diff --git a/docs/ROADMAP.md b/docs/ROADMAP.md
index 57bae2566..17feedeb7 100644
--- a/docs/ROADMAP.md
+++ b/docs/ROADMAP.md
@@ -8,7 +8,7 @@ Each item should be a dedicated `feat/<name>` branch, with tests/examples and a
 2. **Associative arrays** — `int aa[string];` — **in progress / partial** (string keys only; see [assoc-array.md](assoc-array.md))
 3. **Virtual interfaces** + eventing on `vif.clk` — **partial** (see [virtual-interface.md](virtual-interface.md))
 4. **Clocking blocks** — enough for `@(vif.cb)` — **partial** (interface-local `@(bif.cb)`; see [clocking.md](clocking.md))
-5. **`mailbox` / `semaphore` builtins** (or solid class equivalents with blocking put/get)
+5. **`mailbox` / `semaphore` builtins** (or solid class equivalents with blocking put/get) — **partial** (int mailbox + semaphore; see [mailbox-semaphore.md](mailbox-semaphore.md))
 6. **Constraints + `randomize()` / `randomize() with`** — start unconstrained `rand`, then solver
 7. **`$cast` / `$typename` hardening** for factory patterns
 8. **Covergroups** — functional coverage
diff --git a/docs/STATUS.md b/docs/STATUS.md
index 1dde44418..7e0a14978 100644
--- a/docs/STATUS.md
+++ b/docs/STATUS.md
@@ -11,12 +11,13 @@ Last updated: 2026-07-21
 | Area | Status |
 |------|--------|
 | Icarus Verilog tree | Fork of `steveicarus/iverilog` (`master` @ merge of SV array ordering work) |
-| [`uvm/`](../uvm/) | Seeded from IVL_UVM (VerifWorks) — messaging, CLP, stub phases, poor-man’s mailbox/semaphore; **not** Accellera-compatible |
+| [`uvm/`](../uvm/) | Seeded from IVL_UVM (VerifWorks) — messaging, CLP, stub phases, legacy poor-man’s mailbox/semaphore classes; **not** Accellera-compatible. Prefer compiler builtins for `mailbox`/`semaphore` (see STATUS row below). |
 | [`examples/hello_uvm`](../examples/hello_uvm) | Smoke TB for the seeded library (`Makefile` included) |
 | Parameterized classes | **Partial** on `feat/param-classes` (merged): ANSI `class C #(type T = int, parameter int W = 8);` parses into the class scope and elaborates with **defaults**. Explicit specializations `C#(byte)` / overrides not done yet. See [`examples/param_classes`](../examples/param_classes). |
 | Associative arrays | **Partial** on `feat/assoc-array`: string-keyed `int aa[string]` / `int aa[*]` with size/num/exists/delete/foreach/copy. See [`docs/assoc-array.md`](assoc-array.md) and [`examples/assoc_array`](../examples/assoc_array). |
 | Virtual interfaces | **Partial** on `feat/virtual-interface`: `virtual interface T` as class property / TF arg; assign interface instance; member R/W; `@(posedge vif.clk)`. See [`docs/virtual-interface.md`](virtual-interface.md) and [`examples/virtual_interface`](../examples/virtual_interface). |
 | Clocking blocks | **Partial** on `feat/clocking-blocks`: interface-local `clocking`; `@(bif.cb)`; `cb.sig` R/W with `#0` skew. See [`docs/clocking.md`](clocking.md) and [`examples/clocking`](../examples/clocking). |
+| Mailbox / semaphore | **Partial** on `feat/mailbox-semaphore`: compiler builtins; `mailbox #(int)` + blocking/nonblocking put/get/num; `semaphore` get/put/try_get. See [`docs/mailbox-semaphore.md`](mailbox-semaphore.md) and [`examples/mailbox_sem`](../examples/mailbox_sem). |
 | Constraints / randomize | Missing |
 | Covergroups / DPI | Missing |
 
diff --git a/docs/mailbox-semaphore.md b/docs/mailbox-semaphore.md
new file mode 100644
index 000000000..431abfa20
--- /dev/null
+++ b/docs/mailbox-semaphore.md
@@ -0,0 +1,60 @@
+# Mailbox / semaphore (Tier A #5)
+
+Status: **partial** — compiler builtins for Accellera-style IPC (int messages).
+
+## Choice: compiler builtins (not library classes)
+
+IEEE `mailbox` / `semaphore` are built-in classes. A pure SV library cannot implement the required API here because:
+
+- Functions cannot take `output`/`ref` ports (`try_get(v)` needs a side-effecting write).
+- Class `event` properties are not supported yet (needed for efficient wait/wakeup).
+
+This fork therefore implements **native builtins**, similar in shape to `string` / queue method sfuncs: empty `netclass_t` types named `mailbox` / `semaphore`, methods lowered to `$ivl_mailbox$*` / `$ivl_semaphore$*`, and vvp opcodes (`%mbx/*`, `%sem/*`) with real thread suspend/resume.
+
+The older IVL_UVM “poor-man’s” classes (`ivl_uvm_mbx`, `ivl_uvm_semaphore`) remain in `uvm/` for that library’s own tests; new Accellera-shaped code should use the builtins.
+
+## Supported in this slice
+
+```systemverilog
+mailbox #(int) mb = new();   // #(T) accepted; element type is int/vec for v1
+mb.put(7);
+mb.get(v);                   // blocking
+ok = mb.try_put(8);
+ok = mb.try_get(v);          // nonblocking; writes v on success
+n = mb.num();
+
+semaphore sem = new(1);
+sem.get(1);                  // blocking
+sem.put(1);
+ok = sem.try_get(1);
+```
+
+Also: unbounded `new()` / `new(0)`, bounded mailbox `new(N)`, blocking put when full, `peek` / `try_peek`.
+
+## Encoding
+
+| Layer | Role |
+|-------|------|
+| Parse / pform | Builtin typedefs `mailbox` / `semaphore`; `mailbox #(T) x` as var decl (T ignored for now) |
+| Netlist | Empty `netclass_t` “mailbox” / “semaphore” |
+| Elab | `$ivl_mailbox$new/put/get/...`, `$ivl_semaphore$new/get/put/try_get` |
+| Codegen | `%mbx/*`, `%sem/*`, `%box/vec4` / `%unbox/vec4` for int payloads |
+| Runtime | `vvp_mailbox` / `vvp_semaphore` (`vvp/vvp_mailbox.{h,cc}`) |
+
+## Deferred (do not claim)
+
+- True parameterized element types (`mailbox #(string)`, class handles) beyond boxing ints
+- Explicit specialization of user param-classes interacting with mailbox
+- `peek` stress / priority / typed message checking
+- Replacing IVL_UVM’s `ivl_uvm_mbx` usages with the builtin
+
+## Example
+
+[`examples/mailbox_sem/mbx_sem_basic.sv`](../examples/mailbox_sem/mbx_sem_basic.sv) — prints `PASSED`.
+
+```bash
+./install/bin/iverilog -g2012 -o /tmp/mbx_sem.vvp examples/mailbox_sem/mbx_sem_basic.sv
+./install/bin/vvp /tmp/mbx_sem.vvp
+```
+
+See also [STATUS.md](STATUS.md) and [ROADMAP.md](ROADMAP.md).
diff --git a/elab_expr.cc b/elab_expr.cc
index d5eea8d8a..684f6ed32 100644
--- a/elab_expr.cc
+++ b/elab_expr.cc
@@ -2017,6 +2017,25 @@ unsigned PECallFunction::test_width_method_(Design*, NetScope*,
       if (search_results.net && search_results.net->data_type()==IVL_VT_CLASS) {
 	    const netclass_t *class_type = dynamic_cast<const netclass_t*>(search_results.type);
 	    ivl_assert(*this, class_type);
+	    perm_string cname = class_type->get_name();
+	    if (cname == perm_string::literal("mailbox")) {
+		  if (method_name == "num" || method_name == "try_get"
+		      || method_name == "try_peek" || method_name == "try_put") {
+			expr_type_ = IVL_VT_BOOL;
+			expr_width_ = (method_name == "num") ? 32 : 1;
+			min_width_ = expr_width_;
+			signed_flag_ = false;
+			return expr_width_;
+		  }
+	    }
+	    if (cname == perm_string::literal("semaphore")
+		&& method_name == "try_get") {
+		  expr_type_ = IVL_VT_BOOL;
+		  expr_width_ = 1;
+		  min_width_ = 1;
+		  signed_flag_ = false;
+		  return expr_width_;
+	    }
 	    NetScope*method = class_type->method_from_name(method_name);
 
