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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

  2. <html>

  3. <!-- This file documents the GNU Assembler "as".

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  17. <title>M68K-Branch (Using as)</title>

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  27. <link href="M68K_002dopcodes.html#M68K_002dopcodes" rel="up" title="M68K-opcodes">

  28. <link href="M68K_002dChars.html#M68K_002dChars" rel="next" title="M68K-Chars">

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  61. <body lang="en">

  62. <a name="M68K_002dBranch"></a>

  63. <div class="header">

  64. <p>

  65. 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> &nbsp; [<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>

  66. </div>

  67. <hr>

  68. <a name="Branch-Improvement"></a>

  69. <h4 class="subsubsection">9.22.6.1 Branch Improvement</h4>

  70. 

  71. <a name="index-pseudo_002dopcodes_002c-M680x0"></a>

  72. <a name="index-M680x0-pseudo_002dopcodes"></a>

  73. <a name="index-branch-improvement_002c-M680x0"></a>

  74. <a name="index-M680x0-branch-improvement"></a>

  75. <p>Certain pseudo opcodes are permitted for branch instructions.

  76. They expand to the shortest branch instruction that reach the

  77. target.  Generally these mnemonics are made by substituting &lsquo;<samp>j</samp>&rsquo; for

  78. &lsquo;<samp>b</samp>&rsquo; at the start of a Motorola mnemonic.

  79. </p>

  80. <p>The following table summarizes the pseudo-operations.  A <code>*</code> flags

  81. cases that are more fully described after the table:

  82. </p>

  83. <div class="smallexample">

  84. <pre class="smallexample">          Displacement

  85.           +------------------------------------------------------------

  86.           |                68020           68000/10, not PC-relative OK

  87. Pseudo-Op |BYTE    WORD    LONG            ABSOLUTE LONG JUMP    **

  88.           +------------------------------------------------------------

  89.      jbsr |bsrs    bsrw    bsrl            jsr

  90.       jra |bras    braw    bral            jmp

  91. *     jXX |bXXs    bXXw    bXXl            bNXs;jmp

  92. *    dbXX | N/A    dbXXw   dbXX;bras;bral  dbXX;bras;jmp

  93.      fjXX | N/A    fbXXw   fbXXl            N/A

  94. 

  95. XX: condition

  96. NX: negative of condition XX

  97. 

  98. </pre></div>

  99. <div align="center"><code>*</code>&mdash;see full description below

  100. </div><div align="center"><code>**</code>&mdash;this expansion mode is disallowed by &lsquo;<samp>--pcrel</samp>&rsquo;

  101. </div>

  102. <dl compact="compact">

  103. <dt><code>jbsr</code></dt>

  104. <dt><code>jra</code></dt>

  105. <dd><p>These are the simplest jump pseudo-operations; they always map to one

  106. particular machine instruction, depending on the displacement to the

  107. branch target.  This instruction will be a byte or word branch is that

  108. is sufficient.  Otherwise, a long branch will be emitted if available.

  109. If no long branches are available and the &lsquo;<samp>--pcrel</samp>&rsquo; option is not

  110. given, an absolute long jump will be emitted instead.  If no long

  111. branches are available, the &lsquo;<samp>--pcrel</samp>&rsquo; option is given, and a word

  112. branch cannot reach the target, an error message is generated.

  113. </p>

  114. <p>In addition to standard branch operands, <code>as</code> allows these

  115. pseudo-operations to have all operands that are allowed for jsr and jmp,

  116. substituting these instructions if the operand given is not valid for a

  117. branch instruction.

  118. </p>

  119. </dd>

  120. <dt><code>j<var>XX</var></code></dt>

  121. <dd><p>Here, &lsquo;<samp>j<var>XX</var></samp>&rsquo; stands for an entire family of pseudo-operations,

  122. where <var>XX</var> is a conditional branch or condition-code test.  The full

  123. list of pseudo-ops in this family is:

  124. </p><div class="smallexample">

  125. <pre class="smallexample"> jhi   jls   jcc   jcs   jne   jeq   jvc

  126.  jvs   jpl   jmi   jge   jlt   jgt   jle

  127. </pre></div>

  128. 

  129. <p>Usually, each of these pseudo-operations expands to a single branch

  130. instruction.  However, if a word branch is not sufficient, no long branches

  131. are available, and the &lsquo;<samp>--pcrel</samp>&rsquo; option is not given, <code>as</code>

  132. issues a longer code fragment in terms of <var>NX</var>, the opposite condition

  133. to <var>XX</var>.  For example, under these conditions:

  134. </p><div class="smallexample">

  135. <pre class="smallexample">    j<var>XX</var> foo

  136. </pre></div>

  137. <p>gives

  138. </p><div class="smallexample">

  139. <pre class="smallexample">     b<var>NX</var>s oof

  140.      jmp foo

  141.  oof:

  142. </pre></div>

  143. 

  144. </dd>

  145. <dt><code>db<var>XX</var></code></dt>

  146. <dd><p>The full family of pseudo-operations covered here is

  147. </p><div class="smallexample">

  148. <pre class="smallexample"> dbhi   dbls   dbcc   dbcs   dbne   dbeq   dbvc

  149.  dbvs   dbpl   dbmi   dbge   dblt   dbgt   dble

  150.  dbf    dbra   dbt

  151. </pre></div>

  152. 

  153. <p>Motorola &lsquo;<samp>db<var>XX</var></samp>&rsquo; instructions allow word displacements only.  When

  154. a word displacement is sufficient, each of these pseudo-operations expands

  155. to the corresponding Motorola instruction.  When a word displacement is not

  156. sufficient and long branches are available, when the source reads

  157. &lsquo;<samp>db<var>XX</var> foo</samp>&rsquo;, <code>as</code> emits

  158. </p><div class="smallexample">

  159. <pre class="smallexample">     db<var>XX</var> oo1

  160.      bras oo2

  161.  oo1:bral foo

  162.  oo2:

  163. </pre></div>

  164. 

  165. <p>If, however, long branches are not available and the &lsquo;<samp>--pcrel</samp>&rsquo; option is

  166. not given, <code>as</code> emits

  167. </p><div class="smallexample">

  168. <pre class="smallexample">     db<var>XX</var> oo1

  169.      bras oo2

  170.  oo1:jmp foo

  171.  oo2:

  172. </pre></div>

  173. 

  174. </dd>

  175. <dt><code>fj<var>XX</var></code></dt>

  176. <dd><p>This family includes

  177. </p><div class="smallexample">

  178. <pre class="smallexample"> fjne   fjeq   fjge   fjlt   fjgt   fjle   fjf

  179.  fjt    fjgl   fjgle  fjnge  fjngl  fjngle fjngt

  180.  fjnle  fjnlt  fjoge  fjogl  fjogt  fjole  fjolt

  181.  fjor   fjseq  fjsf   fjsne  fjst   fjueq  fjuge

  182.  fjugt  fjule  fjult  fjun

  183. </pre></div>

  184. 

  185. <p>Each of these pseudo-operations always expands to a single Motorola

  186. coprocessor branch instruction, word or long.  All Motorola coprocessor

  187. branch instructions allow both word and long displacements.

  188. </p>

  189. </dd>

  190. </dl>

  191. 

  192. <hr>

  193. <div class="header">

  194. <p>

  195. 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> &nbsp; [<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>

  196. </div>

  197. 

  198. 

  199. 

  200. </body>

  201. </html>