<?xml version="1.0" ?><?xml-stylesheet type="text/xsl" encoding="UTF-8" href="iform.xsl" version="1.0"?><!DOCTYPE instructionsection  PUBLIC '-//ARM//DTD instructionsection //EN'  'iform-p.dtd'><!-- Copyright (c) 2010-2026 Arm Limited or its affiliates. All rights reserved. --><!-- This document is Non-Confidential. This document may only be used and distributed in accordance with the terms of the agreement entered into by Arm and the party that Arm delivered this document to. --><instructionsection id="STADDH_LDADDH" title="STADDH, STADDLH -- A64" type="alias">
  <docvars>
    <docvar key="instr-class" value="general"/>
    <docvar key="isa" value="A64"/>
  </docvars>
  <heading>STADDH, STADDLH</heading>
  <desc>
    <brief>
      <para>Atomic add on halfword, without return</para>
    </brief>
    <authored>
      <para>This instruction
atomically loads a 16-bit halfword from memory, adds the value held in a
register to it, and stores the result back to memory.</para>
      <list type="unordered">
        <listitem>
          <content>
            <instruction>STADDH</instruction> does not have release semantics.</content>
        </listitem>
        <listitem>
          <content>
            <instruction>STADDLH</instruction> stores to memory with release semantics, as described in <xref linkend="ARMARM_BEIHCHEF">Load-Acquire, Store-Release</xref>.</content>
        </listitem>
      </list>
      <para>For information about addressing modes, see
<xref linkend="ARMARM_CHDIIIBB">Load/Store addressing modes</xref>.</para>
    </authored>
  </desc>
  <operationalnotes/>
  <aliasto refiform="ldaddh.xml" iformid="LDADDH">LDADDH, LDADDAH, LDADDALH, LDADDLH</aliasto>
  <classes>
    <iclass name="Integer" oneof="1" id="iclass_integer" no_encodings="2" isa="A64">
      <docvars>
        <docvar key="instr-class" value="general"/>
        <docvar key="isa" value="A64"/>
      </docvars>
      <iclassintro count="2"/>
      <arch_variants>
        <arch_variant feature="FEAT_LSE" name="v8Ap1"/>
      </arch_variants>
      <regdiagram form="32" psname="A64.ldst.memop.LDADDH_32_memop.STADDH" tworows="1">
        <box hibit="31" width="2" name="size" usename="1" settings="2" psbits="xx">
          <c>0</c>
          <c>1</c>
        </box>
        <box hibit="29" width="3" settings="3">
          <c>1</c>
          <c>1</c>
          <c>1</c>
        </box>
        <box hibit="26" name="VR" usename="1" settings="1" psbits="x">
          <c>0</c>
        </box>
        <box hibit="25" width="2" settings="2">
          <c>0</c>
          <c>0</c>
        </box>
        <box hibit="23" name="A" usename="1" settings="1" psbits="x">
          <c>0</c>
        </box>
        <box hibit="22" width="1" name="R" usename="1">
          <c colspan="1"/>
        </box>
        <box hibit="21" width="1" settings="1">
          <c>1</c>
        </box>
        <box hibit="20" width="5" name="Rs" usename="1">
          <c colspan="5"/>
        </box>
        <box hibit="15" name="o3" usename="1" settings="1" psbits="x">
          <c>0</c>
        </box>
        <box hibit="14" width="3" name="opc" usename="1" settings="3" psbits="xxx">
          <c>0</c>
          <c>0</c>
          <c>0</c>
        </box>
        <box hibit="11" width="2" settings="2">
          <c>0</c>
          <c>0</c>
        </box>
        <box hibit="9" width="5" name="Rn" usename="1">
          <c colspan="5"/>
        </box>
        <box hibit="4" width="5" name="Rt" usename="1" settings="5" psbits="xxxxx">
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
          <c>1</c>
        </box>
      </regdiagram>
      <encoding name="STADDH_LDADDH_32_memop" oneofinclass="2" oneof="2" label="No memory ordering" bitdiffs="R == 0">
        <docvars>
