diff --git a/mpact/sim/decoder/InstructionSet.g4 b/mpact/sim/decoder/InstructionSet.g4
index b8e71e5..1c5951e 100644
--- a/mpact/sim/decoder/InstructionSet.g4
+++ b/mpact/sim/decoder/InstructionSet.g4
@@ -142,11 +142,19 @@
   : INCLUDE STRING_LITERAL
   ;
 
-// A bundle_declaration has a name and specifies the set of bundles and/or
-// slots contained within. At least one slot or bundle has to be specified.
-
+// A bundle_declaration has a name and specifies a semantic function, which is
+// is responsible for dispatching the instructions for the bundles and/or slots
+// It also specifies the bundles and/or slots the set of bundles and/or slots
+// it contains. At least one slot or bundle has to be specified.
 bundle_declaration
-  : BUNDLE bundle_name=IDENT '{' bundle_list? slot_list? '}'
+  : BUNDLE bundle_name=IDENT '{' bundle_parts* '}'
+  ;
+
+bundle_parts
+  : include_file_list
+  | bundle_list
+  | slot_list
+  | semfunc_spec ';'
   ;
 
 // A bundle list is a non-empty list of bundle identifiers
diff --git a/mpact/sim/decoder/bin_format_visitor.cc b/mpact/sim/decoder/bin_format_visitor.cc
index 39ba2dc..97351f3 100644
--- a/mpact/sim/decoder/bin_format_visitor.cc
+++ b/mpact/sim/decoder/bin_format_visitor.cc
@@ -62,35 +62,25 @@
 template <typename T>
 static inline T ExtractBits(const uint8_t *data, int data_size, int msb,
                             int width) {
-  if (width == 0) return 0;
-
-  int byte_low = data_size - ((msb - width) >> 3) - 1;
-  int byte_high = data_size - (msb >> 3) - 1;
-  int high_bit = msb & 0x7;
-
-  // If it is only from one byte, extract and return.
-  if (byte_low == byte_high) {
-    uint8_t mask = (1 << (high_bit + 1)) - 1;
-    return (mask & data[byte_high]) >> (high_bit - width + 1);
-  }
-
-  // Extract from the high order byte.
   T val = 0;
-  uint8_t mask = 0xff >> (7 - high_bit);
-  val = (mask & data[byte_high++]);
-  int remainder = width - (1 + high_bit);
 
-  while (remainder >= 8) {
-    val = (val << 8) | data[byte_high++];
-    remainder -= 8;
-  }
+  if (width == 0) return val;
 
-  // Extract any remaining bits from the high end of the last byte.
-  if (remainder > 0) {
-    val <<= remainder;
-    int shift = 8 - remainder;
-    uint8_t mask = 0xff << shift;
-    val |= (data[byte_high] & mask) >> shift;
+  int lsb = msb - width + 1;
+  int byte_low = data_size - (lsb >> 3) - 1;
+
+
+  int blsb = lsb & 0x7;
+  int bits_left = width;
+  int bits_extracted = 0;
+  while (bits_left > 0) {
+    int bwidth = std::min(8 - blsb, bits_left);
+    uint8_t bmask = ((1 << bwidth) - 1) << blsb;
+    val |= ((data[byte_low] & bmask) >> blsb) << bits_extracted;
+    blsb = 0;
+    bits_left -= bwidth;
+    bits_extracted += bwidth;
+    byte_low--;
   }
   return val;
 }
@@ -99,6 +89,38 @@
 
 )foo";
 
+constexpr char kTemplatedInsertBits[] = R"foo(
+namespace internal {
+
+// This function inserts a bitfield width bits wide into the byte vector,
+// starting at bit_index bits from the end of data. The lsb has index 0. The
+// byte vector is data_size bytes long. There is no error checking that T
+// can hold width bits.
+template <typename T>
+static inline void InsertBits(uint8_t *data, int data_size, int msb, int width,
+                              T val) {
+  if (width == 0) return;
+
+  int lsb = msb - width + 1;
+  int byte_low = data_size - (lsb >> 3) - 1;
+  int blsb = lsb & 0x7;
+  while (width > 0) {
+    int bwidth = std::min(8 - blsb, width);
+    T bmask = (1 << bwidth) - 1;
+    uint8_t bval = (val & bmask);
+    bmask <<= blsb;
+    bval <<= blsb;
+    val >>= bwidth;
+    data[byte_low] = (data[byte_low] & ~bmask) | (bval & bmask);
+    blsb = 0;
+    width -= bwidth;
+    byte_low--;
+  }
+}
+
+}  // namespace internal
+)foo";
+
 BinFormatVisitor::BinFormatVisitor() {
   constraint_string_to_type_.emplace("==", ConstraintType::kEq);
   constraint_string_to_type_.emplace("!=", ConstraintType::kNe);
@@ -237,6 +259,7 @@
                   "#include <cstdint>\n"
                   "\n"
                   "#include \"absl/functional/any_invocable.h\"\n"
+                  "#include \"absl/log/log.h\"\n"
                   "\n\n");
   for (auto const &include_file : encoding_info->include_files()) {
     absl::StrAppend(&h_string, "#include ", include_file, "\n");
@@ -327,14 +350,15 @@
                   "#include <iostream>\n"
                   "#include <cstdint>\n\n"
                   "#include \"absl/base/no_destructor.h\"\n"
-                  "#include \"absl/container/flat_hash_map.h\"\n\n"
+                  "#include \"absl/container/flat_hash_map.h\"\n"
+                  "#include \"absl/log/log.h\"\n\n"
                   "#include \"",
                   enum_h_name, "\"\n");
   absl::StrAppend(&cc_string, "#include \"", dot_h_name,
                   "\"\n\n"
                   "#include <cstdint>\n\n"
                   "#include \"absl/base/no_destructor.h\"\n"
-                  "#include \"absl/container/flat_hash_map.h\"\n\n"
+                  "#include \"absl/container/flat_hash_map.h\"\n"
                   "#include \"",
                   enum_h_name, "\"\n");
   for (auto &name_space : encoding_info->decoder()->namespaces()) {
@@ -342,7 +366,8 @@
     absl::StrAppend(&cc_string, name_space_str);
     absl::StrAppend(&h_string, name_space_str);
   }
-  absl::StrAppend(&h_string, "\n");
+  // Write out the templated extractor function used by the other methods.
+  absl::StrAppend(&h_string, "\n", kTemplatedInsertBits, "\n");
   absl::StrAppend(&cc_string, "\n");
   return std::tie(h_string, cc_string);
 }
diff --git a/mpact/sim/decoder/bundle.h b/mpact/sim/decoder/bundle.h
index ae58095..315cd42 100644
--- a/mpact/sim/decoder/bundle.h
+++ b/mpact/sim/decoder/bundle.h
@@ -66,6 +66,10 @@
   InstructionSet *instruction_set() const { return instruction_set_; }
   bool is_marked() const { return is_marked_; }
   void set_is_marked(bool value) { is_marked_ = value; }
+  std::string semfunc_code_string() const { return semfunc_code_string_; }
+  void set_semfunc_code_string(std::string code_string) {
+    semfunc_code_string_ = std::move(code_string);
+  }
 
  private:
   BundleDeclCtx *ctx_;
@@ -76,6 +80,8 @@
   std::string name_;
   // Name in PascalCase.
   std::string pascal_name_;
+  // Semantic function code string.
+  std::string semfunc_code_string_;
   // The slots contained within this bundle, including instance indices.
   std::vector<std::pair<std::string, const std::vector<int>>> slot_uses_;
   // The bundles contained within this bundle.
diff --git a/mpact/sim/decoder/encoding_group.cc b/mpact/sim/decoder/encoding_group.cc
index 30d203f..f722d84 100644
--- a/mpact/sim/decoder/encoding_group.cc
+++ b/mpact/sim/decoder/encoding_group.cc
@@ -159,7 +159,7 @@
       encoding_group->AddEncoding(enc);
     }
     // Avoid useless groups and infinite recursion by deleting any groups that
-    // are empty and where the all the encodings ended up in the same subgroup.
+    // are empty and where all the encodings ended up in the same subgroup.
     if (encoding_group->encoding_vec().empty()) {
       delete encoding_group;
       continue;
@@ -439,17 +439,20 @@
   // For each instruction in the encoding vec, generate the if statement
   // to see if the instruction is matched.
   absl::flat_hash_set<std::string> extracted;
-  // For equal constraints, some can be ignored because those bits are wholly
-  // considered by the parent groups or the discriminator.
+  int count = 0;
+  // For equal constraints, some can be ignored because those bits are
+  // wholly considered by the parent groups or the discriminator.
   for (auto *encoding : encoding_vec_) {
     for (auto *constraint : encoding->equal_constraints()) {
       ProcessConstraint(extracted, constraint, definitions_ptr);
     }
-    EmitEncodingIfStatement(/*indent*/ 0, encoding, opcode_enum, extracted,
-                            definitions_ptr);
+    count += EmitEncodingIfStatement(/*indent*/ 0, encoding, opcode_enum,
+                                     extracted, definitions_ptr);
   }
-  absl::StrAppend(definitions_ptr, "  return std::make_pair(", opcode_enum,
-                  "::kNone, FormatEnum::kNone);\n");
+  if (count > 0) {
+    absl::StrAppend(definitions_ptr, "  return std::make_pair(", opcode_enum,
+                    "::kNone, FormatEnum::kNone);\n");
+  }
 }
 
 void EncodingGroup::ProcessConstraint(
diff --git a/mpact/sim/decoder/format.cc b/mpact/sim/decoder/format.cc
index c602995..5e8de59 100644
--- a/mpact/sim/decoder/format.cc
+++ b/mpact/sim/decoder/format.cc
@@ -143,19 +143,27 @@
 }
 
 // Return the string containing the integer type used to contain the current
-// format. If it is greater than 64 bits, will use a byte array (int8_t *).
+// format. If it is greater than 128 bits, will use a byte array (int8_t *).
+// If it is 65 to 128 bits, will use absl::[u]int128.
+std::string Format::GetUIntType(int bitwidth) const {
+  if (bitwidth > 128) return "uint8_t *";
+  if (bitwidth > 64) return "absl::uint128";
+  return absl::StrCat("uint", GetIntTypeBitWidth(bitwidth), "_t");
+}
+
 std::string Format::GetIntType(int bitwidth) const {
-  if (bitwidth > 64) return "int8_t *";
+  if (bitwidth > 128) return "int8_t *";
+  if (bitwidth > 64) return "absl::int128";
   return absl::StrCat("int", GetIntTypeBitWidth(bitwidth), "_t");
 }
 
-// Return the int type byte width (1, 2, 4, 8) or (-1 if it's bigger), of the
-// integer type that would fit this format.
+// Return the int type byte width (1, 2, 4, 8, 16) or (-1 if it's bigger), of
+// the integer type that would fit this format.
 int Format::GetIntTypeBitWidth(int bitwidth) const {
   auto shift = absl::bit_width(static_cast<unsigned>(bitwidth)) - 1;
   if (absl::popcount(static_cast<unsigned>(bitwidth)) > 1) shift++;
   shift = std::max(shift, 3);
-  if (shift > 6) return -1;
+  if (shift > 7) return -1;
   return 1 << shift;
 }
 