 	    if (method == 0) {
@@ -4126,6 +4145,59 @@ NetExpr* PECallFunction::elaborate_expr_method_(Design*des, NetScope*scope,
 	    NetNet*net = search_results.net;
 	    const netclass_t*class_type = dynamic_cast<const netclass_t*>(search_results.type);
 	    ivl_assert(*this, class_type);
+
+	    perm_string cname = class_type->get_name();
+	    if (cname == perm_string::literal("mailbox")) {
+		  if (method_name == perm_string::literal("num")) {
+			NetESFunc*sys = new NetESFunc("$ivl_mailbox$num",
+						     &netvector_t::atom2u32, 1);
+			sys->set_line(*this);
+			sys->parm(0, sub_expr);
+			return sys;
+		  }
+		  if (method_name == perm_string::literal("try_get")
+		      || method_name == perm_string::literal("try_peek")
+		      || method_name == perm_string::literal("try_put")) {
+			const char*sname =
+			      method_name==perm_string::literal("try_get")
+			      ? "$ivl_mailbox$try_get"
+			      : (method_name==perm_string::literal("try_peek")
+				 ? "$ivl_mailbox$try_peek"
+				 : "$ivl_mailbox$try_put");
+			unsigned nargs = parms_.empty() ? 0 : 1;
+			NetESFunc*sys = new NetESFunc(sname,
+						     &netvector_t::atom2u32,
+						     1 + nargs);
+			sys->set_line(*this);
+			sys->parm(0, sub_expr);
+			if (nargs > 0 && parms_[0].parm) {
+			      NetExpr*a = elab_and_eval(des, scope,
+						       parms_[0].parm,
+						       -1, false, false);
+			      if (a) sys->parm(1, a);
+			}
+			return sys;
+		  }
+	    }
+	    if (cname == perm_string::literal("semaphore")
+		&& method_name == perm_string::literal("try_get")) {
+		  unsigned nargs = parms_.empty() ? 0 : 1;
+		  NetESFunc*sys = new NetESFunc("$ivl_semaphore$try_get",
+						 &netvector_t::atom2u32,
+						 1 + nargs);
+		  sys->set_line(*this);
+		  sys->parm(0, sub_expr);
+		  if (nargs > 0 && parms_[0].parm) {
+			NetExpr*narg = elab_and_eval(des, scope,
+						    parms_[0].parm,
+						    32, false, false,
+						    IVL_VT_LOGIC);
+			sys->parm(1, narg ? narg
+					  : new NetEConst(verinum((uint64_t)1, 32)));
+		  }
+		  return sys;
+	    }
+
 	    return elaborate_class_method_net_(des, scope, net, class_type,
 					       method_name, nullptr);
       }
@@ -7426,6 +7498,33 @@ NetExpr* PENewClass::elaborate_expr_constructor_(Design*des, NetScope*scope,
 
       NetScope *new_scope = ctype->get_constructor();
       if (new_scope == 0) {
+	      // Built-in mailbox/semaphore constructors map to runtime sfuncs.
+	    if (gn_system_verilog()
+		&& ctype->get_name() == perm_string::literal("mailbox")) {
+		  unsigned nargs = parms_.empty() ? 0 : 1;
+		  NetESFunc*mbx = new NetESFunc("$ivl_mailbox$new", ctype, nargs);
+		  mbx->set_line(*this);
+		  if (!parms_.empty() && parms_[0].parm) {
+			NetExpr*barg = elab_and_eval(des, scope, parms_[0].parm, 32,
+						     false, false, IVL_VT_LOGIC);
+			mbx->parm(0, barg ? barg : new NetEConst(verinum((uint64_t)0, 32)));
+		  }
+		  delete obj;
+		  return mbx;
+	    }
+	    if (gn_system_verilog()
+		&& ctype->get_name() == perm_string::literal("semaphore")) {
+		  unsigned nargs = parms_.empty() ? 0 : 1;
+		  NetESFunc*sem = new NetESFunc("$ivl_semaphore$new", ctype, nargs);
+		  sem->set_line(*this);
+		  if (!parms_.empty() && parms_[0].parm) {
+			NetExpr*carg = elab_and_eval(des, scope, parms_[0].parm, 32,
+						     false, false, IVL_VT_LOGIC);
+			sem->parm(0, carg ? carg : new NetEConst(verinum((uint64_t)0, 32)));
+		  }
+		  delete obj;
+		  return sem;
+	    }
 	      // No constructor.
 	    if (parms_.size() > 0) {
 		  cerr << get_fileline() << ": error: "
diff --git a/elab_type.cc b/elab_type.cc
index ef071907a..967c41863 100644
--- a/elab_type.cc
+++ b/elab_type.cc
@@ -40,6 +40,23 @@
 
 using namespace std;
 
+static netclass_t* make_builtin_semaphore_type_()
+{
+      static netclass_t*builtin_semaphore_type = 0;
+      if (!builtin_semaphore_type)
+	    builtin_semaphore_type = new netclass_t(perm_string::literal("semaphore"), 0);
+      return builtin_semaphore_type;
+}
+
+static netclass_t* make_builtin_mailbox_type_()
+{
+      static netclass_t*builtin_mailbox_type = 0;
+      if (!builtin_mailbox_type)
+	    builtin_mailbox_type = new netclass_t(perm_string::literal("mailbox"), 0);
+      return builtin_mailbox_type;
+}
+
+
 /*
  * Elaborations of types may vary depending on the scope that it is
  * done in, so keep a per-scope cache of the results.
@@ -506,6 +523,11 @@ NetScope *typeref_t::find_scope(Design *des, NetScope *s) const
 
 ivl_type_t typedef_t::elaborate_type(Design *des, NetScope *scope)
 {
+      if (name == perm_string::literal("mailbox"))
+	    return make_builtin_mailbox_type_();
+      if (name == perm_string::literal("semaphore"))
+	    return make_builtin_semaphore_type_();
+
       if (!data_type.get()) {
 	    cerr << get_fileline() << ": error: Undefined type `" << name << "`."
 		 << endl;
diff --git a/elaborate.cc b/elaborate.cc
index d585b5198..cacd86f37 100644
--- a/elaborate.cc
+++ b/elaborate.cc
@@ -4582,6 +4582,66 @@ NetProc* PCallTask::elaborate_method_(Design*des, NetScope*scope,
       }
 
       if (const netclass_t*class_type = dynamic_cast<const netclass_t*>(sr.type)) {
+	    perm_string cname = class_type->get_name();
+	    if (cname == perm_string::literal("mailbox")) {
+		  if (method_name == perm_string::literal("put")) {
+			static const std::vector<perm_string> parm_names = {
+			      perm_string::literal("message")
+			};
+			return elaborate_sys_task_method_(des, scope, net, method_name,
+							  "$ivl_mailbox$put", parm_names);
+		  }
+		  if (method_name == perm_string::literal("get")
+		      || method_name == perm_string::literal("peek")) {
+			static const std::vector<perm_string> parm_names = {
+			      perm_string::literal("message")
+			};
+			const char*sname = method_name == perm_string::literal("get")
+			      ? "$ivl_mailbox$get" : "$ivl_mailbox$peek";
+			return elaborate_sys_task_method_(des, scope, net, method_name,
+							  sname, parm_names);
+		  }
+		  if (method_name == perm_string::literal("try_put")
+		      || method_name == perm_string::literal("try_get")
+		      || method_name == perm_string::literal("try_peek")) {
+			static const std::vector<perm_string> parm_names = {
+			      perm_string::literal("message")
+			};
+			const char*sname =
+			      method_name == perm_string::literal("try_put")
+			      ? "$ivl_mailbox$try_put"
+			      : (method_name == perm_string::literal("try_get")
+				 ? "$ivl_mailbox$try_get"
+				 : "$ivl_mailbox$try_peek");
+			return elaborate_sys_task_method_(des, scope, net, method_name,
+							  sname, parm_names);
+		  }
+		  if (method_name == perm_string::literal("num")) {
+			static const std::vector<perm_string> no_parm_names;
+			return elaborate_sys_task_method_(des, scope, net, method_name,
+							  "$ivl_mailbox$num", no_parm_names);
+		  }
+	    }
+	    if (cname == perm_string::literal("semaphore")) {
+		  static const std::vector<perm_string> sem_no_parms;
+		  static const std::vector<perm_string> sem_one_parm = {
+			perm_string::literal("keycount")
+		  };
+		  if (method_name == perm_string::literal("get")
+		      || method_name == perm_string::literal("put")
+		      || method_name == perm_string::literal("try_get")) {
+			const char*sname =
+			      method_name == perm_string::literal("get")
+			      ? "$ivl_semaphore$get"
+			      : (method_name == perm_string::literal("put")
+				 ? "$ivl_semaphore$put"
+				 : "$ivl_semaphore$try_get");
+			return elaborate_sys_task_method_(des, scope, net, method_name,
+							  sname,
+							  parms_.empty() ? sem_no_parms
+									 : sem_one_parm);
+		  }
+	    }
 	    NetScope*task = class_type->method_from_name(method_name);
 	    if (task == 0) {
 		    // If an implicit this was added it is not an error if we
diff --git a/examples/mailbox_sem/mbx_sem_basic.sv b/examples/mailbox_sem/mbx_sem_basic.sv
new file mode 100644
index 000000000..6a9c7484b
--- /dev/null
+++ b/examples/mailbox_sem/mbx_sem_basic.sv
@@ -0,0 +1,82 @@
+// Tier A #5 smoke: built-in mailbox #(int) + semaphore API.
+module mbx_sem_basic;
+  mailbox #(int) mb = new();
+  semaphore sem = new(1);
+  int v, n, ok;
+  int pass;
+
+  initial begin
+    pass = 1;
+
+    mb.put(7);
+    mb.get(v);
+    if (v !== 7) begin
+      $display("FAIL: mb.get got %0d want 7", v);
+      pass = 0;
+    end
+
+    ok = mb.try_put(8);
+    if (ok !== 1) begin
+      $display("FAIL: try_put expected 1 got %0d", ok);
+      pass = 0;
+    end
+
+    ok = mb.try_get(v);
+    if (ok !== 1 || v !== 8) begin
+      $display("FAIL: try_get ok=%0d v=%0d", ok, v);
+      pass = 0;
+    end
+
+    n = mb.num();
+    if (n !== 0) begin
+      $display("FAIL: num expected 0 got %0d", n);
+      pass = 0;
+    end
+
+    ok = mb.try_get(v);
+    if (ok !== 0) begin
+      $display("FAIL: empty try_get expected 0 got %0d", ok);
+      pass = 0;
+    end
+
+    sem.get(1);
+    sem.put(1);
+    ok = sem.try_get(1);
+    if (ok !== 1) begin
+      $display("FAIL: sem.try_get expected 1 got %0d", ok);
+      pass = 0;
+    end
+    ok = sem.try_get(1);
+    if (ok !== 0) begin
+      $display("FAIL: sem.try_get empty expected 0 got %0d", ok);
+      pass = 0;
+    end
+
+    fork
+      begin
+        #5;
+        mb.put(42);
+      end
+      begin
+        mb.get(v);
+        if (v !== 42) begin
+          $display("FAIL: blocking get got %0d", v);
+          pass = 0;
+        end
+      end
+    join
+
+    fork
+      begin
+        #5;
+        sem.put(1);
+      end
+      begin
+        sem.get(1);
+      end
+    join
+
+    if (pass) $display("PASSED");
+    else $display("FAILED");
+  end
+endmodule
diff --git a/netclass.cc b/netclass.cc
index 278fceb43..34ed45a38 100644
--- a/netclass.cc
+++ b/netclass.cc
@@ -165,7 +165,9 @@ bool netclass_t::test_for_missing_initializers() const
 
