diff --git a/erlang/apps/sim/src/sim_core.erl b/erlang/apps/sim/src/sim_core.erl
index 5b44cc1..c0648dd 100644
--- a/erlang/apps/sim/src/sim_core.erl
+++ b/erlang/apps/sim/src/sim_core.erl
@@ -325,6 +325,10 @@ dispatch(Core, Mem, IR, EA) ->
     8#511 -> sim_boolean:handle_SETZ(Core, Mem, IR, EA); % HLLZI = SETZ
     8#512 -> sim_halfword:handle_HLLZM(Core, Mem, IR, EA);
     8#513 -> sim_halfword:handle_HLLZS(Core, Mem, IR, EA);
+    8#520 -> sim_halfword:handle_HLLO(Core, Mem, IR, EA);
+    8#521 -> sim_halfword:handle_HLLOI(Core, Mem, IR, EA);
+    8#522 -> sim_halfword:handle_HLLOM(Core, Mem, IR, EA);
+    8#523 -> sim_halfword:handle_HLLOS(Core, Mem, IR, EA);
     _ ->
       PC = (Core#core.pc_section bsl 18) bor Core#core.pc_offset,
       {Core, Mem, {error, {?MODULE, {dispatch, PC, IR, EA}}}}
diff --git a/erlang/apps/sim/src/sim_halfword.erl b/erlang/apps/sim/src/sim_halfword.erl
index dc18f6a..549ff18 100644
--- a/erlang/apps/sim/src/sim_halfword.erl
+++ b/erlang/apps/sim/src/sim_halfword.erl
@@ -28,6 +28,10 @@
         , handle_HLLI/4
         , handle_HLLM/4
         , handle_HLLS/4
+        , handle_HLLO/4
+        , handle_HLLOI/4
+        , handle_HLLOM/4
+        , handle_HLLOS/4
         , handle_HLLZ/4
         , handle_HLLZM/4
         , handle_HLLZS/4
@@ -134,6 +138,49 @@ handle_HLLZS(Core, Mem, IR, EA) ->
                           fun(Core1, Mem1) -> ?FUNCTION_NAME(Core1, Mem1, IR, EA) end)
   end.
 
+%% HLLO - Half Word Left to Left, Ones
+
+-spec handle_HLLO(#core{}, sim_mem:mem(), IR :: word(), #ea{})
+      -> {#core{}, sim_mem:mem(), {ok, integer()} | {error, {module(), term()}}}.
+handle_HLLO(Core, Mem, IR, EA) ->
+  case sim_core:c(Core, Mem, EA) of
+    {ok, CE} ->
+      AC = IR band 8#17,
+      Word = set_left_ones(get_left(CE)),
+      sim_core:next_pc(sim_core:set_ac(Core, AC, Word), Mem);
+    {error, Reason} ->
+      sim_core:page_fault(Core, Mem, ea_address(EA), read, Reason,
+                          fun(Core1, Mem1) -> ?FUNCTION_NAME(Core1, Mem1, IR, EA) end)
+  end.
+
+-spec handle_HLLOI(#core{}, sim_mem:mem(), IR :: word(), #ea{})
+      -> {#core{}, sim_mem:mem(), {ok, integer()} | {error, {module(), term()}}}.
+handle_HLLOI(Core, Mem, IR, _EA) ->
+  AC = IR band 8#17,
+  Word = (1 bsl 18) - 1,
+  sim_core:next_pc(sim_core:set_ac(Core, AC, Word), Mem).
+
+-spec handle_HLLOM(#core{}, sim_mem:mem(), IR :: word(), #ea{})
+      -> {#core{}, sim_mem:mem(), {ok, integer()} | {error, {module(), term()}}}.
+handle_HLLOM(Core, Mem, IR, EA) ->
+  AC = IR band 8#17,
+  CA = sim_core:get_ac(Core, AC),
+  Word = set_left_ones(get_left(CA)),
+  handle_writeback(Core, Mem, EA, Word).
+
+-spec handle_HLLOS(#core{}, sim_mem:mem(), IR :: word(), #ea{})
+      -> {#core{}, sim_mem:mem(), {ok, integer()} | {error, {module(), term()}}}.
+handle_HLLOS(Core, Mem, IR, EA) ->
+  case sim_core:c(Core, Mem, EA) of
+    {ok, CE} ->
+      AC = IR band 8#17,
+      Word = set_left_ones(get_left(CE)),
+      handle_writeback(Core, Mem, AC, EA, Word);
+    {error, Reason} ->
+      sim_core:page_fault(Core, Mem, ea_address(EA), read, Reason,
+                          fun(Core1, Mem1) -> ?FUNCTION_NAME(Core1, Mem1, IR, EA) end)
+  end.
+
 %% Miscellaneous ===============================================================
 
 handle_writeback(Core, Mem, EA, Word) ->
@@ -164,4 +211,6 @@ get_right(Word) -> Word band ((1 bsl 18) - 1).
 
 set_left(Word, Left) -> get_right(Word) bor (Left bsl 18).
 