@@ -208,7 +216,8 @@
       }
       field_or_format->set_high(declared_width_ - computed_width_ - 1);
       computed_width_ += format->declared_width() * field_or_format->size();
-      extractors_.insert(std::make_pair(format->name(), field_or_format));
+      extractors_.insert(
+          std::make_pair(field_or_format->format_alias(), field_or_format));
     }
     if (computed_width_ != declared_width_) {
       return absl::InternalError(absl::StrCat(
@@ -350,17 +359,17 @@
   std::string h_output;
   int return_width = GetIntTypeBitWidth(field->width);
   std::string result_type_name =
-      absl::StrCat(field->is_signed ? "" : "u", GetIntType(return_width));
-  std::string argument_type_name =
-      absl::StrCat("u", GetIntType(computed_width_));
-  std::string signature =
-      absl::StrCat(result_type_name, " Extract", ToPascalCase(field->name), "(",
-                   argument_type_name, " value)");
+      field->is_signed ? GetIntType(return_width) : GetUIntType(return_width);
+  std::string argument_type_name = GetUIntType(computed_width_);
+  std::string signature = absl::StrCat(
+      result_type_name, " Extract", ToPascalCase(field->name), "(",
+      computed_width_ > 128 ? "const " : "", argument_type_name, " value)");
 
   absl::StrAppend(&h_output, "inline ", signature, " {\n");
 
   // Generate extraction function. For fields it's a simple shift and mask if
-  // the source format width <= 64 bits.
+  // the source format width <= 64 bits. Slightly more involved with format
+  // <= 128 bits. For larger formats use the templated extract helper function.
   std::string expr;
   if (declared_width_ <= 64) {
     uint64_t mask = (1ULL << field->width) - 1;
@@ -369,21 +378,50 @@
     } else {
       expr = absl::StrCat(" (value >> ", field->low, ") & 0x", absl::Hex(mask));
     }
+  } else if (declared_width_ <= 128) {
+    absl::StrAppend(&h_output,
+                    "  absl::uint128 mask = 1;\n"
+                    "  mask = (mask << ",
+                    field->width, ") - 1;\n");
+    if (field->low == 0) {
+      expr = absl::StrCat("value & mask");
+    } else {
+      expr = absl::StrCat(" (value >> ", field->low, ") & mask");
+    }
   } else {
-    // For format width > 64 bits, use the templated extract helper function.
+    // For format width > 128 bits, use the templated extract helper function.
     int byte_size = (declared_width_ + 7) / 8;
     expr = absl::StrCat("internal::ExtractBits<", result_type_name, ">(value, ",
                         byte_size, ", ", field->high, ", ", field->width, ")");
   }
 
   // Add sign-extension if the field is signed.
+  std::string sign_extension;
   if (field->is_signed) {
     int shift = return_width - field->width;
-    absl::StrAppend(&h_output, "  ", result_type_name, " result = (", expr,
-                    ") << ", shift, ";\n  result = result >> ", shift, ";\n",
-                    "  return result;\n}\n\n");
+    sign_extension =
+        absl::StrCat("  ", result_type_name, " result = (", expr, ") << ",
+                     shift, ";\n  result = result >> ", shift, ";\n");
+    expr = "result";
+  }
+  if (declared_width_ <= 64) {
+    absl::StrAppend(&h_output, sign_extension, "  return ", expr, ";\n}\n\n");
+  } else if ((declared_width_ <= 128) && (return_width <= 64)) {
+    absl::StrAppend(&h_output, sign_extension, "  return absl::Uint128Low64(",
+                    expr, ");\n}\n\n");
   } else {
-    absl::StrAppend(&h_output, "  return ", expr, ";\n}\n\n");
+    absl::StrAppend(&h_output, sign_extension, "  return ", expr, ";\n}\n\n");
+  }
+  // If the parent format size is not a power of two, also create an extractor
+  // that takes a uint8_t * parameter.
+  if ((declared_width_ <= 128) &&
+      (absl::popcount(static_cast<unsigned>(declared_width_)) > 1)) {
+    absl::StrAppend(&h_output, "inline ", result_type_name, " Extract",
+                    ToPascalCase(field->name), "(const uint8_t *value) {\n");
+    int byte_size = (declared_width_ + 7) / 8;
+    absl::StrAppend(&h_output, "  return internal::ExtractBits<",
+                    result_type_name, ">(value, ", byte_size, ", ", field->high,
+                    ", ", field->width, ");\n}\n\n");
   }
   return h_output;
 }
@@ -393,9 +431,16 @@
 // it into the right place in the instruction word.
 std::string Format::GenerateFieldInserter(const Field *field) const {
   std::string h_output;
-  absl::StrAppend(&h_output, "static inline uint64_t Insert",
-                  ToPascalCase(field->name),
-                  "(uint64_t value, uint64_t inst_word) {\n");
+  std::string field_type_name;
+  std::string inst_word_type_name = GetUIntType(computed_width_);
+  if (declared_width_ <= 128) {
+    field_type_name = inst_word_type_name;
+  } else {
+    field_type_name = GetUIntType(field->width);
+  }
+  absl::StrAppend(&h_output, "static inline ", inst_word_type_name, " Insert",
+                  ToPascalCase(field->name), "(", field_type_name, " value, ",
+                  inst_word_type_name, " inst_word) {\n");
   if (declared_width_ <= 64) {
     uint64_t mask = ((1ULL << field->width) - 1) << field->low;
     std::string shift;
@@ -404,15 +449,36 @@
     }
     absl::StrAppend(&h_output, "  inst_word = (inst_word & ~0x",
                     absl::Hex(mask), "ULL)", " | ((value", shift, ") & 0x",
-                    absl::Hex(mask), "ULL);\n");
+                    absl::Hex(mask),
+                    "ULL);\n"
+                    "  return inst_word;\n"
+                    "}\n");
+  } else if (declared_width_ <= 128) {
+    absl::StrAppend(&h_output,
+                    "  absl::uint128 mask = 1;\n"
+                    "  mask = (mask << ",
+                    field->width, ") - 1;\n");
+    if (field->low != 0) {
+      absl::StrAppend(&h_output, "  mask = mask << ", field->low, ";\n");
+    }
+    absl::StrAppend(&h_output,
+                    "  inst_word = (inst_word & ~mask) | (value & mask);\n"
+                    "  return inst_word;\n"
+                    "}\n");
+  } else if (field->width <= 128) {
+    int byte_size = (declared_width_ + 7) / 8;
+    absl::StrAppend(&h_output, "  internal::InsertBits(inst_word, ", byte_size,
+                    ", ", field->high, ", ", field->width,
+                    ", value);\n"
+                    "  return inst_word;\n"
+                    "}\n");
   } else {
     absl::StrAppend(
         &h_output,
-        "  #error Support for formats > 64 bits not implemented - yet.");
+        "  LOG(FATAL) << \" Support for fields > 128 bits not implemented - "
+        "yet.\";\n"
+        "  return 0;\n}\n");
   }
-  absl::StrAppend(&h_output,
-                  "  return inst_word;\n"
-                  "}\n");
   return h_output;
 }
 