 NetScope*netclass_t::method_from_name(perm_string name) const
 {
-      NetScope*task = class_scope_->child( hname_t(name) );
+      NetScope*task = 0;
+      if (class_scope_)
+	    task = class_scope_->child( hname_t(name) );
       if ((task == 0) && super_)
 	    task = super_->method_from_name(name);
       return task;
@@ -174,6 +176,9 @@ NetScope*netclass_t::method_from_name(perm_string name) const
 
 NetScope* netclass_t::get_constructor() const
 {
+      if (class_scope_ == 0)
+	    return nullptr;
+
       auto task = class_scope_->child(hname_t(perm_string::literal("new")));
       if (task)
 	    return task;
diff --git a/parse.y b/parse.y
index 331a09717..c3125ba35 100644
--- a/parse.y
+++ b/parse.y
@@ -1102,7 +1102,7 @@ Module::port_t *module_declare_interface_port(const YYLTYPE&loc, char *type,
 %type <porttype> port_direction port_direction_opt clocking_direction
 %type <expr> clocking_skew_opt
 %type <vartype> integer_vector_type integer_vector_type_no_reg
-%type <parmvalue> parameter_value_opt
+%type <parmvalue> parameter_value_opt type_parameter_value
 
 %type <event_exprs> event_expression_list
 %type <event_expr> event_expression
@@ -1609,6 +1609,29 @@ data_declaration /* IEEE1800-2005: A.2.1.3 */
 		       $1, $2);
 	var_lifetime = LexicalScope::INHERITED;
       }
+  /* Built-in / parameterized class types: `mailbox #(int) mb = new();` */
+  | attribute_list_opt K_const_opt variable_lifetime_opt TYPE_IDENTIFIER type_parameter_value list_of_variable_decl_assignments ';'
+      { typeref_t*tmp = new typeref_t($4.type);
+	FILE_NAME(tmp, @4);
+	pform_make_var(@4, $6, tmp, $1, $2);
+	if ($5) {
+	      if ($5->by_order) {
+		    for (std::list<PExpr*>::iterator i = $5->by_order->begin()
+			       ; i != $5->by_order->end() ; ++i)
+			  delete *i;
+		    delete $5->by_order;
+	      }
+	      if ($5->by_name) {
+		    for (std::list<named_pexpr_t>::iterator i = $5->by_name->begin()
+			       ; i != $5->by_name->end() ; ++i)
+			  delete i->parm;
+		    delete $5->by_name;
+	      }
+	      delete $5;
+	}
+	delete[]$4.text;
+	var_lifetime = LexicalScope::INHERITED;
+      }
   | attribute_list_opt K_const_opt K_var variable_lifetime_opt list_of_variable_decl_assignments_with_type ';'
       { data_type_t*data_type = $5.type;
 	if (!data_type) {
@@ -2361,6 +2384,30 @@ type_identifier_variable_decl_assignments_with_type
 	$$.decl_assignments = $3;
 	$$.type = pform_make_parray_type(@2, tmp, $2);
       }
+  /* Built-in parameterized classes: `mailbox #(int) mb = new();` */
+  | TYPE_IDENTIFIER type_parameter_value list_of_variable_decl_assignments
+      { pform_set_type_referenced(@1, $1.text);
+	auto tmp = new typeref_t($1.type);
+	FILE_NAME(tmp, @1);
+	delete[]$1.text;
+	if ($2) {
+	      if ($2->by_order) {
+		    for (std::list<PExpr*>::iterator i = $2->by_order->begin()
+			       ; i != $2->by_order->end() ; ++i)
+			  delete *i;
+		    delete $2->by_order;
+	      }
+	      if ($2->by_name) {
+		    for (std::list<named_pexpr_t>::iterator i = $2->by_name->begin()
+			       ; i != $2->by_name->end() ; ++i)
+			  delete i->parm;
+		    delete $2->by_name;
+	      }
+	      delete $2;
+	}
+	$$.decl_assignments = $3;
+	$$.type = tmp;
+      }
   | package_scope TYPE_IDENTIFIER dimensions_opt list_of_variable_decl_assignments
       { lex_in_package_scope(nullptr);
 	auto tmp = new typeref_t($2.type, $1);
@@ -5866,6 +5913,7 @@ module_item
 		  delete[]$2;
       }
 
+
         | attribute_list_opt
 	  IDENTIFIER parameter_value_opt error ';'
       { yyerror(@2, "error: Invalid module instantiation");
@@ -6400,6 +6448,22 @@ from_exclude : K_from { $$ = false; } | K_exclude { $$ = true; } ;
      The parameter value by name syntax is OVI enhancement BTF-B06 as
      approved by WG1364 on 6/28/1998. */
 
+/* Required `#(...)` type/parameter override list (not optional). */
+type_parameter_value
+  : '#' '(' expression_list_with_nuls ')'
+      { struct parmvalue_t*tmp = new struct parmvalue_t;
+	tmp->by_order = $3;
+	tmp->by_name = 0;
+	$$ = tmp;
+      }
+  | '#' '(' parameter_value_byname_list ')'
+      { struct parmvalue_t*tmp = new struct parmvalue_t;
+	tmp->by_order = 0;
+	tmp->by_name = $3;
+	$$ = tmp;
+      }
+  ;
+
 parameter_value_opt
   : '#' '(' expression_list_with_nuls ')'
       { struct parmvalue_t*tmp = new struct parmvalue_t;
diff --git a/pform.cc b/pform.cc
index 29cfda436..16615fcce 100644
--- a/pform.cc
+++ b/pform.cc
@@ -922,6 +922,28 @@ typedef_t* pform_test_type_identifier(const struct vlltype&loc, const char*txt)
 {
       perm_string name = lex_strings.make(txt);
 