          <docvar key="instr-class" value="general"/>
          <docvar key="isa" value="A64"/>
          <docvar key="mnemonic" value="LDADDH"/>
          <docvar key="alias_mnemonic" value="STADDH"/>
        </docvars>
        <box hibit="22" width="1" name="R">
          <c>0</c>
        </box>
        <asmtemplate><text>STADDH  </text><a hover="Is the 32-bit name of the general-purpose register holding the data value to be operated on with the contents of the memory location, encoded in the &quot;Rs&quot; field." link="WsOrWZR">&lt;Ws&gt;</a><text>, [</text><a hover="Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the &quot;Rn&quot; field." link="XnSP_option">&lt;Xn|SP&gt;</a><text>]</text></asmtemplate>
        <equivalent_to>
          <asmtemplate><a href="ldaddh.xml#LDADDH_32_memop">LDADDH</a><text>  </text><a hover="Is the 32-bit name of the general-purpose register holding the data value to be operated on with the contents of the memory location, encoded in the &quot;Rs&quot; field." href="ldaddh.xml#WsOrWZR">&lt;Ws&gt;</a><text>, WZR, [</text><a hover="Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the &quot;Rn&quot; field." href="ldaddh.xml#XnSP_option">&lt;Xn|SP&gt;</a><text>]</text></asmtemplate>
          <aliascond>Unconditionally</aliascond>
        </equivalent_to>
      </encoding>
      <encoding name="STADDLH_LDADDLH_32_memop" oneofinclass="2" oneof="2" label="Release" bitdiffs="R == 1">
        <docvars>
          <docvar key="instr-class" value="general"/>
          <docvar key="isa" value="A64"/>
          <docvar key="mnemonic" value="LDADDLH"/>
          <docvar key="alias_mnemonic" value="STADDLH"/>
        </docvars>
        <box hibit="22" width="1" name="R">
          <c>1</c>
        </box>
        <asmtemplate><text>STADDLH  </text><a hover="Is the 32-bit name of the general-purpose register holding the data value to be operated on with the contents of the memory location, encoded in the &quot;Rs&quot; field." link="WsOrWZR">&lt;Ws&gt;</a><text>, [</text><a hover="Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the &quot;Rn&quot; field." link="XnSP_option">&lt;Xn|SP&gt;</a><text>]</text></asmtemplate>
        <equivalent_to>
          <asmtemplate><a href="ldaddh.xml#LDADDLH_32_memop">LDADDLH</a><text>  </text><a hover="Is the 32-bit name of the general-purpose register holding the data value to be operated on with the contents of the memory location, encoded in the &quot;Rs&quot; field." href="ldaddh.xml#WsOrWZR">&lt;Ws&gt;</a><text>, WZR, [</text><a hover="Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the &quot;Rn&quot; field." href="ldaddh.xml#XnSP_option">&lt;Xn|SP&gt;</a><text>]</text></asmtemplate>
          <aliascond>Unconditionally</aliascond>
        </equivalent_to>
      </encoding>
    </iclass>
  </classes>
  <explanations scope="all">
    <explanation enclist="STADDH_LDADDH_32_memop, STADDLH_LDADDLH_32_memop" symboldefcount="1">
      <symbol link="WsOrWZR">&lt;Ws&gt;</symbol>
      <account encodedin="Rs">
        <intro>
          <para>Is the 32-bit name of the general-purpose register holding the data value to be operated on with the contents of the memory location, encoded in the &quot;Rs&quot; field.</para>
        </intro>
      </account>
    </explanation>
    <explanation enclist="STADDH_LDADDH_32_memop, STADDLH_LDADDLH_32_memop" symboldefcount="1">
      <symbol link="XnSP_option">&lt;Xn|SP&gt;</symbol>
      <account encodedin="Rn">
        <intro>
          <para>Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the &quot;Rn&quot; field.</para>
        </intro>
      </account>
    </explanation>
  </explanations>
  <timestamp>2026-03-26 20:27:25</timestamp>
  <commit_id>2026-03_rel</commit_id>
</instructionsection>