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<title>const</title>
<link rel=stylesheet href="opcode.css">
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<h1>const</h1>

<h2>Purpose</h2>

<p>
Move the given literal value (sign-extended to 32 bits, if necessary) into the
specified register. 
</p>

<h2>Details</h2>

<table class="instruc">
<thead>
<tr>
  <th>Op &amp; Format</th>
  <th>Mnemonic / Syntax</th>
  <th>Arguments</th>
</tr>
</thead>
<tbody>
<tr>
  <td>12 11n</td>
  <td>const/4 vA, #+B</td>
  <td><code>A:</code> destination register (4 bits)<br/>
    <code>B:</code> signed int (4 bits)</td>
</tr>
<tr>
  <td>13 21s</td>
  <td>const/16 vAA, #+BBBB</td>
  <td><code>A:</code> destination register (8 bits)<br/>
    <code>B:</code> signed int (16 bits)</td>
</tr>
<tr>
  <td>14 31i</td>
  <td>const vAA, #+BBBBBBBB</td>
  <td><code>A:</code> destination register (8 bits)<br/>
    <code>B:</code> arbitrary 32-bit constant</td>
</tr>
<tr>
  <td>15 21h</td>
  <td>const/high16 vAA, #+BBBB0000</td>
  <td><code>A:</code> destination register (8 bits)<br/>
    <code>B:</code> signed int (16 bits)</td>
</tr>
</tbody>
</table>

<h2>Constraints</h2>

<ul>
  <li>
    A must be a valid register index in the current stackframe.
  </li>
</ul>

<h2>Behavior</h2>

<ul>
  <li>
    First, an adjusted value B' is determined as follows:
    <ul>
      <li>
        If we are executing the /high16 variant, then B is left-shifted by 16
        bits, that is, B'=B << 0x10 
      <li>
        Otherwise, if B is a 4 bit or 16 bit constant, it is sign-extended to 32
        bits, that is, B'=sign-extended(B).
      </li>
      <li>
        Otherwise, B'=B.
      </li>
    </ul>
  <li>
    Then, the adjusted value B' is moved into the register A, that is, vA'=B'
  </li>
  <li>
    If v(A-1) is the lower half of a register pair, v(A-1)' becomes undefined.
  </li>
  <li>
    If v(A+1) is the upper half of a register pair, v(A+1)' becomes undefined.
  </li>
</ul>
     
<h2>Exceptions</h2>

<p>
None.
</p>

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