+      // Built-in SystemVerilog classes (IEEE 1800 mailbox/semaphore).
+      if (gn_system_verilog()) {
+	    if (name == lex_strings.make("semaphore")) {
+		  static typedef_t*semaphore_type = 0;
+		  if (!semaphore_type) {
+			semaphore_type = new typedef_t(lex_strings.make("semaphore"));
+			semaphore_type->set_data_type(new type_parameter_t(semaphore_type->name));
+			semaphore_type->set_basic_type(type_restrict_t(type_restrict_t::CLASS));
+		  }
+		  return semaphore_type;
+	    }
+	    if (name == lex_strings.make("mailbox")) {
+		  static typedef_t*mailbox_type = 0;
+		  if (!mailbox_type) {
+			mailbox_type = new typedef_t(lex_strings.make("mailbox"));
+			mailbox_type->set_data_type(new type_parameter_t(mailbox_type->name));
+			mailbox_type->set_basic_type(type_restrict_t(type_restrict_t::CLASS));
+		  }
+		  return mailbox_type;
+	    }
+      }
+
       LexicalScope*cur_scope = lexical_scope;
       do {
 	    LexicalScope::typedef_map_t::iterator cur;
@@ -2558,6 +2580,59 @@ void pform_make_modgates(const struct vlltype&loc,
 			 std::vector<lgate>*gates,
 			 std::list<named_pexpr_t>*attr)
 {
+	// Reinterpret `mailbox #(int) mb;` style no-port instantiations of a
+	// known type identifier as variable declarations (built-in classes).
+      if (gn_system_verilog()) {
+	    typedef_t*decl_type = pform_test_type_identifier(loc, type);
+	    if (decl_type) {
+		  bool declaration_like = true;
+		  std::list<decl_assignment_t*>*decls = new std::list<decl_assignment_t*>;
+		  for (unsigned idx = 0 ; idx < gates->size() ; idx += 1) {
+			lgate&cur = (*gates)[idx];
+			if (cur.parms || cur.parms_by_name) {
+			      declaration_like = false;
+			      break;
+			}
+			decl_assignment_t*decl = new decl_assignment_t;
+			decl->name = { lex_strings.make(cur.name), loc.lexical_pos };
+			if (cur.ranges) {
+			      decl->index.swap(*cur.ranges);
+			      delete cur.ranges;
+			      cur.ranges = 0;
+			}
+			decls->push_back(decl);
+		  }
+		  if (declaration_like) {
+			pform_set_type_referenced(loc, type);
+			typeref_t*dtype = new typeref_t(decl_type);
+			FILE_NAME(dtype, loc);
+			pform_make_var(loc, decls, dtype, attr, false);
+			if (overrides) {
+			      // Built-in mailbox/semaphore ignore specialization args for now.
+			      if (overrides->by_order) {
+				    for (list<PExpr*>::iterator i = overrides->by_order->begin()
+					       ; i != overrides->by_order->end() ; ++i)
+					  delete *i;
+				    delete overrides->by_order;
+			      }
+			      if (overrides->by_name) {
+				    for (list<named_pexpr_t>::iterator i = overrides->by_name->begin()
+					       ; i != overrides->by_name->end() ; ++i)
+					  delete i->parm;
+				    delete overrides->by_name;
+			      }
+			      delete overrides;
+			}
+			delete gates;
+			return;
+		  }
+		  for (list<decl_assignment_t*>::iterator cur = decls->begin()
+			     ; cur != decls->end() ; ++cur)
+			delete *cur;
+		  delete decls;
+	    }
+      }
+
 	// The grammer should not allow module gates to happen outside
 	// an active module. But if really bad input errors combine in
 	// an ugly way with error recovery, then catch this
diff --git a/tgt-vvp/eval_object.c b/tgt-vvp/eval_object.c
index 98e1cd5d4..c42430c78 100644
--- a/tgt-vvp/eval_object.c
+++ b/tgt-vvp/eval_object.c
@@ -794,6 +794,30 @@ int draw_eval_object(ivl_expr_t ex)
 		strstr(ivl_expr_name(ex), "_with") != 0) {
 		  if (draw_queue_method_find_sfunc(ex) == 0) return 0;
 	    }
+	    /* Mailbox constructor: $ivl_mailbox$new([bound]) */
+	    if (strcmp(ivl_expr_name(ex), "$ivl_mailbox$new") == 0) {
+		  long bound = 0;
+		  unsigned parm_count = ivl_expr_parms(ex);
+		  if (parm_count > 0) {
+			ivl_expr_t bexpr = ivl_expr_parm(ex, 0);
+			if (bexpr && ivl_expr_type(bexpr) == IVL_EX_NUMBER)
+			      bound = (long)ivl_expr_uvalue(bexpr);
+		  }
+		  fprintf(vvp_out, "    %%mbx/new %ld;\n", bound);
+		  return 0;
+	    }
+	    /* Semaphore constructor: $ivl_semaphore$new([initial_count]) */
+	    if (strcmp(ivl_expr_name(ex), "$ivl_semaphore$new") == 0) {
+		  long cnt = 0;
+		  unsigned parm_count = ivl_expr_parms(ex);
+		  if (parm_count > 0) {
+			ivl_expr_t cexpr = ivl_expr_parm(ex, 0);
+			if (cexpr && ivl_expr_type(cexpr) == IVL_EX_NUMBER)
+			      cnt = (long)ivl_expr_uvalue(cexpr);
+		  }
+		  fprintf(vvp_out, "    %%sem/new %ld;\n", cnt);
+		  return 0;
+	    }
 	    if (strcmp(ivl_expr_name(ex), "$ivl_vif_new") == 0) {
 		  unsigned n = ivl_expr_parms(ex);
 		  fprintf(vvp_out, "    %%new/vif %u", n);
diff --git a/tgt-vvp/eval_vec4.c b/tgt-vvp/eval_vec4.c
index 93784bff9..e361bee86 100644
--- a/tgt-vvp/eval_vec4.c
+++ b/tgt-vvp/eval_vec4.c
@@ -1194,6 +1194,82 @@ static void draw_sfunc_vec4(ivl_expr_t expr)
 	    return;
       }
 
+
+      /* Mailbox num: $ivl_mailbox$num(mbx) -> int on vec4 */
+      if (strcmp(ivl_expr_name(expr), "$ivl_mailbox$num") == 0) {
+	    ivl_expr_t mbx_e = (parm_count > 0) ? ivl_expr_parm(expr, 0) : 0;
+	    if (mbx_e) draw_eval_object(mbx_e);
+	    fprintf(vvp_out, "    %%mbx/num;\n");
+	    if (ivl_expr_width(expr) > 1)
+		  fprintf(vvp_out, "    %%pad/u %u;\n", ivl_expr_width(expr));
+	    return;
+      }
+
+      /* Mailbox try_put: $ivl_mailbox$try_put(mbx, item) -> bit on vec4 */
+      if (strcmp(ivl_expr_name(expr), "$ivl_mailbox$try_put") == 0) {
+	    ivl_expr_t mbx_e  = (parm_count > 0) ? ivl_expr_parm(expr, 0) : 0;
+	    ivl_expr_t item_e = (parm_count > 1) ? ivl_expr_parm(expr, 1) : 0;
+	    if (mbx_e)  draw_eval_object(mbx_e);
+	    if (item_e) {
+		  if (ivl_expr_value(item_e) == IVL_VT_CLASS) {
+			draw_eval_object(item_e);
+		  } else {
+			unsigned wid = ivl_expr_width(item_e);
+			if (!wid) wid = 32;
+			draw_eval_vec4(item_e);
+			fprintf(vvp_out, "    %%box/vec4 %u;\n", wid);
+		  }
+	    } else {
+		  fprintf(vvp_out, "    %%null;\n");
+	    }
+	    fprintf(vvp_out, "    %%mbx/try_put;\n");
+	    if (ivl_expr_width(expr) > 1)
+		  fprintf(vvp_out, "    %%pad/u %u;\n", ivl_expr_width(expr));
+	    return;
+      }
+
+      /* Mailbox try_get/try_peek */
+      if (strcmp(ivl_expr_name(expr), "$ivl_mailbox$try_get") == 0 ||
+          strcmp(ivl_expr_name(expr), "$ivl_mailbox$try_peek") == 0) {
+	    int is_peek = (strcmp(ivl_expr_name(expr), "$ivl_mailbox$try_peek") == 0);
+	    ivl_expr_t mbx_e  = (parm_count > 0) ? ivl_expr_parm(expr, 0) : 0;
+	    ivl_expr_t item_e = (parm_count > 1) ? ivl_expr_parm(expr, 1) : 0;
+	    if (mbx_e) draw_eval_object(mbx_e);
+	    fprintf(vvp_out, "    %s;\n", is_peek ? "%mbx/try_peek" : "%mbx/try_get");
+	    if (item_e && ivl_expr_type(item_e) == IVL_EX_SIGNAL
+		&& ivl_expr_signal(item_e)) {
+		  ivl_signal_t sig = ivl_expr_signal(item_e);
+		  if (ivl_signal_data_type(sig) == IVL_VT_CLASS) {
+			fprintf(vvp_out, "    %%store/obj v%p_0;\n", sig);
+		  } else {
+			unsigned wid = ivl_signal_width(sig);
+			if (!wid) wid = 32;
+			fprintf(vvp_out, "    %%unbox/vec4 %u;\n", wid);
+			fprintf(vvp_out, "    %%store/vec4 v%p_0, 0, %u;\n", sig, wid);
+		  }
+	    } else {
+		  fprintf(vvp_out, "    %%pop/obj 1, 0;\n");
+	    }
+	    if (ivl_expr_width(expr) > 1)
+		  fprintf(vvp_out, "    %%pad/u %u;\n", ivl_expr_width(expr));
+	    return;
+      }
+
+      /* Semaphore try_get */
+      if (strcmp(ivl_expr_name(expr), "$ivl_semaphore$try_get") == 0) {
+	    ivl_expr_t sem_e = (parm_count > 0) ? ivl_expr_parm(expr, 0) : 0;
+	    if (sem_e) draw_eval_object(sem_e);
+	    if (parm_count >= 2 && ivl_expr_parm(expr, 1)) {
+		  draw_eval_vec4(ivl_expr_parm(expr, 1));
+	    } else {
+		  fprintf(vvp_out, "    %%pushi/vec4 1, 0, 32;\n");
+	    }
+	    fprintf(vvp_out, "    %%sem/try_get;\n");
+	    if (ivl_expr_width(expr) > 1)
+		  fprintf(vvp_out, "    %%pad/u %u;\n", ivl_expr_width(expr));
+	    return;
+      }
+
       draw_vpi_func_call(expr);
 }
 
diff --git a/tgt-vvp/vvp_process.c b/tgt-vvp/vvp_process.c
index b9acef727..7a99a9312 100644
--- a/tgt-vvp/vvp_process.c
+++ b/tgt-vvp/vvp_process.c
@@ -1945,6 +1945,146 @@ static int show_system_task_call(ivl_statement_t net)
 {
       const char*stmt_name = ivl_stmt_name(net);
 
