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<instructionsection id="SSUB8" title="SSUB8 -- AArch32" type="instruction">
  <docvars>
    <docvar key="instr-class" value="general" />
    <docvar key="mnemonic" value="SSUB8" />
  </docvars>
  <heading>SSUB8</heading>
  <desc>
    <brief>
      <para>Signed Subtract 8</para>
    </brief>
    <authored>
      <para>Signed Subtract 8 performs four 8-bit signed integer subtractions, and writes the results to the destination register. It sets <xref linkend="BEIDIGBH">PSTATE</xref>.GE according to the results of the subtractions.</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>
  <operationalnotes>
    <para>If CPSR.DIT is 1, this instruction has passed its condition execution check, and does not use R15 as either its source or destination:</para>
    <list type="unordered">
      <listitem><content>The execution time of this instruction is independent of:<list type="unordered"><listitem><content>The values of the data supplied in any of its registers.</content></listitem><listitem><content>The values of the NZCV flags.</content></listitem></list></content></listitem>
      <listitem><content>The response of this instruction to asynchronous exceptions does not vary based on:<list type="unordered"><listitem><content>The values of the data supplied in any of its registers.</content></listitem><listitem><content>The values of the NZCV flags.</content></listitem></list></content></listitem>
    </list>
  </operationalnotes>
  <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="SSUB8" />
      </docvars>
      <iclassintro count="1"></iclassintro>
      <regdiagram form="32" psname="aarch32/instrs/SSUB8/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="5" settings="5">
          <c>0</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
          <c>0</c>
        </box>
        <box hibit="22" width="3" name="op1" settings="3">
          <c>0</c>
          <c>0</c>
          <c>1</c>
        </box>
        <box hibit="19" width="4" name="Rn" usename="1">
          <c colspan="4"></c>
        </box>
        <box hibit="15" width="4" name="Rd" usename="1">
          <c colspan="4"></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" name="B" settings="1">
          <c>1</c>
        </box>
        <box hibit="6" width="2" name="op2" settings="2">
          <c>1</c>
          <c>1</c>
        </box>
        <box hibit="4" settings="1">
          <c>1</c>
        </box>
        <box hibit="3" width="4" name="Rm" usename="1">
          <c colspan="4"></c>
        </box>
      </regdiagram>
      <encoding name="SSUB8_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="SSUB8" />
        </docvars>
        <asmtemplate><text>SSUB8</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><text> </text><text>{</text><a link="sa_rd" hover="General-purpose destination register (field &quot;Rd&quot;)">&lt;Rd&gt;</a><text>,</text><text>}</text><text> </text><a link="sa_rn" hover="First general-purpose source register (field &quot;Rn&quot;)">&lt;Rn&gt;</a><text>, </text><a link="sa_rm" hover="Second general-purpose source register (field &quot;Rm&quot;)">&lt;Rm&gt;</a></asmtemplate>
      </encoding>
      <ps_section howmany="1">
        <ps name="aarch32/instrs/SSUB8/A1_A.txt" mylink="aarch32.instrs.SSUB8.A1_A.txt" enclabels="" sections="1" secttype="noheading">
          <pstext mayhavelinks="1" section="Decode" rep_section="decode">d = <a link="impl-shared.UInt.1" file="shared_pseudocode.xml" hover="function: integer UInt(bits(N) x)">UInt</a>(Rd);  n = <a link="impl-shared.UInt.1" file="shared_pseudocode.xml" hover="function: integer UInt(bits(N) x)">UInt</a>(Rn);  m = <a link="impl-shared.UInt.1" file="shared_pseudocode.xml" hover="function: integer UInt(bits(N) x)">UInt</a>(Rm);
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE;</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="SSUB8" />
      </docvars>
      <iclassintro count="1"></iclassintro>
      <regdiagram form="16x2" psname="aarch32/instrs/SSUB8/T1_A.txt">
        <box hibit="31" width="9" settings="9">
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>0</c>
          <c>1</c>
          <c>0</c>
          <c>1</c>
        </box>
        <box hibit="22" width="3" name="op1" settings="3">
          <c>1</c>
          <c>0</c>
          <c>0</c>
        </box>
        <box hibit="19" width="4" name="Rn" usename="1">
          <c colspan="4"></c>
        </box>
        <box hibit="15" width="4" settings="4">
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
        </box>
        <box hibit="11" width="4" name="Rd" usename="1">
          <c colspan="4"></c>
        </box>
        <box hibit="7" settings="1">