@@ -421,46 +487,78 @@
 // insert its components into the right places in the instruction word.
 std::string Format::GenerateOverlayInserter(Overlay *overlay) const {
   std::string h_output;
-  absl::StrAppend(&h_output, "static inline uint64_t Insert",
-                  ToPascalCase(overlay->name()),
-                  "(uint64_t value, uint64_t inst_word) {\n");
+  std::string result_type_name = GetUIntType(computed_width_);
+  std::string overlay_type_name;
+  if (computed_width_ <= 128) {
+    overlay_type_name = result_type_name;
+  } else {
+    overlay_type_name = GetUIntType(overlay->declared_width());
+  }
+  absl::StrAppend(&h_output, "static inline ", result_type_name, " Insert",
+                  ToPascalCase(overlay->name()), "(", overlay_type_name,
+                  " value, ", result_type_name, " inst_word) {\n");
   // Mark error if either the overlay or the format is > 64 bits.
-  if (overlay->declared_width() > 64) {
-    absl::StrAppend(&h_output,
-                    "  #error Support for overlays > 64 bits not implemented - "
-                    "yet.\n}\n");
+  if (overlay->declared_width() > 128) {
+    absl::StrAppend(
+        &h_output,
+        "  LOG(FATAL) << \" Support for overlays > 128 bits not implemented - "
+        "yet.\";\n"
+        "  return 0;\n}\n");
     return h_output;
   }
-  if (computed_width_ > 64) {
-    absl::StrAppend(&h_output,
-                    "  #error Support for formats > 64 bits not implemented - "
-                    "yet.\n}\n");
-    return h_output;
+  bool use_mask_variable = false;
+  int remaining = overlay->declared_width();
+  int byte_size = (declared_width_ + 7) / 8;
+  if (declared_width_ <= 128) {
+    absl::StrAppend(&h_output, "  ", result_type_name, " tmp;\n");
+    // Track the leftmost bit in the overlay.
+    if (declared_width_ > 64) {
+      absl::StrAppend(&h_output, "  absl::uint128 mask;\n");
+      use_mask_variable = true;
+    }
+  } else {
+    absl::StrAppend(&h_output, "  ", overlay_type_name, " tmp;\n");
+    if (overlay->declared_width() > 64) {
+      absl::StrAppend(&h_output, "  absl::uint128 mask;\n");
+      use_mask_variable = true;
+    }
   }
-  absl::StrAppend(&h_output, "  uint64_t tmp;\n");
-  // Track the leftmost bit in the overlay.
-  int left = overlay->declared_width();
   for (auto &bits_or_field : overlay->component_vec()) {
     int width = bits_or_field->width();
     // Ignore the bit fields in the overlay.
     if (bits_or_field->high() < 0) {
-      left -= width;
+      remaining -= width;
       continue;
     }
-    uint64_t mask = ((1ULL << width) - 1);
     std::string shift;
-    if (left - width > 0) {
-      shift = absl::StrCat(" >> ", left - width);
+    if (remaining - width > 0) {
+      shift = absl::StrCat(" >> ", remaining - width);
     }
-    // Extract the bits from the overlay value for the current component.
-    absl::StrAppend(&h_output, "  tmp = (value ", shift, ") & 0x",
-                    absl::Hex(mask), "ULL;\n");
+    if (use_mask_variable) {
+      absl::StrAppend(&h_output,
+                      "  mask = 1;\n"
+                      "  mask = (mask << ",
+                      width, ") - 1;\n");
+      absl::StrAppend(&h_output, "  tmp = (value ", shift, ") & mask;\n");
+    } else {
+      uint64_t mask = ((1ULL << width) - 1);
+      // Extract the bits from the overlay value for the current component.
+      absl::StrAppend(&h_output, "  tmp = (value ", shift, ") & 0x",
+                      absl::Hex(mask), "ULL;\n");
+    }
     shift.clear();
     if (bits_or_field->low() != 0) {
       shift = absl::StrCat(" << ", bits_or_field->low());
     }
-    absl::StrAppend(&h_output, "  inst_word |= (tmp ", shift, ");\n");
-    left -= width;
+    if (declared_width_ <= 128) {
+      absl::StrAppend(&h_output, "  inst_word |= (tmp ", shift, ");\n");
+    } else {
+      absl::StrAppend(&h_output, "  internal::InsertBits(inst_word, ",
+                      byte_size, ", ", bits_or_field->high(), ", ", width,
+                      ",  tmp);\n");
+    }
+
+    remaining -= width;
   }
   absl::StrAppend(&h_output, "  return inst_word;\n}\n");
   return h_output;
@@ -472,27 +570,116 @@
 std::string Format::GenerateFormatInserter(std::string_view format_alias,
                                            const Format *format, int high,
                                            int size) const {
+  if (size > 1) {
+    return GenerateReplicatedFormatInserter(format_alias, format, high, size);
+  }
+  return GenerateSingleFormatInserter(format_alias, format, high);
+}
+
+std::string Format::GenerateReplicatedFormatInserter(
+    std::string_view format_alias, const Format *format, int high,
+    int size) const {
   std::string h_output;
-  std::string target_type_name = absl::StrCat("u", GetIntType(computed_width_));
-  absl::StrAppend(&h_output, "static inline uint64_tInsert",
-                  ToPascalCase(format_alias),
-                  "(uint64_t value, uint64_t inst_word) {\n");
-  if (declared_width_ > 64) {
-    absl::StrAppend(&h_output,
-                    "  #error Support for formats > 64 bits not implemented - "
-                    "yet.\n}\n");
+  std::string target_type_name = GetUIntType(declared_width_);
+  std::string format_type_name;
+
+  if (declared_width_ <= 128) {
+    format_type_name = target_type_name;
+  } else {
+    format_type_name = GetUIntType(format->declared_width());
+  }
+  absl::StrAppend(&h_output, "static inline ", target_type_name, " Insert",
+                  ToPascalCase(format_alias), "(", "int index, ",
+                  format_type_name, " value, ", target_type_name,
+                  " inst_word) {\n");
+  if (format->declared_width() > 128) {
+    absl::StrAppend(
+        &h_output,
+        "  LOG(FATAL) << \" Support for formats > 128 bits not implemented - "
+        "yet.\";\n"
+        "  return 0;\n}\n");
     return h_output;
   }
   int width = format->declared_width();
   int low = high - width + 1;
-  uint64_t mask = (1ULL << width) << low;
+  if (declared_width_ <= 64) {
+    uint64_t mask = (1ULL << width) - 1;
+    absl::StrAppend(&h_output, "  int low = ", low, " - (index * ", width,
+                    ");\n"
+                    "  return (inst_word & (~0x",
+                    absl::Hex(mask), "ULL << low))", " | ((value << low) & (0x",
+                    absl::Hex(mask), "ULL << low));\n}\n");
+  } else if (declared_width_ <= 128) {
+    absl::StrAppend(
+        &h_output, "  int low = ", low, " - (index * ", width,
+        ");\n"
+        "  absl::uint128 mask = 1;\n"
+        "  mask = (mask << ",
+        width,
+        ") - 1;\n"
+        "  mask <<= low;\n"
+        "  return (inst_word & ~mask) | (value << low) & mask;\n}\n");
+  } else {
+    int byte_size = (declared_width_ + 7) / 8;
+    absl::StrAppend(&h_output, "  internal::InsertBits(inst_word, ", byte_size,
+                    ", ", high, " - (index * ", width, "), ", width,
+                    ", value);\n"
+                    "  return inst_word;\n}\n");
+  }
+  return h_output;
+}
+
+std::string Format::GenerateSingleFormatInserter(std::string_view format_alias,
+                                                 const Format *format,
+                                                 int high) const {
+  std::string h_output;
+  std::string target_type_name = GetUIntType(declared_width_);
+  std::string format_type_name;
+  if (declared_width_ <= 128) {
+    format_type_name = target_type_name;
+  } else {
+    format_type_name = GetUIntType(format->declared_width());
+  }
+
+  absl::StrAppend(&h_output, "static inline ", target_type_name, " Insert",
+                  ToPascalCase(format_alias), "(", format_type_name, " value, ",
+                  target_type_name, " inst_word) {\n");
+  if (format->declared_width() > 128) {
+    absl::StrAppend(
+        &h_output,
+        "  LOG(FATAL) << \" Support for formats > 128 bits not implemented - "
+        "yet.\";\n"
+        "  return 0;\n}\n");
+    return h_output;
+  }
+  int width = format->declared_width();
+  int low = high - width + 1;
   std::string shift;
   if (low != 0) {
     shift = absl::StrCat(" << ", low);
   }
-  absl::StrAppend(&h_output, "  return (inst_word & (~0x", absl::Hex(mask),
-                  "ULL))", " | ((value ", shift, ") & 0x", absl::Hex(mask),
-                  "ULL);\n}\n");
+  if (declared_width_ <= 64) {
+    uint64_t mask = ((1ULL << width) - 1) << low;
+    absl::StrAppend(&h_output, "  return (inst_word & (~0x", absl::Hex(mask),
+                    "ULL))", " | ((value ", shift, ") & 0x", absl::Hex(mask),
+                    "ULL);\n}\n");
+  } else if (declared_width_ <= 128) {
+    absl::StrAppend(&h_output,
+                    "  absl::uint128 mask = 1;\n"
+                    "  mask = (mask << ",
+                    width, ") - 1;\n");
+    if (low > 0) {
+      absl::StrAppend(&h_output, "  mask = mask << ", low, ";\n");
+    }
+    absl::StrAppend(&h_output, "  return (inst_word & ~mask) | (value ", shift,
+                    ") & mask;\n}\n");
+  } else {
+    int byte_size = (declared_width_ + 7) / 8;
+    absl::StrAppend(&h_output, "  internal::InsertBits(inst_word, ", byte_size,
+                    ", ", high, ", ", width,
+                    ", value);\n"
+                    "  return inst_word;\n}\n");
+  }
   return h_output;
 }
 
@@ -504,23 +691,23 @@
   std::string h_output;  // For each format generate am extractor.
   int width = format->declared_width();
   // An extraction can only be for 64 bits or less.
-  if (width > 64) {
+  if (width > 128) {
     encoding_info_->error_listener()->semanticError(
         nullptr,
         absl::StrCat("Cannot generate a format extractor for format '",
-                     format->name(), "': format is wider than 64 bits"));
+                     format->name(), "': format is wider than 128 bits"));
     return "";
   }
-  std::string return_type = absl::StrCat("u", GetIntType(width));
+  std::string return_type = GetUIntType(width);
   std::string signature = absl::StrCat("inline ", return_type, " Extract",
                                        ToPascalCase(format_alias), "(");
-  if (declared_width_ <= 64) {
-    // If the source format is <= 64 bits, then use an int type.
-    std::string arg_type = absl::StrCat("u", GetIntType(declared_width_));
+  if (declared_width_ <= 128) {
+    // If the source format is <= 128 bits, then use an int type.
+    std::string arg_type = GetUIntType(declared_width_);
     absl::StrAppend(&signature, arg_type, " value");
   } else {
     // Otherwise use a pointer to uint8_t type.
-    absl::StrAppend(&signature, "uint8_t *value");
+    absl::StrAppend(&signature, "const uint8_t *value");
   }
   // If the format has multiple instances add an index parameter.
   if (size > 1) {
@@ -530,18 +717,33 @@
   // Now start the body.
   absl::StrAppend(&h_output, signature, " {\n");
   std::string expr;
-  if (declared_width_ <= 64) {
-    // If the source format can be stored in a uint64_t or smaller.
-    uint64_t mask = (1ULL << width) - 1;
+  if (declared_width_ <= 128) {
+    // If the source format can be stored in a uint128 or smaller.
     int low = high - width + 1;
-    int shift_amount = GetIntTypeBitWidth(declared_width_) - low;
     std::string shift;
     if (size > 1) {
-      shift = absl::StrCat("(", shift_amount, " - (index - 1) * ", width, ")");
+      shift = absl::StrCat("(", low, " + (index - 1) * ", width, ")");
     } else {
-      shift = absl::StrCat(shift_amount);
+      shift = absl::StrCat(low);
     }
-    expr = absl::StrCat("(value >> ", shift, ") & 0x", absl::Hex(mask), ";\n");
+    if (declared_width_ <= 64) {
+      uint64_t mask = (1ULL << width) - 1;
+      expr =
+          absl::StrCat("(value >> ", shift, ") & 0x", absl::Hex(mask), ";\n");
+      absl::StrAppend(&h_output, "  return ", expr, ";\n}\n\n");
+    } else {
+      absl::StrAppend(&h_output,
+                      "  absl::uint128 mask = 1;\n"
+                      "  mask = (mask << ",
+                      width, ") - 1;\n");
+      expr = absl::StrCat("(value >> ", shift, ") & mask");
+      if (width <= 64) {
+        absl::StrAppend(&h_output, "  return absl::Uint128Low64(", expr,
+                        ");\n}\n\n");
+      } else {
+        absl::StrAppend(&h_output, "  return ", expr, ";\n}\n\n");
+      }
+    }
   } else {
     // If the source format is stored in uint8_t[].
     int byte_size = (declared_width_ + 7) / 8;
@@ -551,8 +753,23 @@
       absl::StrAppend(&expr, " - (index * ", width, ")");
     }
     absl::StrAppend(&expr, ", ", width, ")");
+    absl::StrAppend(&h_output, "  return ", expr, ";\n}\n\n");
   }
-  absl::StrAppend(&h_output, "  return ", expr, ";\n}\n\n");
+  // If the parent format size is not a power of two, also create an extractor
+  // that takes a uint8_t * parameter.
+  if ((declared_width_ <= 128) &&
+      (absl::popcount(static_cast<unsigned>(declared_width_)) > 1)) {
+    absl::StrAppend(&h_output, "inline ", return_type, " Extract",
+                    ToPascalCase(format_alias), "(const uint8_t *value) {\n");
+    int byte_size = (declared_width_ + 7) / 8;
+    expr = absl::StrCat("internal::ExtractBits<", return_type, ">(value, ",
+                        byte_size, ", ", high);
+    if (size > 1) {
+      absl::StrAppend(&expr, " - (index * ", width, ")");
+    }
+    absl::StrAppend(&expr, ", ", width, ")");
+    absl::StrAppend(&h_output, "  return ", expr, ";\n}\n\n");
+  }
   return h_output;
 }
 
