt fjgl fjgle fjnge fjngl fjngle fjngt |
| fjnle fjnlt fjoge fjogl fjogt fjole fjolt |
| fjor fjseq fjsf fjsne fjst fjueq fjuge |
| fjugt fjule fjult fjun |
| </pre></div> |
| |
| <p>Each of these pseudo-operations always expands to a single Motorola |
| coprocessor branch instruction, word or long. All Motorola coprocessor |
| branch instructions allow both word and long displacements. |
| </p> |
| </dd> |
| </dl> |
| |
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