          <c>0</c>
        </box>
        <box hibit="6" name="U" settings="1">
          <c>0</c>
        </box>
        <box hibit="5" name="H" settings="1">
          <c>0</c>
        </box>
        <box hibit="4" name="S" settings="1">
          <c>0</c>
        </box>
        <box hibit="3" width="4" name="Rm" usename="1">
          <c colspan="4"></c>
        </box>
      </regdiagram>
      <encoding name="SSUB8_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="SSUB8" />
        </docvars>
        <asmtemplate><text>SSUB8</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><text> </text><text>{</text><a link="sa_rd" hover="General-purpose destination register (field &quot;Rd&quot;)">&lt;Rd&gt;</a><text>,</text><text>}</text><text> </text><a link="sa_rn" hover="First general-purpose source register (field &quot;Rn&quot;)">&lt;Rn&gt;</a><text>, </text><a link="sa_rm" hover="Second general-purpose source register (field &quot;Rm&quot;)">&lt;Rm&gt;</a></asmtemplate>
      </encoding>
      <ps_section howmany="1">
        <ps name="aarch32/instrs/SSUB8/T1_A.txt" mylink="aarch32.instrs.SSUB8.T1_A.txt" enclabels="" sections="1" secttype="noheading">
          <pstext mayhavelinks="1" section="Decode" rep_section="decode">d = <a link="impl-shared.UInt.1" file="shared_pseudocode.xml" hover="function: integer UInt(bits(N) x)">UInt</a>(Rd);  n = <a link="impl-shared.UInt.1" file="shared_pseudocode.xml" hover="function: integer UInt(bits(N) x)">UInt</a>(Rn);  m = <a link="impl-shared.UInt.1" file="shared_pseudocode.xml" hover="function: integer UInt(bits(N) x)">UInt</a>(Rm);
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13</pstext>
        </ps>
      </ps_section>
    </iclass>
  </classes>
  <explanations scope="all">
    <explanation enclist="SSUB8_A1, SSUB8_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="SSUB8_A1, SSUB8_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>
    <explanation enclist="SSUB8_A1, SSUB8_T1" symboldefcount="1">
      <symbol link="sa_rd">&lt;Rd&gt;</symbol>
      <account encodedin="Rd">
        <intro>
          <para>Is the general-purpose destination register, encoded in the "Rd" field.</para>
        </intro>
      </account>
    </explanation>
    <explanation enclist="SSUB8_A1, SSUB8_T1" symboldefcount="1">
      <symbol link="sa_rn">&lt;Rn&gt;</symbol>
      <account encodedin="Rn">
        <intro>
          <para>Is the first general-purpose source register, encoded in the "Rn" field.</para>
        </intro>
      </account>
    </explanation>
    <explanation enclist="SSUB8_A1, SSUB8_T1" symboldefcount="1">
      <symbol link="sa_rm">&lt;Rm&gt;</symbol>
      <account encodedin="Rm">
        <intro>
          <para>Is the second general-purpose source register, encoded in the "Rm" field.</para>
        </intro>
      </account>
    </explanation>
  </explanations>
  <ps_section howmany="1">
    <ps name="aarch32/instrs/SSUB8/Op_A.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();
    diff1 = <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[n]&lt;7:0&gt;) - <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[m]&lt;7:0&gt;);
    diff2 = <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[n]&lt;15:8&gt;) - <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[m]&lt;15:8&gt;);
    diff3 = <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[n]&lt;23:16&gt;) - <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[m]&lt;23:16&gt;);
    diff4 = <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[n]&lt;31:24&gt;) - <a link="impl-shared.SInt.1" file="shared_pseudocode.xml" hover="function: integer SInt(bits(N) x)">SInt</a>(<a link="impl-aarch32.R.read.1" file="shared_pseudocode.xml" hover="accessor: bits(32) R[integer n]">R</a>[m]&lt;31:24&gt;);
    <a link="impl-aarch32.R.write.1" file="shared_pseudocode.xml" hover="accessor: R[integer n] = bits(32) value">R</a>[d]&lt;7:0&gt;   = diff1&lt;7:0&gt;;
    <a link="impl-aarch32.R.write.1" file="shared_pseudocode.xml" hover="accessor: R[integer n] = bits(32) value">R</a>[d]&lt;15:8&gt;  = diff2&lt;7:0&gt;;
    <a link="impl-aarch32.R.write.1" file="shared_pseudocode.xml" hover="accessor: R[integer n] = bits(32) value">R</a>[d]&lt;23:16&gt; = diff3&lt;7:0&gt;;
    <a link="impl-aarch32.R.write.1" file="shared_pseudocode.xml" hover="accessor: R[integer n] = bits(32) value">R</a>[d]&lt;31:24&gt; = diff4&lt;7:0&gt;;
    PSTATE.GE&lt;0&gt;  = if diff1 &gt;= 0 then '1' else '0';
    PSTATE.GE&lt;1&gt;  = if diff2 &gt;= 0 then '1' else '0';
    PSTATE.GE&lt;2&gt;  = if diff3 &gt;= 0 then '1' else '0';
    PSTATE.GE&lt;3&gt;  = if diff4 &gt;= 0 then '1' else '0';</pstext>
    </ps>
  </ps_section>
</instructionsection>