@@ -560,9 +777,11 @@
 std::string Format::GenerateOverlayExtractor(Overlay *overlay) const {
   std::string h_output;
 
-  std::string return_type = absl::StrCat(overlay->is_signed() ? "" : "u",
-                                         GetIntType(overlay->declared_width()));
-  std::string arg_type = absl::StrCat("u", GetIntType(declared_width_));
+  std::string return_type = overlay->is_signed()
+                                ? GetIntType(overlay->declared_width())
+                                : GetUIntType(overlay->declared_width());
+
+  std::string arg_type = GetUIntType(declared_width_);
   std::string signature =
       absl::StrCat("inline ", return_type, " Extract",
                    ToPascalCase(overlay->name()), "(", arg_type, " value)");
@@ -587,9 +806,15 @@
                     "  result = result >> ",
                     shift, ";\n");
   }
-  absl::StrAppend(&h_output,
-                  "  return result;\n"
-                  "}\n\n");
+  if ((declared_width_ > 64) && (overlay->declared_width() <= 64)) {
+    absl::StrAppend(&h_output,
+                    "  return UInt128Low64(result();\n"
+                    "}\n\n");
+  } else {
+    absl::StrAppend(&h_output,
+                    "  return result;\n"
+                    "}\n\n");
+  }
   return h_output;
 }
 
diff --git a/mpact/sim/decoder/format.h b/mpact/sim/decoder/format.h
index 1ebfbbe..c986510 100644
--- a/mpact/sim/decoder/format.h
+++ b/mpact/sim/decoder/format.h
@@ -15,6 +15,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <map>
 #include <string>
 #include <tuple>
 #include <vector>
@@ -171,9 +172,16 @@
   std::string GenerateFormatInserter(std::string_view format_alias,
                                      const Format *format, int high,
                                      int size) const;
+  std::string GenerateReplicatedFormatInserter(std::string_view format_alias,
+                                               const Format *format, int high,
+                                               int size) const;
+  std::string GenerateSingleFormatInserter(std::string_view format_alias,
+                                           const Format *format,
+                                           int high) const;
   std::string GenerateOverlayInserter(Overlay *overlay) const;
   // Return string representation of the int type that contains bitwidth bits.
   std::string GetIntType(int bitwidth) const;
+  std::string GetUIntType(int bitwidth) const;
   int GetIntTypeBitWidth(int bitwidth) const;
 
   std::string name_;
diff --git a/mpact/sim/decoder/instruction_set.cc b/mpact/sim/decoder/instruction_set.cc
index 93c61d8..a6d91cf 100644
--- a/mpact/sim/decoder/instruction_set.cc
+++ b/mpact/sim/decoder/instruction_set.cc
@@ -343,6 +343,9 @@
                   Indent(absl::StrCat("  ", pascal_name(), "InstructionSet(")),
                   factory_class_name,
                   " *factory);\n"
+                  "  virtual ~",
+                  pascal_name(),
+                  "InstructionSet();\n"
                   "  Instruction *Decode(uint64_t address, ",
                   encoding_type,
                   " *encoding);\n"
@@ -387,6 +390,10 @@
                     ToPascalCase(slot_name), "Slot(arch_state_);\n");
   }
   absl::StrAppend(&output, "}\n");
+  // Destructor.
+  absl::StrAppend(&output, class_name, "::~", class_name, "() {\n");
+  absl::StrAppend(&output, "  // empty for now.\n");
+  absl::StrAppend(&output, "}\n");
   // Generate the top level decode function body.
   absl::StrAppend(&output, "Instruction *", class_name,
                   "::Decode(uint64_t address, ", encoding_type,
@@ -887,7 +894,9 @@
                     position,
                     ") {\n"
                     "    return absl::InvalidArgumentError(\n"
-                    "        absl::StrCat(\"Invalid number of operands (\", "
+                    "        absl::StrCat(\"",
+                    opcode->pascal_name(),
+                    ": Invalid number of operands (\", "
                     "num_args, \") - expected ",
                     position,
                     "\"));\n"
diff --git a/mpact/sim/decoder/instruction_set_visitor.cc b/mpact/sim/decoder/instruction_set_visitor.cc
index dae84eb..406c6dd 100644
--- a/mpact/sim/decoder/instruction_set_visitor.cc
+++ b/mpact/sim/decoder/instruction_set_visitor.cc
@@ -191,7 +191,7 @@
                                           instruction_set->namespaces());
   dec_dot_h_file.close();
   // Decoder .cc file.
-  dec_dot_cc_file << GenerateCcFileProlog(dec_dot_h_name,
+  dec_dot_cc_file << GenerateCcFileProlog(dec_dot_h_name, /*use_includes=*/true,
                                           instruction_set->namespaces());
   dec_dot_cc_file << instruction_set->GenerateClassDefinitions(
       dec_dot_h_name, encoding_type_name);
@@ -201,7 +201,7 @@
   // Enum files.
   enum_h_file << GenerateSimpleHdrProlog(ToHeaderGuard(enum_h_name),
                                          instruction_set->namespaces());
-  enum_cc_file << GenerateCcFileProlog(enum_h_name,
+  enum_cc_file << GenerateCcFileProlog(enum_h_name, /*use_includes=*/false,
                                        instruction_set->namespaces());
   auto [h_output, cc_output] = instruction_set->GenerateEnums(enum_h_name);
   enum_h_file << h_output;
@@ -575,10 +575,70 @@
   Bundle *bundle =
       new Bundle(ctx->bundle_name->getText(), instruction_set, ctx);
   instruction_set->AddBundle(bundle);
-  context_file_map_[ctx->bundle_list()] = context_file_map_.at(ctx);
-  VisitBundleList(ctx->bundle_list(), bundle);
-  context_file_map_[ctx->slot_list()] = context_file_map_.at(ctx);
-  VisitSlotList(ctx->slot_list(), bundle);
+  int num_slot_lists = 0;
+  int num_bundle_lists = 0;
+  int num_include_file_lists = 0;
+  int num_semfunc_specs = 0;
+  for (auto *part : ctx->bundle_parts()) {
+    if (part->slot_list() != nullptr) {
+      if (num_slot_lists > 0) {
+        error_listener()->semanticError(file_names_[context_file_map_.at(ctx)],
+                                        part->start,
+                                        "Multiple slot lists in bundle");
+        return;
+      }
+      context_file_map_[part->slot_list()] = context_file_map_.at(ctx);
+      VisitSlotList(part->slot_list(), bundle);
+      num_slot_lists++;
+      continue;
+    }
+    if (part->bundle_list() != nullptr) {
+      if (num_bundle_lists > 0) {
+        error_listener()->semanticError(file_names_[context_file_map_.at(ctx)],
+                                        part->start,
+                                        "Multiple bundle lists in bundle");
+        return;
+      }
+      context_file_map_[part->bundle_list()] = context_file_map_.at(ctx);
+      VisitBundleList(part->bundle_list(), bundle);
+      num_bundle_lists++;
+      continue;
+    }
+    if (part->include_file_list() != nullptr) {
+      if (num_include_file_lists > 0) {
+        error_listener()->semanticError(
+            file_names_[context_file_map_.at(ctx)], part->start,
+            "Multiple include file lists in bundle");
+        return;
+      }
+      for (auto *include_file : part->include_file_list()->include_file()) {
+        // Insert the string - the call will always succeed, but the insertion
+        // does not happen if it already exists.
+        include_files_.insert(include_file->STRING_LITERAL()->getText());
+      }
+      num_include_file_lists++;
+      continue;
+    }
+    if (part->semfunc_spec() != nullptr) {
+      if (num_semfunc_specs > 0) {
+        error_listener()->semanticError(file_names_[context_file_map_.at(ctx)],
+                                        part->start,
+                                        "Multiple semfunc specs in bundle");
+        return;
+      }
+      std::string string_literal =
+          part->semfunc_spec()->STRING_LITERAL(0)->getText();
+      // Strip double quotes.
+      std::string code_string =
+          string_literal.substr(1, string_literal.length() - 2);
+      bundle->set_semfunc_code_string(code_string);
+      num_semfunc_specs++;
+      continue;
+    }
+    error_listener()->semanticError(file_names_[context_file_map_.at(ctx)],
+                                    part->start, "Unhandled bundle part type");
+    return;
+  }
 }
 
 void InstructionSetVisitor::VisitSlotDeclaration(
@@ -2235,7 +2295,7 @@
       "  virtual ResourceOperandInterface * "
       "GetComplexResourceOperand",
       "(SlotEnum slot, int entry, OpcodeEnum opcode, ComplexResourceEnum "
-      "resource_op, int begin, int end) { return {}; }\n");
+      "resource_op, int begin, int end) { return nullptr; }\n");
   absl::StrAppend(
       &output,
       "  virtual std::vector<ResourceOperandInterface *> "
@@ -2319,6 +2379,7 @@
                   "#include <string>\n"
                   "#include <vector>\n"
                   "\n"
+                  "#include \"absl/container/flat_hash_map.h\"\n"
                   "#include \"absl/status/status.h\"\n"
                   "#include \"absl/status/statusor.h\"\n"
                   "#include \"absl/strings/string_view.h\"\n"
@@ -2370,7 +2431,7 @@
 }
 
