diff --git a/erlang/apps/lib/src/libelf.erl b/erlang/apps/lib/src/libelf.erl
index 951b849..ebbef28 100644
--- a/erlang/apps/lib/src/libelf.erl
+++ b/erlang/apps/lib/src/libelf.erl
@@ -707,7 +707,7 @@ read_Word(FP)  -> read_uint36(FP).
 
 read_sint36(FP) ->
   case read_uint36(FP) of
-    {ok, UInt36} -> {ok, (UInt36 bxor (1 bsl 35)) - (1 bsl 35)}; % sign-extend
+    {ok, UInt36} -> {ok, sext:sext(UInt36, 36)};
     {error, _Reason} = Error -> Error
   end.
 
diff --git a/erlang/apps/lib/src/sext.erl b/erlang/apps/lib/src/sext.erl
new file mode 100644
index 0000000..dcf085f
--- /dev/null
+++ b/erlang/apps/lib/src/sext.erl
@@ -0,0 +1,30 @@
+%%% -*- erlang-indent-level: 2 -*-
+%%%
+%%% Sign-extend integers of given widths.
+%%% Copyright (C) 2013-2025  Mikael Pettersson
+%%%
+%%% This file is part of pdp10-tools.
+%%%
+%%% pdp10-tools is free software: you can redistribute it and/or modify
+%%% it under the terms of the GNU General Public License as published by
+%%% the Free Software Foundation, either version 3 of the License, or
+%%% (at your option) any later version.
+%%%
+%%% pdp10-tools is distributed in the hope that it will be useful,
+%%% but WITHOUT ANY WARRANTY; without even the implied warranty of
+%%% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+%%% GNU General Public License for more details.
+%%%
+%%% You should have received a copy of the GNU General Public License
+%%% along with pdp10-tools.  If not, see <http://www.gnu.org/licenses/>.
+
+-module(sext).
+
+-export([ sext/2
+        ]).
+
+-spec sext(non_neg_integer(), pos_integer()) -> integer().
+sext(Word, Width) ->
+  SignBit = 1 bsl (Width - 1),
+  Max = (1 bsl Width) - 1,
+  ((Word band Max) bxor SignBit) - SignBit.
diff --git a/erlang/apps/od/src/od.erl b/erlang/apps/od/src/od.erl
index aa1c110..034048e 100644
--- a/erlang/apps/od/src/od.erl
+++ b/erlang/apps/od/src/od.erl
@@ -1,7 +1,7 @@
 %%% -*- erlang-indent-level: 2 -*-
 %%%
 %%% 'od' for files with 9-bit bytes
-%%% Copyright (C) 2013-2023  Mikael Pettersson
+%%% Copyright (C) 2013-2025  Mikael Pettersson
 %%%
 %%% This file is part of pdp10-tools.
 %%%
@@ -333,7 +333,7 @@ print_number(Opts, UInt) ->
   OutputType = Opts#options.output_type,
   Number =
     case OutputType of
-      $d -> sign_extend(UInt, 9 * Opts#options.bytes_per_datum);
+      $d -> sext:sext(UInt, 9 * Opts#options.bytes_per_datum);
       _ -> UInt
     end,
   {Base, Pad} =
@@ -345,11 +345,6 @@ print_number(Opts, UInt) ->
     end,
   io:format(" ~*.*.*B", [Opts#options.chars_per_datum, Base, Pad, Number]).
 
-sign_extend(UInt, NrBits) ->
-  Max = (1 bsl NrBits) - 1,
-  SignBit = 1 bsl (NrBits - 1),
-  ((UInt band Max) bxor SignBit) - SignBit.
-
 bytes_to_uint(B1, B2) -> % PDP10 has big-endian byte order
   ((B1 band 16#1FF) bsl 9) bor (B2 band 16#1FF).
 
diff --git a/erlang/apps/sim/src/sim_arithmetic.erl b/erlang/apps/sim/src/sim_arithmetic.erl
index af4261e..b5d6a4f 100644
--- a/erlang/apps/sim/src/sim_arithmetic.erl
+++ b/erlang/apps/sim/src/sim_arithmetic.erl
@@ -1,7 +1,7 @@
 %%% -*- erlang-indent-level: 2 -*-
 %%%
 %%% simulator for pdp10-elf
-%%% Copyright (C) 2020  Mikael Pettersson
+%%% Copyright (C) 2020-2025  Mikael Pettersson
 %%%
 %%% This file is part of pdp10-tools.
 %%%
@@ -933,9 +933,7 @@ sub1(Word) ->
 %% Sign-extend a uint36_t() to the full width of its representation type.
 -spec sext36(uint36_t()) -> integer().
 sext36(X) ->
-  UInt36Sbit = 1 bsl (36 - 1),
-  UInt36Max = (1 bsl 36) - 1,
-  ((X band UInt36Max) bxor UInt36Sbit) - UInt36Sbit.
+  sext:sext(X, 36).
 
 set_non_zero_ac(Core, IR, Word) ->
   case IR band 8#17 of
diff --git a/erlang/apps/sim/src/sim_core.erl b/erlang/apps/sim/src/sim_core.erl
index c351302..9c3d9ae 100644
--- a/erlang/apps/sim/src/sim_core.erl
+++ b/erlang/apps/sim/src/sim_core.erl
@@ -1,7 +1,7 @@
 %%% -*- erlang-indent-level: 2 -*-
 %%%
 %%% simulator for pdp10-elf
-%%% Copyright (C) 2018-2020  Mikael Pettersson
+%%% Copyright (C) 2018-2025  Mikael Pettersson
 %%%
 %%% This file is part of pdp10-tools.
 %%%
@@ -606,9 +606,7 @@ bit36(X, BIT) -> bits36(X, BIT, BIT).
 %% Sign-extend a uint18_t() to the full width of its representation type.
 -spec sext18(uint18_t()) -> integer().
 sext18(X) ->
-  UInt18Sbit = 1 bsl (18 - 1),
-  UInt18Max = (1 bsl 18) - 1,
-  ((X band UInt18Max) bxor UInt18Sbit) - UInt18Sbit.
+  sext:sext(X, 18).
 
 %% This implements Word := C(E).
 -spec c(#core{}, sim_mem:mem(), #ea{})