+
+#define EMIT_MBX_PUSH_ITEM(item_expr) \
+      do { \
+	    ivl_expr_t _item = (item_expr); \
+	    if (_item) { \
+		  if (ivl_expr_value(_item) == IVL_VT_CLASS) { \
+			draw_eval_object(_item); \
+		  } else { \
+			unsigned _wid = ivl_expr_width(_item); \
+			if (!_wid) _wid = 32; \
+			draw_eval_vec4(_item); \
+			fprintf(vvp_out, "    %%box/vec4 %u;\n", _wid); \
+		  } \
+	    } else { \
+		  fprintf(vvp_out, "    %%null;\n"); \
+	    } \
+      } while(0)
+
+#define EMIT_MBX_STORE_ITEM(net_arg_idx) \
+      do { \
+	    if (ivl_stmt_parm_count(net) > (net_arg_idx)) { \
+		  ivl_expr_t _item = ivl_stmt_parm(net, net_arg_idx); \
+		  if (_item && ivl_expr_type(_item) == IVL_EX_SIGNAL) { \
+			ivl_signal_t _sig = ivl_expr_signal(_item); \
+			if (_sig && ivl_signal_data_type(_sig) == IVL_VT_CLASS) { \
+			      fprintf(vvp_out, "    %%store/obj v%p_0;\n", _sig); \
+			} else if (_sig) { \
+			      unsigned _wid = ivl_signal_width(_sig); \
+			      if (!_wid) _wid = 32; \
+			      fprintf(vvp_out, "    %%unbox/vec4 %u;\n", _wid); \
+			      fprintf(vvp_out, "    %%store/vec4 v%p_0, 0, %u;\n", _sig, _wid); \
+			} else { \
+			      fprintf(vvp_out, "    %%pop/obj 1, 0;\n"); \
+			} \
+		  } else { \
+			fprintf(vvp_out, "    %%pop/obj 1, 0;\n"); \
+		  } \
+	    } else { \
+		  fprintf(vvp_out, "    %%pop/obj 1, 0;\n"); \
+	    } \
+      } while(0)
+
+      if (strcmp(stmt_name, "$ivl_mailbox$put") == 0) {
+	    ivl_expr_t mbx  = ivl_stmt_parm(net, 0);
+	    ivl_expr_t item = ivl_stmt_parm_count(net) >= 2 ? ivl_stmt_parm(net, 1) : 0;
+	    if (mbx) draw_eval_object(mbx);
+	    EMIT_MBX_PUSH_ITEM(item);
+	    fprintf(vvp_out, "    %%mbx/put;\n");
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_mailbox$get") == 0) {
+	    ivl_expr_t mbx = ivl_stmt_parm(net, 0);
+	    if (mbx) draw_eval_object(mbx);
+	    fprintf(vvp_out, "    %%mbx/get;\n");
+	    EMIT_MBX_STORE_ITEM(1);
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_mailbox$peek") == 0) {
+	    ivl_expr_t mbx = ivl_stmt_parm(net, 0);
+	    if (mbx) draw_eval_object(mbx);
+	    fprintf(vvp_out, "    %%mbx/peek;\n");
+	    EMIT_MBX_STORE_ITEM(1);
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_mailbox$try_put") == 0) {
+	    ivl_expr_t mbx  = ivl_stmt_parm(net, 0);
+	    ivl_expr_t item = ivl_stmt_parm_count(net) >= 2 ? ivl_stmt_parm(net, 1) : 0;
+	    if (mbx) draw_eval_object(mbx);
+	    EMIT_MBX_PUSH_ITEM(item);
+	    fprintf(vvp_out, "    %%mbx/try_put;\n");
+	    fprintf(vvp_out, "    %%pop/vec4 1;\n");
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_mailbox$try_get") == 0) {
+	    ivl_expr_t mbx = ivl_stmt_parm(net, 0);
+	    if (mbx) draw_eval_object(mbx);
+	    fprintf(vvp_out, "    %%mbx/try_get;\n");
+	    fprintf(vvp_out, "    %%pop/vec4 1;\n");
+	    EMIT_MBX_STORE_ITEM(1);
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_mailbox$try_peek") == 0) {
+	    ivl_expr_t mbx = ivl_stmt_parm(net, 0);
+	    if (mbx) draw_eval_object(mbx);
+	    fprintf(vvp_out, "    %%mbx/try_peek;\n");
+	    fprintf(vvp_out, "    %%pop/vec4 1;\n");
+	    EMIT_MBX_STORE_ITEM(1);
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_mailbox$num") == 0) {
+	    ivl_expr_t mbx = ivl_stmt_parm(net, 0);
+	    if (mbx) draw_eval_object(mbx);
+	    fprintf(vvp_out, "    %%mbx/num;\n");
+	    fprintf(vvp_out, "    %%pop/vec4 1;\n");
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_semaphore$get") == 0) {
+	    ivl_expr_t sem = ivl_stmt_parm(net, 0);
+	    if (sem) draw_eval_object(sem);
+	    if (ivl_stmt_parm_count(net) >= 2 && ivl_stmt_parm(net, 1)) {
+		  draw_eval_vec4(ivl_stmt_parm(net, 1));
+	    } else {
+		  fprintf(vvp_out, "    %%pushi/vec4 1, 0, 32;\n");
+	    }
+	    fprintf(vvp_out, "    %%sem/get;\n");
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_semaphore$put") == 0) {
+	    ivl_expr_t sem = ivl_stmt_parm(net, 0);
+	    if (sem) draw_eval_object(sem);
+	    if (ivl_stmt_parm_count(net) >= 2 && ivl_stmt_parm(net, 1)) {
+		  draw_eval_vec4(ivl_stmt_parm(net, 1));
+	    } else {
+		  fprintf(vvp_out, "    %%pushi/vec4 1, 0, 32;\n");
+	    }
+	    fprintf(vvp_out, "    %%sem/put;\n");
+	    return 0;
+      }
+
+      if (strcmp(stmt_name, "$ivl_semaphore$try_get") == 0) {
+	    ivl_expr_t sem = ivl_stmt_parm(net, 0);
+	    if (sem) draw_eval_object(sem);
+	    if (ivl_stmt_parm_count(net) >= 2 && ivl_stmt_parm(net, 1)) {
+		  draw_eval_vec4(ivl_stmt_parm(net, 1));
+	    } else {
+		  fprintf(vvp_out, "    %%pushi/vec4 1, 0, 32;\n");
+	    }
+	    fprintf(vvp_out, "    %%sem/try_get;\n");
+	    fprintf(vvp_out, "    %%pop/vec4 1;\n");
+	    return 0;
+      }
+
       if (strcmp(stmt_name,"$ivl_vif_wait") == 0) {
 	    ivl_expr_t vif = ivl_stmt_parm(net, 0);
 	    ivl_expr_t edge_ex = ivl_stmt_parm(net, 1);
diff --git a/uvm/ivl_uvm_src/ivl_uvm_mbx.svh b/uvm/ivl_uvm_src/ivl_uvm_mbx.svh
index f07f234ec..d3bdc146f 100644
--- a/uvm/ivl_uvm_src/ivl_uvm_mbx.svh
+++ b/uvm/ivl_uvm_src/ivl_uvm_mbx.svh
@@ -3,7 +3,10 @@
   `define IVL_UVM_MBX_T int
 `endif
 
-// Poor man's mailbox
+// Poor man's mailbox (legacy IVL_UVM helper).
+// Prefer the compiler builtin `mailbox #(int)` when using -g2012 on this fork
+// (see docs/mailbox-semaphore.md). This class remains for IVL_UVM tests that
+// still instantiate ivl_uvm_mbx.
 // Default size = 1 in SV
 // Here, only size-1 is supported as of now
 // Also no parameterized classes in Icarus (yet), so using a `define
diff --git a/vvp/Makefile.in b/vvp/Makefile.in
index 5049e0f23..41ef14252 100644
--- a/vvp/Makefile.in
+++ b/vvp/Makefile.in
@@ -147,7 +147,7 @@ CORE_OBJ = lib_main.o \
            substitute.o \
            symbols.o ufunc.o codes.o vthread.o schedule.o \
            statistics.o tables.o udp.o vvp_island.o vvp_net.o vvp_net_sig.o \
-           vvp_object.o vvp_cobject.o vvp_darray.o vvp_aarray.o vvp_vif.o event.o logic.o delay.o \
+           vvp_object.o vvp_cobject.o vvp_darray.o vvp_aarray.o vvp_vif.o vvp_mailbox.o event.o logic.o delay.o \
            words.o island_tran.o
 