 std::string InstructionSetVisitor::GenerateCcFileProlog(
-    absl::string_view hdr_file_name,
+    absl::string_view hdr_file_name, bool use_includes,
     const std::vector<std::string> &namespaces) {
   std::string output;
   // Include files.
@@ -2378,9 +2439,10 @@
   absl::StrAppend(&output,
                   "\n#include <array>\n\n"
                   "#include \"absl/strings/str_format.h\"\n\n");
-
-  for (auto &include_file : include_files_) {
-    absl::StrAppend(&output, "#include ", include_file, "\n");
+  if (use_includes) {
+    for (auto &include_file : include_files_) {
+      absl::StrAppend(&output, "#include ", include_file, "\n");
+    }
   }
   absl::StrAppend(&output, "\n");
   // Namespaces.
diff --git a/mpact/sim/decoder/instruction_set_visitor.h b/mpact/sim/decoder/instruction_set_visitor.h
index e0f19b7..a07edc5 100644
--- a/mpact/sim/decoder/instruction_set_visitor.h
+++ b/mpact/sim/decoder/instruction_set_visitor.h
@@ -187,6 +187,7 @@
   std::string GenerateHdrFileEpilog(absl::string_view guard_name,
                                     const std::vector<std::string> &namespaces);
   std::string GenerateCcFileProlog(absl::string_view hdr_file_name,
+                                   bool use_includes,
                                    const std::vector<std::string> &namespaces);
   std::string GenerateNamespaceEpilog(
       const std::vector<std::string> &namespaces);
diff --git a/mpact/sim/decoder/mpact_sim_isa.bzl b/mpact/sim/decoder/mpact_sim_isa.bzl
index 5a86d18..db01738 100644
--- a/mpact/sim/decoder/mpact_sim_isa.bzl
+++ b/mpact/sim/decoder/mpact_sim_isa.bzl
@@ -198,6 +198,8 @@
         lib_deps.append("@com_google_absl//absl/container:flat_hash_map")
     if "@com_google_absl//absl/functional:any_invocable" not in deps:
         lib_deps.append("@com_google_absl//absl/functional:any_invocable")
+    if "@com_google_absl//absl/log" not in deps:
+        lib_deps.append("@com_google_absl//absl/log")
     if "@com_google_absl//absl/strings:str_format" not in deps:
         lib_deps.append("@com_google_absl//absl/strings:str_format")
     if "@com_google_mpact-sim//mpact/sim/generic:arch_state" not in deps:
diff --git a/mpact/sim/decoder/slot.cc b/mpact/sim/decoder/slot.cc
index 135539f..06b48a8 100644
--- a/mpact/sim/decoder/slot.cc
+++ b/mpact/sim/decoder/slot.cc
@@ -488,7 +488,8 @@
       encoder,
       " *encoder_;\n"
       "  std::vector<RE2 *> regex_vec_;\n"
-      "  RE2::Set regex_set_;\n");
+      "  RE2::Set regex_set_;\n"
+      "  absl::flat_hash_map<int, int> index_to_opcode_map_;\n");
   absl::StrAppend(&cc_output, class_name, "::", class_name, "(",
                   instruction_set_->pascal_name(),
                   "EncoderInterfaceBase *encoder) :\n"
@@ -514,12 +515,16 @@
   for (auto const &[name, inst_ptr] : instruction_map_) {
     auto [regex, opnd_locators] = GenerateRegEx(inst_ptr, formats);
     max_args = std::max(max_args, opnd_locators.size());
+    std::string opcode_name =
+        absl::StrCat("OpcodeEnum::k", ToPascalCase(inst_ptr->opcode()->name()));
     absl::StrAppend(&cc_output, "  regex_vec_.push_back(new RE2(", regex,
                     "));\n"
                     "  index = regex_set_.Add(",
                     regex,
                     ", &error);\n"
-                    "  if (index == -1) return absl::InternalError(error);\n");
+                    "  if (index == -1) return absl::InternalError(error);\n"
+                    "  index_to_opcode_map_.insert({index, static_cast<int>(",
+                    opcode_name, ")", "});\n");
   }
   absl::StrAppend(&h_output, "  std::string args[", max_args,
                   "];\n"
@@ -576,12 +581,13 @@
   for (auto index : matches) {
     std::vector<std::string> values;
     if (!Extract(text, index, values)) continue;
+    int opcode_index = index_to_opcode_map_.at(index);
 )",
-      "    auto result = encode_fcns[index](encoder_, SlotEnum::k",
+      "    auto result = encode_fcns[opcode_index](encoder_, SlotEnum::k",
       pascal_name(),
       ", entry, \n"
       "                                     "
-      "static_cast<OpcodeEnum>(index), address, values, resolver, "
+      "static_cast<OpcodeEnum>(opcode_index), address, values, resolver, "
       "relocations);\n",
       R"(
     if (!result.status().ok()) {
@@ -1235,14 +1241,15 @@
                       absl::StrCat(pascal_name(), "SlotSetOperandsNull"),
                       encoding_type, default_instruction_->opcode()));
   absl::StrAppend(
-      &output, "    {OperandSetter{", pascal_name(),
+      &output, "    {static_cast<int>(OpcodeEnum::kNone), {OperandSetter{",
+      pascal_name(),
       "SlotSetOperandsNull},\n"
       "    ",
       GenerateDisassemblySetter(default_instruction_), ",\n", "    ",
       GenerateResourceSetter(default_instruction_, encoding_type), ",\n",
       "    ", GenerateAttributeSetter(default_instruction_), ",\n",
       "    SemFuncSetter{", default_instruction_->semfunc_code_string(), "}, ",
-      default_instruction_->opcode()->instruction_size(), "},\n");
+      default_instruction_->opcode()->instruction_size(), "}},\n");
   for (auto const &[unused, inst_ptr] : instruction_map_) {
     auto *instruction = inst_ptr;
     std::string opcode_name = instruction->opcode()->pascal_name();
@@ -1281,12 +1288,13 @@
       }
       sep = ", ";
     }
-    absl::StrAppend(&output, "    {OperandSetter{", operands_str, "},\n",
-                    "    ", GenerateDisassemblySetter(instruction), ",\n",
-                    "    ", GenerateResourceSetter(instruction, encoding_type),
-                    ",\n", "    ", GenerateAttributeSetter(instruction), ",\n",
+    absl::StrAppend(&output, "    {static_cast<int>(OpcodeEnum::k", opcode_name,
+                    "), {OperandSetter{", operands_str, "},\n", "    ",
+                    GenerateDisassemblySetter(instruction), ",\n", "    ",
+                    GenerateResourceSetter(instruction, encoding_type), ",\n",
+                    "    ", GenerateAttributeSetter(instruction), ",\n",
                     "    SemFuncSetter{", code_str, "}, ",
-                    instruction->opcode()->instruction_size(), "},\n");
+                    instruction->opcode()->instruction_size(), "}},\n");
   }
   return output;
 }
@@ -1305,18 +1313,19 @@
                   class_name, "(ArchState *arch_state);\n");
   // Emit Decode function generated that decodes the slot and creates and
   // initializes an instruction object, as well as private data members.
-  absl::StrAppend(&output, "  Instruction *Decode(uint64_t address, ",
-                  encoding_type, "* isa_encoding, SlotEnum, int entry);\n",
-                  "\n"
-                  " private:\n"
-                  "  ArchState *arch_state_;\n"
-                  "  std::array<InstructionInfo, ",
-                  instruction_map_.size() + 1, "> instruction_info_", ";\n",
-                  "  static constexpr SlotEnum slot_ = SlotEnum::k",
-                  pascal_name(),
-                  ";\n"
-                  "};\n"
-                  "\n");
+  absl::StrAppend(
+      &output, "  Instruction *Decode(uint64_t address, ", encoding_type,
+      "* isa_encoding, SlotEnum, int entry);\n",
+      "\n"
+      " private:\n"
+      "  ArchState *arch_state_;\n"
+      "  absl::flat_hash_map<int, InstructionInfo> instruction_info_;\n",
+      //"  std::array<InstructionInfo, ",
+      // instruction_map_.size() + 1, "> instruction_info_", ";\n",
+      "  static constexpr SlotEnum slot_ = SlotEnum::k", pascal_name(),
+      ";\n"
+      "};\n"
+      "\n");
   return output;
 }
 
diff --git a/mpact/sim/generic/decoder_interface.h b/mpact/sim/generic/decoder_interface.h
index f555f26..86173eb 100644
--- a/mpact/sim/generic/decoder_interface.h
+++ b/mpact/sim/generic/decoder_interface.h
@@ -27,7 +27,7 @@
 class DecoderInterface {
  public:
   // Return a decoded instruction for the given address. If there are errors
-  // in the instruciton decoding, the decoder should still produce an
+  // in the instruction decoding, the decoder should still produce an
   // instruction that can be executed, but its semantic action function should
   // set an error condition in the simulation when executed.
   virtual Instruction *DecodeInstruction(uint64_t address) = 0;
diff --git a/mpact/sim/generic/fifo.h b/mpact/sim/generic/fifo.h
index f5e841b..35cd9a8 100644
--- a/mpact/sim/generic/fifo.h
+++ b/mpact/sim/generic/fifo.h
@@ -335,7 +335,7 @@
   std::string op_name_;
 };
 
-// This is a parial specialization of the Source operand class. This is used
+// This is a partial specialization of the Source operand class. This is used
 // when the element type stored in the data buffer is not an integral type. This
 // is primarily for when the fifo element type really doesn't model a register
 // value per se, but a more complex structure such as a dma descriptor. In this
diff --git a/mpact/sim/generic/instruction.h b/mpact/sim/generic/instruction.h
index 60ab5d8..af4e7eb 100644
--- a/mpact/sim/generic/instruction.h
+++ b/mpact/sim/generic/instruction.h
@@ -176,7 +176,7 @@
     disasm_string_ = std::move(disasm);
   }
 
-  std::string AsString() const;
+  virtual std::string AsString() const;
 
   // Setter and getter for the integer attributes.
   absl::Span<const int> Attributes() const { return attributes_; }
diff --git a/mpact/sim/generic/instruction_helpers.h b/mpact/sim/generic/instruction_helpers.h
index 47ba835..b3d3b31 100644
--- a/mpact/sim/generic/instruction_helpers.h
+++ b/mpact/sim/generic/instruction_helpers.h
@@ -34,7 +34,7 @@
 // destination operand. This version supports different types for the result and
 // each of the two source operands.
 template <typename Result, typename Argument1, typename Argument2>
-inline void BinaryOp(Instruction *instruction,
+inline void BinaryOp(const Instruction *instruction,
                      std::function<Result(Argument1, Argument2)> operation) {
   Argument1 lhs = generic::GetInstructionSource<Argument1>(instruction, 0);
   Argument2 rhs = generic::GetInstructionSource<Argument2>(instruction, 1);
@@ -49,7 +49,7 @@
 // destination operand. This version supports different types for the result
 // and the operands, but the two source operands must have the same type.
 template <typename Result, typename Argument>
-inline void BinaryOp(Instruction *instruction,
+inline void BinaryOp(const Instruction *instruction,
                      std::function<Result(Argument, Argument)> operation) {
   Argument lhs = generic::GetInstructionSource<Argument>(instruction, 0);
   Argument rhs = generic::GetInstructionSource<Argument>(instruction, 1);
@@ -64,7 +64,7 @@
 // destination operand. This version requires both result and source operands
 // to have the same type.
 template <typename Result>
-inline void BinaryOp(Instruction *instruction,
+inline void BinaryOp(const Instruction *instruction,
                      std::function<Result(Result, Result)> operation) {
   Result lhs = generic::GetInstructionSource<Result>(instruction, 0);
   Result rhs = generic::GetInstructionSource<Result>(instruction, 1);
@@ -79,7 +79,7 @@
 // destination operand. This version supports the result and argument having
 // different types.
 template <typename Result, typename Argument>
-inline void UnaryOp(Instruction *instruction,
+inline void UnaryOp(const Instruction *instruction,
                     std::function<Result(Argument)> operation) {
   Argument lhs = generic::GetInstructionSource<Argument>(instruction, 0);
   Result dest_value = operation(lhs);
@@ -93,7 +93,7 @@
 // destination operand. This version requires that the result and argument have
 // the same type.
 template <typename Result>
-inline void UnaryOp(Instruction *instruction,
+inline void UnaryOp(const Instruction *instruction,
                     std::function<Result(Result)> operation) {
   Result lhs = generic::GetInstructionSource<Result>(instruction, 0);
   Result dest_value = operation(lhs);
@@ -107,7 +107,7 @@
 template <typename Result, typename Argument1, typename Argument2,
           typename Argument3>
 inline void TernaryVectorOp(
-    Instruction *instruction,
+    const Instruction *instruction,
     std::function<Result(Argument1, Argument2, Argument3)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -131,7 +131,7 @@
 // the arguments have to all have the same type.
 template <typename Result, typename Argument>
 inline void TernaryVectorOp(
-    Instruction *instruction,
+    const Instruction *instruction,
     std::function<Result(Argument, Argument, Argument)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -151,7 +151,7 @@
 // requires the result and arguments to have the same type.
 template <typename Result>
 inline void TernaryVectorOp(
-    Instruction *instruction,
+    const Instruction *instruction,
     std::function<Result(Result, Result, Result)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -171,7 +171,7 @@
 // allows for different types for the result and each argument.
 template <typename Result, typename Argument1, typename Argument2>
 inline void BinaryVectorOp(
-    Instruction *instruction,
+    const Instruction *instruction,
     std::function<Result(Argument1, Argument2)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -191,7 +191,7 @@
 // the arguments have to have the same type.
 template <typename Result, typename Argument>
 inline void BinaryVectorOp(
-    Instruction *instruction,
+    const Instruction *instruction,
     std::function<Result(Argument, Argument)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -209,7 +209,7 @@
 // two operand vector instruction semantic functions. This version
 // requires the result and arguments to have the same type.
 template <typename Result>
-inline void BinaryVectorOp(Instruction *instruction,
+inline void BinaryVectorOp(const Instruction *instruction,
                            std::function<Result(Result, Result)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -227,7 +227,7 @@
 // single operand vector instruction semantic functions. This version
 // allows the result and argument to have different types.
 template <typename Result, typename Argument>
-inline void UnaryVectorOp(Instruction *instruction,
+inline void UnaryVectorOp(const Instruction *instruction,
                           std::function<Result(Argument)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
@@ -244,7 +244,7 @@
 // single operand vector instruction semantic functions. This version
 // requires the result and argument to have the same type.
 template <typename Result>
-inline void UnaryVectorOp(Instruction *instruction,
+inline void UnaryVectorOp(const Instruction *instruction,
                           std::function<Result(Result)> operation) {
   auto *dst = instruction->Destination(0);
   auto *db = dst->AllocateDataBuffer();
diff --git a/mpact/sim/generic/operand_interface.h b/mpact/sim/generic/operand_interface.h
index 60714f8..e39adc5 100644
--- a/mpact/sim/generic/operand_interface.h
+++ b/mpact/sim/generic/operand_interface.h
@@ -32,7 +32,7 @@
 namespace sim {
 namespace generic {
 
