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<instructionsection id="ERET" title="ERET -- AArch32" type="instruction">
  <docvars>
    <docvar key="instr-class" value="general" />
    <docvar key="mnemonic" value="ERET" />
  </docvars>
  <heading>ERET</heading>
  <desc>
    <brief>
      <para>Exception Return</para>
    </brief>
    <authored>
      <para>Exception Return.</para>
      <para>The PE branches to the address held in the register holding the preferred return address, and restores <xref linkend="BEIDIGBH">PSTATE</xref> from SPSR_&lt;current_mode&gt;.</para>
      <para>The register holding the preferred return address is:</para>
      <list type="unordered">
        <listitem><content><xref linkend="AArch32.elr_hyp">ELR_hyp</xref>, when executing in Hyp mode.</content></listitem>
        <listitem><content>LR, when executing in a mode other than Hyp mode, User mode, or System mode.</content></listitem>
      </list>
      <para>The PE checks SPSR_&lt;current_mode&gt; for an illegal return event. See <xref linkend="CHDDDJDB">Illegal return events from AArch32 state</xref>.</para>
      <para>Exception Return is <arm-defined-word>constrained unpredictable</arm-defined-word> in User mode and System mode.</para>
      <para>In Debug state, the T1 encoding of <instruction>ERET</instruction> executes the DRPS operation.</para>
    </authored>
    <encodingnotes>
      <para>For more information about the <arm-defined-word>constrained unpredictable</arm-defined-word> behavior of this instruction, see <xref linkend="CJAEGDJC">Architectural Constraints on UNPREDICTABLE behaviors</xref>.</para>
    </encodingnotes>
  </desc>
  <alias_list howmany="0"></alias_list>
  <classes>
    <classesintro count="2">
      <txt>It has encodings from the following instruction sets:</txt>
      <txt> A32 (</txt>
      <a href="#iclass_a1">A1</a>
      <txt>)</txt>
      <txt> and </txt>
      <txt> T32 (</txt>
      <a href="#iclass_t1">T1</a>
      <txt>)</txt>
      <txt>.</txt>
    </classesintro>
    <iclass name="A1" oneof="2" id="iclass_a1" no_encodings="1" isa="A32">
      <docvars>
        <docvar key="armarmheading" value="A1" />
        <docvar key="instr-class" value="general" />
        <docvar key="isa" value="A32" />
        <docvar key="mnemonic" value="ERET" />
      </docvars>
      <iclassintro count="1"></iclassintro>
      <regdiagram form="32" psname="aarch32/instrs/ERET/A1_A.txt" tworows="1">
        <box hibit="31" width="4" name="cond" usename="1" settings="4" constraint="!= 1111">
          <c colspan="4">!= 1111</c>
        </box>
        <box hibit="27" width="8" settings="8">
          <c>0</c>
          <c>0</c>
          <c>0</c>
          <c>1</c>
          <c>0</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
        </box>
        <box hibit="19" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="18" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="17" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="16" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="15" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="14" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="13" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="12" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="11" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="10" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="9" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="8" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="7" width="4" settings="4">
          <c>0</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
        </box>
        <box hibit="3" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="2" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="1" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="0" settings="1">
          <c>(0)</c>
        </box>
      </regdiagram>
      <encoding name="ERET_A1" oneofinclass="1" oneof="2" label="A1">
        <docvars>
          <docvar key="armarmheading" value="A1" />
          <docvar key="instr-class" value="general" />
          <docvar key="isa" value="A32" />
          <docvar key="mnemonic" value="ERET" />
        </docvars>
        <asmtemplate><text>ERET</text><text>{</text><a link="sa_c" hover="See {xref{ARMARM_Babbefhf}{Standard assembler syntax fields}}">&lt;c&gt;</a><text>}</text><text>{</text><a link="sa_q" hover="See {xref{ARMARM_Babbefhf}{Standard assembler syntax fields}}">&lt;q&gt;</a><text>}</text></asmtemplate>
      </encoding>
      <ps_section howmany="1">
        <ps name="aarch32/instrs/ERET/A1_A.txt" mylink="aarch32.instrs.ERET.A1_A.txt" enclabels="" sections="1" secttype="noheading">
          <pstext mayhavelinks="1" section="Decode" rep_section="decode">// No additional decoding required</pstext>
        </ps>
      </ps_section>
    </iclass>
    <iclass name="T1" oneof="2" id="iclass_t1" no_encodings="1" isa="T32">
      <docvars>
        <docvar key="armarmheading" value="T1" />
        <docvar key="instr-class" value="general" />
        <docvar key="isa" value="T32" />
        <docvar key="mnemonic" value="ERET" />
      </docvars>
      <iclassintro count="1"></iclassintro>
      <regdiagram form="16x2" psname="aarch32/instrs/ERET/T1_A.txt">
        <box hibit="31" width="12" settings="12">
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