 VPI_OBJ = vpi_modules.o vpi_bit.o vpi_callback.o vpi_cobject.o vpi_const.o vpi_darray.o \
diff --git a/vvp/codes.h b/vvp/codes.h
index 1a9e33d12..b702bb8d6 100644
--- a/vvp/codes.h
+++ b/vvp/codes.h
@@ -172,6 +172,20 @@ extern bool of_NANDR(vthread_t thr, vvp_code_t code);
 extern bool of_NEW_COBJ(vthread_t thr, vvp_code_t code);
 extern bool of_NEW_DARRAY(vthread_t thr, vvp_code_t code);
 extern bool of_NEW_VIF(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_NEW(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_PUT(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_GET(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_PEEK(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_TRY_PUT(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_TRY_GET(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_TRY_PEEK(vthread_t thr, vvp_code_t code);
+extern bool of_MBX_NUM(vthread_t thr, vvp_code_t code);
+extern bool of_SEM_NEW(vthread_t thr, vvp_code_t code);
+extern bool of_SEM_GET(vthread_t thr, vvp_code_t code);
+extern bool of_SEM_PUT(vthread_t thr, vvp_code_t code);
+extern bool of_SEM_TRY_GET(vthread_t thr, vvp_code_t code);
+extern bool of_BOX_VEC4(vthread_t thr, vvp_code_t code);
+extern bool of_UNBOX_VEC4(vthread_t thr, vvp_code_t code);
 extern bool of_NOOP(vthread_t thr, vvp_code_t code);
 extern bool of_NOR(vthread_t thr, vvp_code_t code);
 extern bool of_NORR(vthread_t thr, vvp_code_t code);
diff --git a/vvp/compile.cc b/vvp/compile.cc
index c52aac891..82de8aac1 100644
--- a/vvp/compile.cc
+++ b/vvp/compile.cc
@@ -114,6 +114,7 @@ static const struct opcode_table_s opcode_table[] = {
       { "%assign/wr/e",of_ASSIGN_WRE,1,{OA_VPI_PTR, OA_NONE, OA_NONE} },
       { "%blend",    of_BLEND,   0,  {OA_NONE,  OA_NONE,     OA_NONE} },
       { "%blend/wr", of_BLEND_WR,0,  {OA_NONE,  OA_NONE,     OA_NONE} },
+      { "%box/vec4",  of_BOX_VEC4,  1, {OA_NUMBER, OA_NONE,  OA_NONE} },
       { "%breakpoint", of_BREAKPOINT, 0,  {OA_NONE, OA_NONE, OA_NONE} },
       { "%callf/obj",       of_CALLF_OBJ,       2,{OA_CODE_PTR2,OA_VPI_PTR, OA_NONE} },
       { "%callf/real",      of_CALLF_REAL,      2,{OA_CODE_PTR2,OA_VPI_PTR, OA_NONE} },
@@ -225,6 +226,14 @@ static const struct opcode_table_s opcode_table[] = {
       { "%load/vec4",  of_LOAD_VEC4, 1,{OA_FUNC_PTR,OA_NONE,  OA_NONE} },
       { "%load/vec4a", of_LOAD_VEC4A,2,{OA_ARR_PTR, OA_BIT1, OA_NONE} },
       { "%max/wr", of_MAX_WR, 0,  {OA_NONE,     OA_NONE,     OA_NONE} },
+      { "%mbx/get",      of_MBX_GET,      0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%mbx/new",      of_MBX_NEW,      1, {OA_NUMBER, OA_NONE, OA_NONE} },
+      { "%mbx/num",      of_MBX_NUM,      0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%mbx/peek",     of_MBX_PEEK,     0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%mbx/put",      of_MBX_PUT,      0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%mbx/try_get",  of_MBX_TRY_GET,  0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%mbx/try_peek", of_MBX_TRY_PEEK, 0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%mbx/try_put",  of_MBX_TRY_PUT,  0, {OA_NONE,   OA_NONE, OA_NONE} },
       { "%min/wr", of_MIN_WR, 0,  {OA_NONE,     OA_NONE,     OA_NONE} },
       { "%mod",    of_MOD,    0,  {OA_NONE,     OA_NONE,     OA_NONE} },
       { "%mod/s",  of_MOD_S,  0,  {OA_NONE,     OA_NONE,     OA_NONE} },
@@ -335,6 +344,10 @@ static const struct opcode_table_s opcode_table[] = {
       { "%rsort/obj", of_RSORT_OBJ, 1, {OA_FUNC_PTR, OA_NONE, OA_NONE} },
       { "%rsort/prop/obj", of_RSORT_PROP_OBJ, 1, {OA_NUMBER, OA_NONE, OA_NONE} },
       { "%scopy",  of_SCOPY,  0,  {OA_NONE,     OA_NONE,     OA_NONE} },
+      { "%sem/get",      of_SEM_GET,      0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%sem/new",      of_SEM_NEW,      1, {OA_NUMBER, OA_NONE, OA_NONE} },
+      { "%sem/put",      of_SEM_PUT,      0, {OA_NONE,   OA_NONE, OA_NONE} },
+      { "%sem/try_get",  of_SEM_TRY_GET,  0, {OA_NONE,   OA_NONE, OA_NONE} },
       { "%set/dar/obj/real",of_SET_DAR_OBJ_REAL,1,{OA_NUMBER,OA_NONE,OA_NONE} },
       { "%set/dar/obj/str", of_SET_DAR_OBJ_STR, 1,{OA_NUMBER,OA_NONE,OA_NONE} },
       { "%set/dar/obj/vec4",of_SET_DAR_OBJ_VEC4,1,{OA_NUMBER,OA_NONE,OA_NONE} },
@@ -389,6 +402,7 @@ static const struct opcode_table_s opcode_table[] = {
       { "%test_nul/a",   of_TEST_NUL_A,   2,{OA_ARR_PTR, OA_BIT1,    OA_NONE} },
       { "%test_nul/obj", of_TEST_NUL_OBJ, 0,{OA_NONE,    OA_NONE,    OA_NONE} },
       { "%test_nul/prop",of_TEST_NUL_PROP,2,{OA_NUMBER,  OA_BIT1,    OA_NONE} },
+      { "%unbox/vec4",of_UNBOX_VEC4,1, {OA_NUMBER, OA_NONE,  OA_NONE} },
       { "%vif/load/vec4", of_VIF_LOAD_VEC4, 2,{OA_NUMBER, OA_BIT1,  OA_NONE} },
       { "%vif/store/vec4",of_VIF_STORE_VEC4,2,{OA_NUMBER, OA_BIT1,  OA_NONE} },
       { "%vif/wait",     of_VIF_WAIT,      2,{OA_NUMBER, OA_BIT1,   OA_NONE} },
diff --git a/vvp/vthread.cc b/vvp/vthread.cc
index 1248db3ea..c422145bc 100644
--- a/vvp/vthread.cc
+++ b/vvp/vthread.cc
@@ -29,6 +29,7 @@
 # include  "vvp_darray.h"
 # include  "vvp_aarray.h"
 # include  "vvp_vif.h"
+# include  "vvp_mailbox.h"
 # include  "class_type.h"
 #ifdef CHECK_WITH_VALGRIND
 # include  "vvp_cleanup.h"
@@ -8399,3 +8400,235 @@ bool of_REAP_UFUNC(vthread_t thr, vvp_code_t cp)
 