-// The predicte operand interface is intended primarily as the interface to
+// The predicate operand interface is intended primarily as the interface to
 // read the value of instruction predicates. It is separated from source
 // predicates to avoid mixing it in with the source operands needed for modeling
 // the instruction semantics.
diff --git a/mpact/sim/util/asm/BUILD b/mpact/sim/util/asm/BUILD
index 1620657..1ba2ac2 100644
--- a/mpact/sim/util/asm/BUILD
+++ b/mpact/sim/util/asm/BUILD
@@ -21,17 +21,19 @@
 
 cc_library(
     name = "asm",
+    srcs = ["resolver.cc"],
     hdrs = [
         "opcode_assembler_interface.h",
+        "resolver.h",
         "resolver_interface.h",
     ],
     deps = [
         "@com_github_serge1_elfio//:elfio",
+        "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/functional:any_invocable",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
-        "@com_google_absl//absl/strings:str_format",
     ],
 )
 
@@ -44,7 +46,6 @@
         "@com_github_serge1_elfio//:elfio",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/container:flat_hash_set",
-        "@com_google_absl//absl/functional:any_invocable",
         "@com_google_absl//absl/functional:bind_front",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/status",
diff --git a/mpact/sim/util/asm/opcode_assembler_interface.h b/mpact/sim/util/asm/opcode_assembler_interface.h
index 80782d1..1663787 100644
--- a/mpact/sim/util/asm/opcode_assembler_interface.h
+++ b/mpact/sim/util/asm/opcode_assembler_interface.h
@@ -15,12 +15,14 @@
 #ifndef MPACT_SIM_UTIL_ASM_OPCODE_ASSEMBLER_INTERFACE_H_
 #define MPACT_SIM_UTIL_ASM_OPCODE_ASSEMBLER_INTERFACE_H_
 
+#include <cstddef>
 #include <cstdint>
 #include <string>
 #include <vector>
 
 #include "absl/functional/any_invocable.h"
 #include "absl/status/status.h"
+#include "absl/status/statusor.h"
 #include "absl/strings/string_view.h"
 #include "elfio/elf_types.hpp"
 #include "mpact/sim/util/asm/resolver_interface.h"
@@ -59,12 +61,13 @@
   // Takes the current address, the text for the assembly instruction (including
   // any label definitions), and a symbol resolver interface.Return ok status if
   // the text is successfully encoded into the bytes vector. Symbols for any
-  // labels are added using the callback function interface.
-  virtual absl::Status Encode(uint64_t address, absl::string_view text,
-                              AddSymbolCallback add_symbol_callback,
-                              ResolverInterface *resolver,
-                              std::vector<uint8_t> &bytes,
-                              std::vector<RelocationInfo> &relocations) = 0;
+  // labels are added using the callback function interface. The method returns
+  // the increment to the address after the instruction is encoded.
+  virtual absl::StatusOr<size_t> Encode(
+      uint64_t address, absl::string_view text,
+      AddSymbolCallback add_symbol_callback, ResolverInterface *resolver,
+      std::vector<uint8_t> &bytes,
+      std::vector<RelocationInfo> &relocations) = 0;
 };
 
 }  // namespace assembler
diff --git a/mpact/sim/util/asm/resolver.cc b/mpact/sim/util/asm/resolver.cc
new file mode 100644
index 0000000..f8ec781
--- /dev/null
+++ b/mpact/sim/util/asm/resolver.cc
@@ -0,0 +1,52 @@
+#include "mpact/sim/util/asm/resolver.h"
+
+#include <cstdint>
+#include <string>
+
+#include "absl/container/flat_hash_map.h"
+#include "absl/status/status.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "elfio/elf_types.hpp"
+#include "elfio/elfio_section.hpp"
+
+namespace mpact::sim::util::assembler {
+
+absl::StatusOr<uint64_t> ZeroResolver::Resolve(absl::string_view text) {
+  // Any symbol name should be added to the symbol table as an undefined
+  // symbol if it is not already there. When the symbol is defined, the
+  // symbol table will be updated. In the case of generating an executable
+  // ELF file, any unresolved symbols will result in an error. When generating
+  // an object file, any unresolved symbols will remain in the symbol table
+  // and must be handled by the linker.
+  add_symbol_fcn_(text);
+  // Return 0 for any symbol name.
+  return 0;
+}
+
+SymbolResolver::SymbolResolver(
+    int elf_file_class, ELFIO::section *symtab,
+    const absl::flat_hash_map<std::string, ELFIO::Elf_Word> &symbol_indices)
+    : elf_file_class_(elf_file_class),
+      symtab_(symtab),
+      symbol_indices_(symbol_indices) {}
+
+absl::StatusOr<uint64_t> SymbolResolver::Resolve(absl::string_view text) {
+  auto iter = symbol_indices_.find(text);
+  if (iter == symbol_indices_.end()) {
+    return absl::InvalidArgumentError(
+        absl::StrCat("SymbolResolver: Symbol '", text, "' not found"));
+  }
+  auto index = iter->second;
+  if (elf_file_class_ == ELFCLASS64) {
+    auto *sym = reinterpret_cast<const ELFIO::Elf64_Sym *>(symtab_->get_data());
+    return sym[index].st_value;
+  } else if (elf_file_class_ == ELFCLASS32) {
+    auto *sym = reinterpret_cast<const ELFIO::Elf32_Sym *>(symtab_->get_data());
+    return sym[index].st_value;
+  }
+  return absl::InternalError("Unsupported ELF file class");
+}
+
+}  // namespace mpact::sim::util::assembler
diff --git a/mpact/sim/util/asm/resolver.h b/mpact/sim/util/asm/resolver.h
new file mode 100644
index 0000000..3adb3ff
--- /dev/null
+++ b/mpact/sim/util/asm/resolver.h
@@ -0,0 +1,67 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MPACT_SIM_UTIL_ASM_RESOLVER_H_
+#define MPACT_SIM_UTIL_ASM_RESOLVER_H_
+
+#include <cstdint>
+#include <string>
+
+#include "absl/container/flat_hash_map.h"
+#include "absl/functional/any_invocable.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/string_view.h"
+#include "elfio/elf_types.hpp"
+#include "elfio/elfio.hpp"  // IWYU pragma: keep
+#include "elfio/elfio_section.hpp"
+#include "mpact/sim/util/asm/resolver_interface.h"
+
+namespace mpact::sim::util::assembler {
+
+// A symbol resolver that always returns 0 for any symbol name. This is used
+// for the first pass of parsing the assembly code, when we are just creating
+// the symbols and computing the sizes of the sections.
+class ZeroResolver : public ResolverInterface {
+ public:
+  // Constructor takes a callback function that will be called for each symbol
+  // name encountered so that it can be added to the symbol table.
+  template <typename T>
+  ZeroResolver(T add_symbol_fcn) : add_symbol_fcn_(add_symbol_fcn) {}
+  absl::StatusOr<uint64_t> Resolve(absl::string_view text) override;
+
+ private:
+  absl::AnyInvocable<void(absl::string_view)> add_symbol_fcn_;
+};
+
+// A symbol resolver that uses the symbol table and the symbol indices to
+// resolve symbol names to values.
+class SymbolResolver : public ResolverInterface {
+ public:
+  SymbolResolver(
+      int elf_file_class, ELFIO::section *symtab,
+      const absl::flat_hash_map<std::string, ELFIO::Elf_Word> &symbol_indices);
+  absl::StatusOr<uint64_t> Resolve(absl::string_view text) override;
+
+ private:
+  // Elf file class.
+  int elf_file_class_ = 0;
+  // The symbol table ELF section.
+  ELFIO::section *symtab_;
+  // Map from symbol name to symbol index in the symbol table.
+  const absl::flat_hash_map<std::string, ELFIO::Elf_Word> &symbol_indices_;
+};
+
+}  // namespace mpact::sim::util::assembler
+
+#endif  // MPACT_SIM_UTIL_ASM_RESOLVER_H_
diff --git a/mpact/sim/util/asm/simple_assembler.cc b/mpact/sim/util/asm/simple_assembler.cc
index 1622c79..d0b4d54 100644
--- a/mpact/sim/util/asm/simple_assembler.cc
+++ b/mpact/sim/util/asm/simple_assembler.cc
@@ -17,15 +17,14 @@
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
+#include <functional>
 #include <istream>
 #include <ostream>
 #include <string>
-#include <utility>
 #include <vector>
 