          <c>0</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
          <c>1</c>
        </box>
        <box hibit="19" width="4" name="Rn" settings="4">
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
        </box>
        <box hibit="15" width="2" settings="2">
          <c>1</c>
          <c>0</c>
        </box>
        <box hibit="13" settings="1">
          <c>(0)</c>
        </box>
        <box hibit="12" settings="1">
          <c>0</c>
        </box>
        <box hibit="11" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="10" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="9" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="8" settings="1">
          <c>(1)</c>
        </box>
        <box hibit="7" width="8" name="imm8" settings="8">
          <c>0</c>
          <c>0</c>
          <c>0</c>
          <c>0</c>
          <c>0</c>
          <c>0</c>
          <c>0</c>
          <c>0</c>
        </box>
      </regdiagram>
      <encoding name="ERET_T1" oneofinclass="1" oneof="2" label="T1">
        <docvars>
          <docvar key="armarmheading" value="T1" />
          <docvar key="instr-class" value="general" />
          <docvar key="isa" value="T32" />
          <docvar key="mnemonic" value="ERET" />
        </docvars>
        <asmtemplate><text>ERET</text><text>{</text><a link="sa_c" hover="See {xref{ARMARM_Babbefhf}{Standard assembler syntax fields}}">&lt;c&gt;</a><text>}</text><text>{</text><a link="sa_q" hover="See {xref{ARMARM_Babbefhf}{Standard assembler syntax fields}}">&lt;q&gt;</a><text>}</text></asmtemplate>
      </encoding>
      <ps_section howmany="1">
        <ps name="aarch32/instrs/ERET/T1_A.txt" mylink="aarch32.instrs.ERET.T1_A.txt" enclabels="" sections="1" secttype="noheading">
          <pstext mayhavelinks="1" section="Decode" rep_section="decode">if <a link="impl-aarch32.InITBlock.0" file="shared_pseudocode.xml" hover="function: boolean InITBlock()">InITBlock</a>() &amp;&amp; !<a link="impl-aarch32.LastInITBlock.0" file="shared_pseudocode.xml" hover="function: boolean LastInITBlock()">LastInITBlock</a>() then UNPREDICTABLE;</pstext>
        </ps>
      </ps_section>
    </iclass>
  </classes>
  <explanations scope="all">
    <explanation enclist="ERET_A1, ERET_T1" symboldefcount="1">
      <symbol link="sa_c">&lt;c&gt;</symbol>
      <account encodedin="">
        <intro>
          <para>See <xref linkend="Babbefhf">Standard assembler syntax fields</xref>.</para>
        </intro>
      </account>
    </explanation>
    <explanation enclist="ERET_A1, ERET_T1" symboldefcount="1">
      <symbol link="sa_q">&lt;q&gt;</symbol>
      <account encodedin="">
        <intro>
          <para>See <xref linkend="Babbefhf">Standard assembler syntax fields</xref>.</para>
        </intro>
      </account>
    </explanation>
  </explanations>
  <ps_section howmany="1">
    <ps name="aarch32/instrs/ERET/Op_AS.txt" mylink="execute" enclabels="" sections="1" secttype="Operation">
      <pstext mayhavelinks="1" section="Execute" rep_section="execute">if <a link="impl-aarch32.ConditionPassed.0" file="shared_pseudocode.xml" hover="function: boolean ConditionPassed()">ConditionPassed</a>() then
    EncodingSpecificOperations();
    if !<a link="impl-shared.Halted.0" file="shared_pseudocode.xml" hover="function: boolean Halted()">Halted</a>() then
        if PSTATE.M IN {<a link="M32_User" file="shared_pseudocode.xml" hover="constant bits(5) M32_User = '10000'">M32_User</a>,<a link="M32_System" file="shared_pseudocode.xml" hover="constant bits(5) M32_System = '11111'">M32_System</a>} then
            UNPREDICTABLE;                        // UNDEFINED or NOP
        else
            new_pc_value = if PSTATE.EL == <a link="EL2" file="shared_pseudocode.xml" hover="constant bits(2) EL2 = '10'">EL2</a> then ELR_hyp else <a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[14];
            <a link="AArch32.ExceptionReturn.2" file="shared_pseudocode.xml" hover="function: AArch32.ExceptionReturn(bits(32) new_pc_in, bits(32) spsr)">AArch32.ExceptionReturn</a>(new_pc_value, <a link="impl-shared.SPSR.read.0" file="shared_pseudocode.xml" hover="accessor: bits(N) SPSR[]">SPSR</a>[]);
    else                                          // Perform DRPS operation in Debug state
        if PSTATE.M == <a link="M32_User" file="shared_pseudocode.xml" hover="constant bits(5) M32_User = '10000'">M32_User</a> then
            UNDEFINED;
        elsif PSTATE.M == <a link="M32_System" file="shared_pseudocode.xml" hover="constant bits(5) M32_System = '11111'">M32_System</a> then
            UNPREDICTABLE;                        // UNDEFINED or NOP
        else
            <a link="impl-shared.SynchronizeContext.0" file="shared_pseudocode.xml" hover="function: SynchronizeContext()">SynchronizeContext</a>();
            <a link="impl-shared.DebugRestorePSR.0" file="shared_pseudocode.xml" hover="function: DebugRestorePSR()">DebugRestorePSR</a>();</pstext>
    </ps>
  </ps_section>
  <constrained_unpredictables ps_block="Operation">
    <cu_case>
      <cu_cause>
        <pstext mayhavelinks="1">PSTATE.M IN {M32_User,M32_System}</pstext>
      </cu_cause>
      <cu_type constraint="Constraint_UNDEF" />
      <cu_type constraint="Constraint_NOP" />
    </cu_case>
  </constrained_unpredictables>
</instructionsection>