       return true;
 }
+
+
+/*
+ * Push an object onto a thread's object stack (used when waking mailbox/semaphore waiters).
+ */
+void vthread_push_obj_item(vthread_t thr, const vvp_object_t& obj)
+{
+      thr->push_object(obj);
+}
+
+/* ================================================================
+ * Mailbox opcodes
+ * ================================================================ */
+
+bool of_MBX_NEW(vthread_t thr, vvp_code_t cp)
+{
+      size_t bound = cp->number;
+      vvp_object_t mbx(new vvp_mailbox(bound));
+      thr->push_object(mbx);
+      return true;
+}
+
+bool of_MBX_PUT(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t item_obj;
+      thr->pop_object(item_obj);
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      if (!mbx) return true;
+
+      bool done = mbx->put(thr, item_obj);
+      if (!done) return false;
+      return true;
+}
+
+bool of_MBX_GET(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      if (!mbx) {
+	    thr->push_object(vvp_object_t());
+	    return true;
+      }
+
+      vvp_object_t item;
+      bool done = mbx->get(thr, item);
+      if (!done) return false;
+      thr->push_object(item);
+      return true;
+}
+
+bool of_MBX_PEEK(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      if (!mbx) {
+	    thr->push_object(vvp_object_t());
+	    return true;
+      }
+
+      vvp_object_t item;
+      bool done = mbx->peek(thr, item);
+      if (!done) return false;
+      thr->push_object(item);
+      return true;
+}
+
+bool of_MBX_TRY_PUT(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t item_obj;
+      thr->pop_object(item_obj);
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      bool ok = mbx ? mbx->try_put(item_obj) : false;
+      vvp_vector4_t res(1, ok ? BIT4_1 : BIT4_0);
+      thr->push_vec4(res);
+      return true;
+}
+
+bool of_MBX_TRY_GET(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      vvp_object_t item;
+      bool ok = mbx ? mbx->try_get(item) : false;
+      thr->push_object(ok ? item : vvp_object_t());
+      vvp_vector4_t res(1, ok ? BIT4_1 : BIT4_0);
+      thr->push_vec4(res);
+      return true;
+}
+
+bool of_MBX_TRY_PEEK(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      vvp_object_t item;
+      bool ok = mbx ? mbx->try_peek(item) : false;
+      thr->push_object(ok ? item : vvp_object_t());
+      vvp_vector4_t res(1, ok ? BIT4_1 : BIT4_0);
+      thr->push_vec4(res);
+      return true;
+}
+
+bool of_MBX_NUM(vthread_t thr, vvp_code_t)
+{
+      vvp_object_t mbx_obj;
+      thr->pop_object(mbx_obj);
+
+      vvp_mailbox*mbx = mbx_obj.peek<vvp_mailbox>();
+      unsigned long cnt = mbx ? (unsigned long)mbx->num() : 0UL;
+      vvp_vector4_t res(32);
+      for (unsigned idx = 0 ; idx < 32 ; ++idx)
+	    res.set_bit(idx, (cnt >> idx) & 1 ? BIT4_1 : BIT4_0);
+      thr->push_vec4(res);
+      return true;
+}
+
+/* ================================================================
+ * Semaphore opcodes
+ * ================================================================ */
+
+bool of_SEM_NEW(vthread_t thr, vvp_code_t cp)
+{
+      size_t cnt = cp->number;
+      vvp_object_t sem(new vvp_semaphore(cnt));
+      thr->push_object(sem);
+      return true;
+}
+
+bool of_SEM_GET(vthread_t thr, vvp_code_t)
+{
+      unsigned long nval = 0;
+      {
+	    const vvp_vector4_t& nv = thr->peek_vec4(0);
+	    unsigned nbits = nv.size() < 32 ? nv.size() : 32;
+	    for (unsigned i = 0 ; i < nbits ; ++i)
+		  if (nv.value(i) == BIT4_1) nval |= (1UL << i);
+      }
+      thr->pop_vec4(1);
+
+      vvp_object_t sem_obj;
+      thr->pop_object(sem_obj);
+
+      vvp_semaphore*sem = sem_obj.peek<vvp_semaphore>();
+      if (!sem) return true;
+
+      bool done = sem->get(thr, nval ? nval : 1);
+      if (!done) return false;
+      return true;
+}
+
+bool of_SEM_PUT(vthread_t thr, vvp_code_t)
+{
+      unsigned long nval = 0;
+      {
+	    const vvp_vector4_t& nv = thr->peek_vec4(0);
+	    unsigned nbits = nv.size() < 32 ? nv.size() : 32;
+	    for (unsigned i = 0 ; i < nbits ; ++i)
+		  if (nv.value(i) == BIT4_1) nval |= (1UL << i);
+      }
+      thr->pop_vec4(1);
+
+      vvp_object_t sem_obj;
+      thr->pop_object(sem_obj);
+
+      vvp_semaphore*sem = sem_obj.peek<vvp_semaphore>();
+      if (sem) sem->put(nval ? nval : 1);
+      return true;
+}
+
+bool of_SEM_TRY_GET(vthread_t thr, vvp_code_t)
+{
+      unsigned long nval = 0;
+      {
+	    const vvp_vector4_t& nv = thr->peek_vec4(0);
+	    unsigned nbits = nv.size() < 32 ? nv.size() : 32;
+	    for (unsigned i = 0 ; i < nbits ; ++i)
+		  if (nv.value(i) == BIT4_1) nval |= (1UL << i);
+      }
+      thr->pop_vec4(1);
+
+      vvp_object_t sem_obj;
+      thr->pop_object(sem_obj);
+
+      vvp_semaphore*sem = sem_obj.peek<vvp_semaphore>();
+      bool ok = sem ? sem->try_get(nval ? nval : 1) : false;
+      vvp_vector4_t res(1, ok ? BIT4_1 : BIT4_0);
+      thr->push_vec4(res);
+      return true;
+}
+
+bool of_BOX_VEC4(vthread_t thr, vvp_code_t cp)
+{
+      unsigned wid = cp->number;
+      vvp_vector4_t v(wid);
+      const vvp_vector4_t& top = thr->peek_vec4(0);
+      unsigned bits = top.size() < wid ? top.size() : wid;
+      for (unsigned i = 0 ; i < bits ; ++i)
+	    v.set_bit(i, top.value(i));
+      thr->pop_vec4(1);
+      vvp_object_t box(new vvp_boxed_vec4(v));
+      thr->push_object(box);
+      return true;
+}
+
+bool of_UNBOX_VEC4(vthread_t thr, vvp_code_t cp)
+{
+      unsigned wid = cp->number;
+      vvp_object_t obj;
+      thr->pop_object(obj);
+      vvp_vector4_t res(wid, BIT4_0);
+      if (vvp_boxed_vec4*box = obj.peek<vvp_boxed_vec4>()) {
+	    const vvp_vector4_t& v = box->get_value();
+	    unsigned bits = v.size() < wid ? v.size() : wid;
+	    for (unsigned i = 0 ; i < bits ; ++i)
+		  res.set_bit(i, v.value(i));
+      }
+      thr->push_vec4(res);
+      return true;
+}
diff --git a/vvp/vthread.h b/vvp/vthread.h
index 1658c6839..ec616020c 100644
--- a/vvp/vthread.h
+++ b/vvp/vthread.h
@@ -136,4 +136,8 @@ extern const vvp_vector4_t& vthread_get_vec4_stack(struct vthread_s*thr, unsigne
 /* This is used to actually delete a thread once we are done with it. */
 extern void vthread_delete(vthread_t thr);
 