 #include "absl/container/flat_hash_map.h"
 #include "absl/container/flat_hash_set.h"
-#include "absl/functional/any_invocable.h"
 #include "absl/functional/bind_front.h"
 #include "absl/log/log.h"
 #include "absl/status/status.h"
@@ -34,11 +33,13 @@
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
 #include "elfio/elf_types.hpp"
+#include "elfio/elfio.hpp"  // IWYU pragma: keep
 #include "elfio/elfio_section.hpp"
 #include "elfio/elfio_segment.hpp"
 #include "elfio/elfio_strings.hpp"
 #include "elfio/elfio_symbols.hpp"
 #include "mpact/sim/util/asm/opcode_assembler_interface.h"
+#include "mpact/sim/util/asm/resolver.h"
 #include "mpact/sim/util/asm/resolver_interface.h"
 #include "re2/re2.h"
 
@@ -47,69 +48,6 @@
 namespace util {
 namespace assembler {
 
-// A symbol resolver that always returns 0 for any symbol name. This is used
-// for the first pass of parsing the assembly code, when we are just creating
-// the symbols and computing the sizes of the sections.
-class ZeroResolver : public ResolverInterface {
- public:
-  // Constructor takes a callback function that will be called for each symbol
-  // name encountered so that it can be added to the symbol table.
-  template <typename T>
-  ZeroResolver(T add_symbol_fcn) : add_symbol_fcn_(add_symbol_fcn) {}
-  absl::StatusOr<uint64_t> Resolve(absl::string_view text) override {
-    // Any symbol name should be added to the symbol table as an undefined
-    // symbol if it is not already there. When the symbol is defined, the
-    // symbol table will be updated. In the case of generating an executable
-    // ELF file, any unresolved symbols will result in an error. When generating
-    // an object file, any unresolved symbols will remain in the symbol table
-    // and must be handled by the linker.
-    add_symbol_fcn_(text);
-    // Return 0 for any symbol name.
-    return 0;
-  }
-
- private:
-  absl::AnyInvocable<void(absl::string_view)> add_symbol_fcn_;
-};
-
-// A symbol resolver that uses the symbol table and the symbol indices to
-// resolve symbol names to values.
-class SymbolResolver : public ResolverInterface {
- public:
-  SymbolResolver(
-      int elf_file_class, ELFIO::section *symtab,
-      const absl::flat_hash_map<std::string, ELFIO::Elf_Word> &symbol_indices)
-      : elf_file_class_(elf_file_class),
-        symtab_(symtab),
-        symbol_indices_(symbol_indices) {}
-  absl::StatusOr<uint64_t> Resolve(absl::string_view text) override {
-    auto iter = symbol_indices_.find(text);
-    if (iter == symbol_indices_.end()) {
-      return absl::InvalidArgumentError(
-          absl::StrCat("SymbolResolver: Symbol '", text, "' not found"));
-    }
-    auto index = iter->second;
-    if (elf_file_class_ == ELFCLASS64) {
-      auto *sym =
-          reinterpret_cast<const ELFIO::Elf64_Sym *>(symtab_->get_data());
-      return sym[index].st_value;
-    } else if (elf_file_class_ == ELFCLASS32) {
-      auto *sym =
-          reinterpret_cast<const ELFIO::Elf32_Sym *>(symtab_->get_data());
-      return sym[index].st_value;
-    }
-    return absl::InternalError("Unsupported ELF file class");
-  }
-
- private:
-  // Elf file class.
-  int elf_file_class_ = 0;
-  // The symbol table ELF section.
-  ELFIO::section *symtab_;
-  // Map from symbol name to symbol index in the symbol table.
-  const absl::flat_hash_map<std::string, ELFIO::Elf_Word> &symbol_indices_;
-};
-
 // Helper functions for parsing the assembly code.
 namespace {
 
@@ -354,15 +292,23 @@
 }
 
 SimpleAssembler::~SimpleAssembler() {
-  delete symbol_resolver_;
   delete symbol_accessor_;
+  symbol_accessor_ = nullptr;
   delete string_accessor_;
+  string_accessor_ = nullptr;
 }
 
-absl::Status SimpleAssembler::Parse(std::istream &is) {
+absl::Status SimpleAssembler::Parse(std::istream &is,
+                                    ResolverInterface *zero_resolver) {
   // A trivial symbol resolver that always returns 0.
-  ZeroResolver zero_resolver(
-      absl::bind_front(&SimpleAssembler::SimpleAddSymbol, this));
+  bool own_zero_resolver = false;
+  std::function<void()> cleanup = []() {};
+  if (zero_resolver == nullptr) {
+    zero_resolver = new ZeroResolver(
+        absl::bind_front(&SimpleAssembler::SimpleAddSymbol, this));
+    own_zero_resolver = true;
+    cleanup = [zero_resolver]() { delete zero_resolver; };
+  }
   // First pass of parsing the input stream. This will add symbols to the symbol
   // table and compute the sizes of all instructions and the sections. The
   // section_address_map_ will keep track of the current location within each
@@ -409,10 +355,10 @@
         // Pass the full line into the parse functions, they are responsible
         // for handling the labels in pass one.
         if (statement[0] == '.') {
-          status = ParseAsmDirective(line, address, &zero_resolver, byte_vector,
+          status = ParseAsmDirective(line, address, zero_resolver, byte_vector,
                                      relo_vector);
         } else {
-          status = ParseAsmStatement(line, address, &zero_resolver, byte_vector,
+          status = ParseAsmStatement(line, address, zero_resolver, byte_vector,
                                      relo_vector);
         }
         if (!status.ok()) return status;
@@ -428,15 +374,20 @@
       continue;
     }
     // Parse failure.
+    cleanup();
     return absl::AbortedError(absl::StrCat("Parse failure: '", line, "'"));
   }
 
-  if (!is.eof()) return absl::InternalError("Input stream entered bad state");
+  if (!is.eof()) {
+    cleanup();
+    return absl::InternalError("Input stream entered bad state");
+  }
 
   // Add undefined symbols to the symbol table.
   for (auto const &symbol : undefined_symbols_) {
     auto status = AddSymbol(symbol, 0, 0, STT_NOTYPE, 0, 0, nullptr);
     if (!status.ok()) {
+      cleanup();
       return absl::InternalError(absl::StrCat(
           "Failed to add undefined symbol '", symbol, "': ", status.message()));
     }
@@ -446,12 +397,15 @@
   if (bss_section_ != nullptr) {
     bss_section_->set_size(section_address_map_[bss_section_]);
   }
+  cleanup();
   return absl::OkStatus();
 }
 
-absl::Status SimpleAssembler::CreateExecutable(uint64_t base_address,
-                                               uint64_t entry_point) {
-  return CreateExecutable(base_address, absl::StrCat(entry_point));
+absl::Status SimpleAssembler::CreateExecutable(
+    uint64_t base_address, uint64_t entry_point,
+    ResolverInterface *symbol_resolver) {
+  return CreateExecutable(base_address, absl::StrCat(entry_point),
+                          symbol_resolver);
 }
 
 // Helper function to update the symbol table entries for an executable file.
@@ -501,8 +455,10 @@
   delete[] symbols;
 }
 
-absl::Status SimpleAssembler::CreateExecutable(uint64_t base_address,
-                                               const std::string &entry_point) {
+absl::Status SimpleAssembler::CreateExecutable(
+    uint64_t base_address, const std::string &entry_point,
+    ResolverInterface *symbol_resolver) {
+  LOG(INFO) << "CreateExecutable";
   if (!undefined_symbols_.empty()) {
     std::string message;
     absl::StrAppend(
@@ -513,6 +469,7 @@
     }
     return absl::InvalidArgumentError(message);
   }
+  LOG(INFO) << "set type to ET_EXEC";
   writer_.set_type(ET_EXEC);
   // Section sizes are now known. So let's compute the layout and update all
   // the symbol values/addresses before the next pass.
@@ -525,7 +482,10 @@
   uint64_t text_segment_start = 0;
   if (text_section_ != nullptr) {
     text_segment_start = base_address & ~4095ULL;
-    ELFIO::segment *text_segment = writer_.segments.add();
+    text_segment = writer_.segments.add();
+    if (text_segment == nullptr) {
+      return absl::InternalError("Failed to create elf segment for text");
+    }
     text_segment->set_type(PT_LOAD);
     text_segment->set_virtual_address(text_segment_start);
     text_segment->set_physical_address(text_segment_start);
@@ -542,6 +502,9 @@
         ~4095ULL;
 
     ELFIO::segment *data_segment = writer_.segments.add();
+    if (data_segment == nullptr) {
+      return absl::InternalError("Failed to create elf segment for data");
+    }
     data_segment->set_type(PT_LOAD);
     data_segment->set_virtual_address(data_segment_start);
     data_segment->set_physical_address(data_segment_start);
@@ -575,12 +538,21 @@
   section_address_map_[data_section_] = data_segment_start;
   section_address_map_[bss_section_] = bss_segment_start;
 
+  std::function<void()> cleanup = []() {};
+  if (symbol_resolver == nullptr) {
+    symbol_resolver =
+        new SymbolResolver(elf_file_class_, symtab_, symbol_indices_);
+    cleanup = [symbol_resolver]() { delete symbol_resolver; };
+  }
   // Pass in the relocation vector to the second pass of parsing, but ignore
   // the values, since we are creating an executable file, and all the symbols
   // are resolved.
   std::vector<RelocationInfo> relo_vector;
-  auto status = ParsePassTwo(relo_vector);
-  if (!status.ok()) return status;
+  auto status = ParsePassTwo(relo_vector, symbol_resolver);
+  if (!status.ok()) {
+    cleanup();
+    return status;
+  }
 
   // Add sections to the segments. First segment gets the text section. The
   // second segment gets the data and bss sections.
@@ -595,12 +567,16 @@
                                     bss_section_->get_addr_align());
   }
 
-  auto res = SimpleTextToInt<uint64_t>(entry_point, symbol_resolver_);
-  if (!res.ok()) return res.status();
+  auto res = SimpleTextToInt<uint64_t>(entry_point, symbol_resolver);
+  if (!res.ok()) {
+    cleanup();
+    return res.status();
+  }
   uint64_t entry_point_value = res.value();
 
   symbol_accessor_->arrange_local_symbols();
   writer_.set_entry(entry_point_value);
+  cleanup();
   return absl::OkStatus();
 }
 
@@ -648,7 +624,8 @@
   symtab_->set_info(last_local + 1);
 }
 
-absl::Status SimpleAssembler::CreateRelocatable() {
+absl::Status SimpleAssembler::CreateRelocatable(
+    ResolverInterface *symbol_resolver) {
   writer_.set_type(ET_REL);
   // Reset the section address map to zero since we are creating a relocatable
   // file.
@@ -679,10 +656,19 @@
     UpdateSymtabHeaderInfo<ELFIO::Elf32_Sym>();
   }
 