+set_left_ones(Left) -> (Left bsl 18) bor ((1 bsl 18) - 1).
+
 set_left_zeros(Left) -> Left bsl 18.
diff --git a/erlang/apps/sim/test/sim_halfword_tests.erl b/erlang/apps/sim/test/sim_halfword_tests.erl
index 143def2..cc4c8df 100644
--- a/erlang/apps/sim/test/sim_halfword_tests.erl
+++ b/erlang/apps/sim/test/sim_halfword_tests.erl
@@ -53,6 +53,10 @@
 -define(OP_HLLZI, 8#511).
 -define(OP_HLLZM, 8#512).
 -define(OP_HLLZS, 8#513).
+-define(OP_HLLO,  8#520).
+-define(OP_HLLOI, 8#521).
+-define(OP_HLLOM, 8#522).
+-define(OP_HLLOS, 8#523).
 
 %% 2.8 Half-Word Data Transmission =============================================
 
@@ -195,6 +199,64 @@ hllzs_no_ac_test() ->
          , {#ea{section = 1, offset = 0, islocal = false}, ?COMMA2(0, 1)} % AC0 = 0,,1
          ]).
 
+%% HLLO - Half Word Left to Left, Ones
+
+hllo_test() ->
+  Prog =
+    [ {1, 8#100, ?INSN(?OP_MOVEI, 1, 0, 0, 1)}     % 1,,100/ MOVEI 1,1
+    , {1, 8#101, ?INSN(?OP_HLLO, 1, 0, 0, 8#200)}  % 1,,101/ HLLO 1,200
+    , {1, 8#102, ?INSN_INVALID}                    % 1,,102/ <invalid>
+    , {1, 8#200, ?COMMA2(1, 0)}                    % 1,,200/ 1,,0
+    ],
+  expect(Prog, [], {1, 8#102}, ?DEFAULT_FLAGS,
+         [ {#ea{section = 1, offset = 1, islocal = false}, ?COMMA2(1, -1)} % AC1 = 1,,-1
+         ]).
+
+hlloi_test() ->
+  Prog =
+    [ {1, 8#100, ?INSN(?OP_MOVEI, 1, 0, 0, 1)}     % 0,,100/ MOVEI 1,1
+    , {1, 8#101, ?INSN(?OP_HLLOI, 1, 0, 0, 0)}     % 0,,101/ HLLOI 1,0
+    , {1, 8#102, ?INSN_INVALID}                    % 0,,102/ <invalid>
+    ],
+  expect(Prog, [], {1, 8#102}, ?DEFAULT_FLAGS,
+         [ {#ea{section = 1, offset = 1, islocal = false}, ?COMMA2(0, -1)} % AC1 = 0,,-1
+         ]).
+
+hllom_test() ->
+  Prog =
+    [ {1, 8#100, ?INSN(?OP_MOVSI, 1, 0, 0, 1)}     % 1,,100/ MOVSI 1,1
+    , {1, 8#101, ?INSN(?OP_HLLOM, 1, 0, 0, 8#200)} % 1,,101/ HLLOM 1,200
+    , {1, 8#102, ?INSN_INVALID}                    % 1,,102/ <invalid>
+    , {1, 8#200, ?COMMA2(0, 1)}                    % 1,,200/ 0,,1
+    ],
+  expect(Prog, [], {1, 8#102}, ?DEFAULT_FLAGS,
+         [ {#ea{section = 1, offset = 8#200, islocal = false}, ?COMMA2(1, -1)} % C(1,,200) = 1,,-1
+         ]).
+
+hllos_ac_test() ->
+  Prog =
+    [ {1, 8#100, ?INSN(?OP_MOVEI, 1, 0, 0, 1)}     % 1,,100/ MOVEI 1,1
+    , {1, 8#101, ?INSN(?OP_HLLOS, 1, 0, 0, 8#200)} % 1,,101/ HLLOS 1,200
+    , {1, 8#102, ?INSN_INVALID}                    % 1,,102/ <invalid>
+    , {1, 8#200, ?COMMA2(1, 1)}                    % 1,,200/ 1,,1
+    ],
+  expect(Prog, [], {1, 8#102}, ?DEFAULT_FLAGS,
+         [ {#ea{section = 1, offset = 8#200, islocal = false}, ?COMMA2(1, -1)} % C(1,,200) = 1,,-1
+         , {#ea{section = 1, offset = 1, islocal = false}, ?COMMA2(1, -1)} % AC1 = 1,,-1
+         ]).
+
+hllos_no_ac_test() ->
+  Prog =
+    [ {1, 8#100, ?INSN(?OP_MOVEI, 0, 0, 0, 1)}     % 1,,100/ MOVEI 0,1
+    , {1, 8#101, ?INSN(?OP_HLLOS, 0, 0, 0, 8#200)} % 1,,101/ HLLOS 0,200
+    , {1, 8#102, ?INSN_INVALID}                    % 1,,102/ <invalid>
+    , {1, 8#200, ?COMMA2(1, 0)}                    % 1,,200/ 1,,0
+    ],
+  expect(Prog, [], {1, 8#102}, ?DEFAULT_FLAGS,
+         [ {#ea{section = 1, offset = 8#200, islocal = false}, ?COMMA2(1, -1)} % C(1,,200) = 1,,-1
+         , {#ea{section = 1, offset = 0, islocal = false}, ?COMMA2(0, 1)} % AC0 = 0,,1
+         ]).
+
 %% Common code to run short sequences ==========================================
 
 expect(Prog, ACs, ExpectedPC, ExpectedFlags, ExpectedEs) ->