diff --git a/erlang/apps/lib/src/pdp10_extint.erl b/erlang/apps/lib/src/pdp10_extint.erl
new file mode 100644
index 0000000..1118dbf
--- /dev/null
+++ b/erlang/apps/lib/src/pdp10_extint.erl
@@ -0,0 +1,54 @@
+%%% -*- erlang-indent-level: 2 -*-
+%%%
+%%% converts between 18 and 36-bit integers and sequences of nonets
+%%% Copyright (C) 2013-2019  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(pdp10_extint).
+
+-export([ uint18_to_ext/1
+        , uint18_from_ext/1
+        , uint36_to_ext/1
+        , uint36_from_ext/1
+        ]).
+
+-include_lib("lib/include/pdp10_stdint.hrl").
+
+%% The PDP10 is big-endian, so the conversions here are big-endian.
+%% The bytes (nonets) are numbered in storage order.
+
+-spec uint18_to_ext(uint18_t()) -> [uint9_t()].
+uint18_to_ext(U18) ->
+  [_B0 = (U18 bsr 9) band ?PDP10_UINT9_MAX, _B1 = U18 band ?PDP10_UINT9_MAX].
+
+-spec uint18_from_ext([uint9_t()]) -> uint18_t().
+uint18_from_ext([B0, B1]) ->
+  ((B0 band ?PDP10_UINT9_MAX) bsl 9) bor (B1 band ?PDP10_UINT9_MAX).
+
+-spec uint36_to_ext(uint36_t()) -> [uint9_t()].
+uint36_to_ext(U36) ->
+  [_B0 = (U36 bsr 27) band ?PDP10_UINT9_MAX,
+   _B1 = (U36 bsr 18) band ?PDP10_UINT9_MAX,
+   _B2 = (U36 bsr  9) band ?PDP10_UINT9_MAX,
+   _B3 =  U36         band ?PDP10_UINT9_MAX].
+
+-spec uint36_from_ext([uint9_t()]) -> uint36_t().
+uint36_from_ext([B0, B1, B2, B3]) ->
+  ((B0 band ?PDP10_UINT9_MAX) bsl 27) bor
+  ((B1 band ?PDP10_UINT9_MAX) bsl 18) bor
+  ((B2 band ?PDP10_UINT9_MAX) bsl  9) bor
+   (B3 band ?PDP10_UINT9_MAX).