+/* Push an object onto a (possibly suspended) thread object stack. */
+class vvp_object_t;
+extern void vthread_push_obj_item(vthread_t thr, const vvp_object_t& obj);
+
 #endif /* IVL_vthread_H */
diff --git a/vvp/vvp_mailbox.cc b/vvp/vvp_mailbox.cc
new file mode 100644
index 000000000..ef252bafa
--- /dev/null
+++ b/vvp/vvp_mailbox.cc
@@ -0,0 +1,201 @@
+/*
+ * Copyright (c) 2026 Icarus UVM fork contributors
+ *
+ *    This source code is free software; you can redistribute it
+ *    and/or modify it in source code form under the terms of the GNU
+ *    General Public License as published by the Free Software
+ *    Foundation; either version 2 of the License, or (at your option)
+ *    any later version.
+ *
+ * Runtime implementation of SystemVerilog built-in mailbox and semaphore.
+ */
+
+#include "vvp_mailbox.h"
+#include "vthread.h"
+#include "schedule.h"
+#include <cassert>
+
+/* ================================================================
+ * vvp_mailbox
+ * ================================================================ */
+
+vvp_mailbox::vvp_mailbox(size_t bound)
+: bound_(bound)
+{
+}
+
+vvp_mailbox::~vvp_mailbox()
+{
+}
+
+vvp_object* vvp_mailbox::duplicate() const
+{
+      vvp_mailbox* copy = new vvp_mailbox(bound_);
+      copy->items_ = items_;
+      /* Note: waiters are NOT copied – a duplicate mailbox has no
+       * pending waiters. */
+      return copy;
+}
+
+bool vvp_mailbox::try_put(const vvp_object_t& item)
+{
+      if (full())
+	    return false;
+      items_.push_back(item);
+      resume_get_waiters_();
+      return true;
+}
+
+bool vvp_mailbox::try_get(vvp_object_t& item)
+{
+      if (empty())
+	    return false;
+      item = items_.front();
+      items_.pop_front();
+      resume_put_waiters_();
+      return true;
+}
+
+bool vvp_mailbox::try_peek(vvp_object_t& item)
+{
+      if (empty())
+	    return false;
+      item = items_.front();
+      return true;
+}
+
+bool vvp_mailbox::put(vthread_t thr, const vvp_object_t& item)
+{
+      if (!full()) {
+	    items_.push_back(item);
+	    resume_get_waiters_();
+	    return true;  /* completed immediately */
+      }
+      /* Mailbox is full – suspend the thread. Store the item in the
+       * waiter record so it is available when space opens up. */
+      put_waiter_t w;
+      w.thr  = thr;
+      w.item = item;
+      put_waiters_.push_back(w);
+      return false;             /* thread suspended */
+}
+
+bool vvp_mailbox::get(vthread_t thr, vvp_object_t& item_out)
+{
+      if (!empty()) {
+	    item_out = items_.front();
+	    items_.pop_front();
+	    resume_put_waiters_();
+	    return true;
+      }
+      get_waiter_t w;
+      w.thr = thr;
+      w.is_peek = false;
+      get_waiters_.push_back(w);
+      return false;
+}
+
+bool vvp_mailbox::peek(vthread_t thr, vvp_object_t& item_out)
+{
+      if (!empty()) {
+	    item_out = items_.front();
+	    return true;
+      }
+      get_waiter_t w;
+      w.thr = thr;
+      w.is_peek = true;
+      get_waiters_.push_back(w);
+      return false;
+}
+
+void vvp_mailbox::resume_get_waiters_()
+{
+      while (!get_waiters_.empty() && !empty()) {
+	    get_waiter_t waiter = get_waiters_.front();
+	    get_waiters_.erase(get_waiters_.begin());
+	    /* Push the retrieved item onto the waiting thread's object
+	     * stack so it is available when the thread resumes at the
+	     * instruction following %mbx/get or %mbx/peek. */
+	    vthread_push_obj_item(waiter.thr, items_.front());
+	    if (!waiter.is_peek) {
+		  items_.pop_front();
+		  resume_put_waiters_();
+	    }
+	    schedule_vthread(waiter.thr, 0, true);
+      }
+}
+
+void vvp_mailbox::resume_put_waiters_()
+{
+      while (!put_waiters_.empty() && !full()) {
+	    put_waiter_t w = put_waiters_.front();
+	    put_waiters_.erase(put_waiters_.begin());
+	    items_.push_back(w.item);
+	    schedule_vthread(w.thr, 0, true);
+	    resume_get_waiters_();
+      }
+}
+
+/* ================================================================
+ * vvp_semaphore
+ * ================================================================ */
+
+vvp_semaphore::vvp_semaphore(size_t initial_count)
+: count_(initial_count)
+{
+}
+
+vvp_semaphore::~vvp_semaphore()
+{
+}
+
+vvp_object* vvp_semaphore::duplicate() const
+{
+      return new vvp_semaphore(count_);
+}
+
+bool vvp_semaphore::try_get(size_t n)
+{
+      if (count_ >= n) {
+	    count_ -= n;
+	    return true;
+      }
+      return false;
+}
+
+void vvp_semaphore::put(size_t n)
+{
+      count_ += n;
+      resume_waiters_();
+}
+
+bool vvp_semaphore::get(vthread_t thr, size_t n)
+{
+      if (count_ >= n) {
+	    count_ -= n;
+	    return true;
+      }
+      waiter_t w;
+      w.thr = thr;
+      w.n   = n;
+      waiters_.push_back(w);
+      return false;
+}
+
+void vvp_semaphore::resume_waiters_()
+{
+      bool progress = true;
+      while (progress) {
+	    progress = false;
+	    for (size_t idx = 0; idx < waiters_.size(); ++idx) {
+		  if (count_ >= waiters_[idx].n) {
+			count_ -= waiters_[idx].n;
+			vthread_t w = waiters_[idx].thr;
+			waiters_.erase(waiters_.begin() + (long)idx);
+			schedule_vthread(w, 0, true);
+			progress = true;
+			break;
+		  }
+	    }
+      }
+}
diff --git a/vvp/vvp_mailbox.h b/vvp/vvp_mailbox.h
new file mode 100644
index 000000000..579b77bd2
--- /dev/null
+++ b/vvp/vvp_mailbox.h
@@ -0,0 +1,157 @@
+/*
+ * Copyright (c) 2026 Icarus UVM fork contributors
+ *
+ *    This source code is free software; you can redistribute it
+ *    and/or modify it in source code form under the terms of the GNU
+ *    General Public License as published by the Free Software
+ *    Foundation; either version 2 of the License, or (at your option)
+ *    any later version.
+ *
+ * Runtime objects for SystemVerilog built-in mailbox and semaphore.
+ */
+#ifndef VVP_MAILBOX_H
+#define VVP_MAILBOX_H
+
+#include "vvp_object.h"
+#include "vvp_net.h"
+#include <deque>
+#include <vector>
+
+struct vthread_s;
+typedef vthread_s* vthread_t;
+
+/*
+ * vvp_mailbox -- runtime representation of SystemVerilog's
+ * built-in parameterized mailbox.
+ *
+ * A mailbox is an unbounded (or bounded) typed FIFO. When the FIFO
+ * is empty, get() and peek() suspend the calling thread.  When the
+ * FIFO is at capacity, put() suspends the calling thread.
+ *
+ * We store contents as vvp_object_t (like queue_object). String and
+ * vec4 mailboxes would need separate classes; for UVM the mailbox
+ * element type is always an object.
+ */
+class vvp_mailbox : public vvp_object {
+    public:
+      explicit vvp_mailbox(size_t bound = 0);  /* 0 = unbounded */
+      ~vvp_mailbox() override;
+
+      vvp_object* duplicate() const override;
+      void shallow_copy(const vvp_object*) override {}
+
+      size_t bound() const { return bound_; }
+      size_t num()   const { return items_.size(); }
+      bool   full()  const { return bound_ && items_.size() >= bound_; }
+      bool   empty() const { return items_.empty(); }
+
+      /* Non-blocking operations.  Return true on success. */
+      bool try_put(const vvp_object_t& item);
+      bool try_get(vvp_object_t& item);
+      bool try_peek(vvp_object_t& item);
+
+      /*
+       * Blocking operations: return true if the operation completed
+       * immediately, or false if thr was suspended and will be
+       * rescheduled when the condition changes.
+       *
+       * For get/peek: when returning false the thread is added to
+       * get_waiters_.  On resumption (via resume_get_waiters_) the
+       * retrieved item has already been pushed onto the thread's
+       * object stack by vthread_push_obj_item().
+       *
+       * For put: when returning false the item is stored in the
+       * put_waiter_t record; it is NOT pushed to the thread stack.
+       */
+      bool put(vthread_t thr, const vvp_object_t& item);
+      bool get(vthread_t thr, vvp_object_t& item_out);
+      bool peek(vthread_t thr, vvp_object_t& item_out);
+
+    private:
+      void resume_get_waiters_();
+      void resume_put_waiters_();
+
+      size_t bound_;                      /* 0 = unbounded */
+      std::deque<vvp_object_t> items_;
+
+      /* Threads suspended waiting for items (get/peek). */
+      struct get_waiter_t {
+	    vthread_t thr;
+	    bool is_peek;
+      };
+      std::vector<get_waiter_t> get_waiters_;
+
+      /* Threads suspended waiting for space (put).
+       * The item to be put is stored here until space is available. */
+      struct put_waiter_t {
+	    vthread_t thr;
+	    vvp_object_t item;
+      };
+      std::vector<put_waiter_t> put_waiters_;
+};
+
+/*
+ * vvp_semaphore -- runtime representation of SystemVerilog's
+ * built-in semaphore.
+ *
+ * A semaphore has an integer key count. get(N) blocks until at least
+ * N keys are available, then decrements by N.  put(N) adds N keys
+ * and wakes any waiters.  try_get(N) is non-blocking.
+ */
+class vvp_semaphore : public vvp_object {
+    public:
+      explicit vvp_semaphore(size_t initial_count = 0);
+      ~vvp_semaphore() override;
+
+      vvp_object* duplicate() const override;
+      void shallow_copy(const vvp_object*) override {}
+
+      size_t count() const { return count_; }
+
+      /* try_get: if count >= n, decrement and return true; else false. */
+      bool try_get(size_t n = 1);
+
+      /* put: add n keys and wake suspended threads. */
+      void put(size_t n = 1);
+
+      /* get: block thr until count >= n, then decrement.
+       * Returns true if immediately satisfied, false if thr is suspended. */
+      bool get(vthread_t thr, size_t n = 1);
+
+    private:
+      void resume_waiters_();
+
+      size_t count_;
+
+      struct waiter_t {
+	    vthread_t thr;
+	    size_t n;
+      };
+      std::vector<waiter_t> waiters_;
+};
+
+/*
+ * vvp_boxed_vec4 -- wraps a vec4 (integer/bit) value as a vvp_object_t so
+ * that it can be stored in a mailbox alongside class objects.  Used when
+ * mailbox #(T) is instantiated with a non-class element type (e.g. int).
+ */
+class vvp_boxed_vec4 : public vvp_object {
+    public:
+      explicit vvp_boxed_vec4(const vvp_vector4_t& v) : value_(v) {}
+      ~vvp_boxed_vec4() override {}
+
+      vvp_object* duplicate() const override {
+	    return new vvp_boxed_vec4(value_);
+      }
+      void shallow_copy(const vvp_object* src) override {
+	    if (const vvp_boxed_vec4* s = dynamic_cast<const vvp_boxed_vec4*>(src))
+		  value_ = s->value_;
+      }
+
+      const vvp_vector4_t& get_value() const { return value_; }
+
+    private:
+      vvp_vector4_t value_;
+};
+
+#endif /* VVP_MAILBOX_H */