+  std::function<void()> cleanup = []() {};
+  if (symbol_resolver == nullptr) {
+    symbol_resolver =
+        new SymbolResolver(elf_file_class_, symtab_, symbol_indices_);
+    cleanup = [symbol_resolver]() { delete symbol_resolver; };
+  }
   // Parse the source again, collect relocations.
   std::vector<RelocationInfo> relo_vector;
-  auto status = ParsePassTwo(relo_vector);
-  if (!status.ok()) return status;
+  auto status = ParsePassTwo(relo_vector, symbol_resolver);
+  if (!status.ok()) {
+    cleanup();
+    return status;
+  }
 
   // Handle relocations if there are any.
   if (!relo_vector.empty()) {
@@ -694,10 +680,12 @@
     }
     for (auto const &[section_index, relo_vec] : relo_map) {
       if (section_index == 0) {
+        cleanup();
         return absl::InternalError(
             "Relocation entry with section index 0 not supported");
       }
       if (!section_index_map_.contains(section_index)) {
+        cleanup();
         return absl::InternalError(
             absl::StrCat("Section index not found: ", section_index));
       }
@@ -722,22 +710,23 @@
         status = AddRelocationEntries<ELFIO::Elf32_Rela>(
             relo_vec, symbol_indices_, rela_section);
       } else {
+        cleanup();
         return absl::InternalError(
             absl::StrCat("Unsupported ELF file class: ", elf_file_class_));
       }
-      if (!status.ok()) return status;
+      if (!status.ok()) {
+        cleanup();
+        return status;
+      }
     }
   }
+  cleanup();
   return absl::OkStatus();
 }
 
 absl::Status SimpleAssembler::ParsePassTwo(
-    std::vector<RelocationInfo> &relo_vector) {
-  // For the second pass, we need a symbol resolver that uses the symbol
-  // table and the symbol indices.
-  symbol_resolver_ =
-      new SymbolResolver(elf_file_class_, symtab_, symbol_indices_);
-
+    std::vector<RelocationInfo> &relo_vector,
+    ResolverInterface *symbol_resolver) {
   // Now fill in the sections. Parse each of the lines saved in the first
   // pass.
   for (auto const &line : lines_) {
@@ -747,10 +736,10 @@
     auto relo_size = relo_vector.size();
     auto address = section_address_map_[section];
     if (line[0] == '.') {
-      auto status = ParseAsmDirective(line, address, symbol_resolver_,
+      auto status = ParseAsmDirective(line, address, symbol_resolver,
                                       byte_vector, relo_vector);
     } else {
-      auto status = ParseAsmStatement(line, address, symbol_resolver_,
+      auto status = ParseAsmStatement(line, address, symbol_resolver,
                                       byte_vector, relo_vector);
     }
     if (!status.ok()) return status;
@@ -763,6 +752,9 @@
     if (byte_vector.empty()) continue;
     // Add data to the section, but first make sure it's not bss.
     if (section != bss_section_) {
+      if (section == nullptr) {
+        return absl::InternalError("Data is added to a null section");
+      }
       section->append_data(reinterpret_cast<const char *>(byte_vector.data()),
                            byte_vector.size());
     }
@@ -861,7 +853,7 @@
     auto values = res.value();
     size = values.size() * sizeof(int64_t);
     ConvertToBytes<int64_t>(values, byte_values);
-  } else if (match == "section") {
+  } else if (match == "sect") {
     // .section <name>,<type>
     // TODO(torerik): Implement.
     return absl::UnimplementedError("Section directive not implemented");
@@ -958,12 +950,12 @@
     std::vector<uint8_t> &byte_values,
     std::vector<RelocationInfo> &relocations) {
   // Call the target specific assembler to encode the statement.
-  auto status = opcode_assembler_if_->Encode(
+  auto result = opcode_assembler_if_->Encode(
       address, line,
       absl::bind_front(&SimpleAssembler::AddSymbolToCurrentSection, this),
       resolver, byte_values, relocations);
-  if (!status.ok()) return status;
-  section_address_map_[current_section_] += byte_values.size();
+  if (!result.ok()) return result.status();
+  section_address_map_[current_section_] += result.value();
   return absl::OkStatus();
 }
 
diff --git a/mpact/sim/util/asm/simple_assembler.h b/mpact/sim/util/asm/simple_assembler.h
index e711fc7..982b5bf 100644
--- a/mpact/sim/util/asm/simple_assembler.h
+++ b/mpact/sim/util/asm/simple_assembler.h
@@ -74,7 +74,8 @@
   virtual ~SimpleAssembler();
 
   // Parse the input stream as assembly.
-  absl::Status Parse(std::istream &is);
+  absl::Status Parse(std::istream &is,
+                     ResolverInterface *zero_resolver = nullptr);
   // Add the symbol to the symbol table for the current section. See ELFIO
   // documentation for details of the meaning of the parameters.
   absl::Status AddSymbolToCurrentSection(const std::string &name,
@@ -90,15 +91,26 @@
   // The text segment will be laid out starting at base address, followed by
   // the data segment.
   absl::Status CreateExecutable(uint64_t base_address,
-                                const std::string &entry_point);
-  absl::Status CreateExecutable(uint64_t base_address, uint64_t entry_point);
+                                const std::string &entry_point,
+                                ResolverInterface *symbol_resolver = nullptr);
+  absl::Status CreateExecutable(uint64_t base_address, uint64_t entry_point,
+                                ResolverInterface *symbol_resolver = nullptr);
   // Create a relocatable ELF file.
-  absl::Status CreateRelocatable();
+  absl::Status CreateRelocatable(ResolverInterface *symbol_resolver = nullptr);
   // Write the ELF file to the given output stream.
   absl::Status Write(std::ostream &os);
   // Access the ELF writer.
   ELFIO::elfio &writer() { return writer_; }
 
+  // Add a symbol reference to the symbol table if it is not already defined.
+  void SimpleAddSymbol(absl::string_view name);
+
+  // Getters.
+  absl::flat_hash_map<std::string, ELFIO::Elf_Word> &symbol_indices() {
+    return symbol_indices_;
+  }
+  ELFIO::section *symtab() { return symtab_; }
+
  private:
   // Helper function to update the symbol table entries.
   template <typename SymbolType>
@@ -110,7 +122,8 @@
   template <typename SymbolType>
   void UpdateSymtabHeaderInfo();
   // Perform second pass of parsing.
-  absl::Status ParsePassTwo(std::vector<RelocationInfo> &relo_vector);
+  absl::Status ParsePassTwo(std::vector<RelocationInfo> &relo_vector,
+                            ResolverInterface *symbol_resolver);
   // Parse and process an assembly directive.
   absl::Status ParseAsmDirective(absl::string_view line, uint64_t address,
                                  ResolverInterface *resolver,
@@ -125,8 +138,6 @@
   absl::Status AddSymbol(const std::string &name, ELFIO::Elf64_Addr value,
                          ELFIO::Elf_Xword size, uint8_t type, uint8_t binding,
                          uint8_t other, ELFIO::section *section);
-  // Add a symbol reference to the symbol table if it is not already defined.
-  void SimpleAddSymbol(absl::string_view name);
   // Append the data to the current section.
   absl::Status AppendData(const char *data, size_t size);
 
@@ -157,9 +168,6 @@
   // Map that tracks the current address of each section.
   absl::flat_hash_map<ELFIO::section *, uint64_t> section_address_map_;
 
-  // Current symbol resolver (looks up symbols in the symbol table and returns
-  // their values).
-  ResolverInterface *symbol_resolver_ = nullptr;
   std::vector<std::string> lines_;
   // Section pointers.
   ELFIO::section *text_section_ = nullptr;
diff --git a/mpact/sim/util/asm/test/BUILD b/mpact/sim/util/asm/test/BUILD
index 3c5e77a..692c7fe 100644
--- a/mpact/sim/util/asm/test/BUILD
+++ b/mpact/sim/util/asm/test/BUILD
@@ -74,9 +74,6 @@
         "//mpact/sim/util/asm",
         "@com_google_absl//absl/base:no_destructor",
         "@com_google_absl//absl/container:flat_hash_map",
-        "@com_google_absl//absl/functional:any_invocable",
-        "@com_google_absl//absl/log",
-        "@com_google_absl//absl/log:check",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
@@ -89,7 +86,6 @@
     size = "small",
     srcs = ["riscv64x_asm_test.cc"],
     deps = [
-        ":riscv64x_bin_fmt",
         ":riscv64x_encoder",
         ":riscv64x_isa",
         "//mpact/sim/util/asm",
@@ -97,10 +93,8 @@
         "@com_github_serge1_elfio//:elfio",
         "@com_google_absl//absl/base:no_destructor",
         "@com_google_absl//absl/container:flat_hash_map",
-        "@com_google_absl//absl/log",
         "@com_google_absl//absl/log:check",
         "@com_google_absl//absl/status",
-        "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
         "@com_google_absl//absl/types:span",
         "@com_google_googletest//:gtest_main",
diff --git a/mpact/sim/util/asm/test/riscv64x_asm_test.cc b/mpact/sim/util/asm/test/riscv64x_asm_test.cc
index 91d9d93..3091c09 100644
--- a/mpact/sim/util/asm/test/riscv64x_asm_test.cc
+++ b/mpact/sim/util/asm/test/riscv64x_asm_test.cc
@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <cstddef>
 #include <cstdint>
 #include <sstream>
 #include <string>
@@ -54,10 +55,11 @@
   RiscV64XAssembler(Riscv64xSlotMatcher* matcher)
       : label_re_("^(\\S+)\\s*:"), matcher_(matcher) {};
   ~RiscV64XAssembler() override = default;
-  absl::Status Encode(uint64_t address, absl::string_view text,
-                      AddSymbolCallback add_symbol_callback,
-                      ResolverInterface* resolver, std::vector<uint8_t>& bytes,
-                      std::vector<RelocationInfo>& relocations) override {
+  absl::StatusOr<size_t> Encode(
+      uint64_t address, absl::string_view text,
+      AddSymbolCallback add_symbol_callback, ResolverInterface* resolver,
+      std::vector<uint8_t>& bytes,
+      std::vector<RelocationInfo>& relocations) override {
     // First check to see if there is a label, if so, add it to the symbol table
     // with the current address.
     std::string label;
@@ -78,7 +80,7 @@
     for (int i = 0; i < size / 8; ++i) {
       bytes.push_back(u.b[i]);
     }
-    return absl::OkStatus();
+    return bytes.size();
   }
 
  private:
diff --git a/mpact/sim/util/program_loader/elf_program_loader.cc b/mpact/sim/util/program_loader/elf_program_loader.cc
index 2ee8544..8371a9c 100644
--- a/mpact/sim/util/program_loader/elf_program_loader.cc
+++ b/mpact/sim/util/program_loader/elf_program_loader.cc
@@ -18,8 +18,6 @@
 
 #include <cstdint>
 #include <cstring>
-#include <fstream>
-#include <ios>
 #include <string>
 #include <utility>