diff --git a/erlang/apps/sim/src/sim_arithmetic.erl b/erlang/apps/sim/src/sim_arithmetic.erl
new file mode 100644
index 0000000..fe8f4ff
--- /dev/null
+++ b/erlang/apps/sim/src/sim_arithmetic.erl
@@ -0,0 +1,78 @@
+%%% -*- erlang-indent-level: 2 -*-
+%%%
+%%% simulator for pdp10-elf
+%%% Copyright (C) 2020 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 .
+%%%
+%%%=============================================================================
+%%%
+%%% 2.6 Arithmetic Testing
+
+-module(sim_arithmetic).
+
+-export([ handle_AOBJN/4
+ , handle_AOBJP/4
+ ]).
+
+-include("sim_core.hrl").
+
+%% 2.6.1 Add One to Both Halves of AC and Jump =================================
+
+%% AOBJP - Add One to Both Halves of AC and Jump if Positive
+
+-spec handle_AOBJP(#core{}, sim_mem:mem(), IR :: word(), #ea{})
+ -> {#core{}, sim_mem:mem(), {ok, integer()} | {error, {module(), term()}}}.
+handle_AOBJP(Core, Mem, IR, EA) ->
+ %% Note: KA10 incorrectly does a 36-bit add with 8#000001000001 here instead
+ %% of adjusting the two halves separately.
+ AC = IR band 8#17,
+ CA = sim_core:get_ac(Core, AC),
+ Left = CA bsr 18,
+ Right = CA band ((1 bsl 18) - 1),
+ Left1 = (Left + 1) band ((1 bsl 18) - 1),
+ Right1 = (Right + 1) band ((1 bsl 18) - 1),
+ CA1 = (Left1 bsl 18) bor Right1,
+ Core1 = sim_core:set_ac(Core, AC, CA1),
+ case Left1 band (1 bsl 17) of
+ 0 -> jump(Core1, Mem, EA);
+ _ -> sim_core:next_pc(Core1, Mem)
+ end.
+
+%% AOBJN - Add One to Both Halves of AC and Jump if Negative
+
+-spec handle_AOBJN(#core{}, sim_mem:mem(), IR :: word(), #ea{})
+ -> {#core{}, sim_mem:mem(), {ok, integer()} | {error, {module(), term()}}}.
+handle_AOBJN(Core, Mem, IR, EA) ->
+ %% Note: KA10 incorrectly does a 36-bit add with 8#000001000001 here instead
+ %% of adjusting the two halves separately.
+ AC = IR band 8#17,
+ CA = sim_core:get_ac(Core, AC),
+ Left = CA bsr 18,
+ Right = CA band ((1 bsl 18) - 1),
+ Left1 = (Left + 1) band ((1 bsl 18) - 1),
+ Right1 = (Right + 1) band ((1 bsl 18) - 1),
+ CA1 = (Left1 bsl 18) bor Right1,
+ Core1 = sim_core:set_ac(Core, AC, CA1),
+ case Left1 band (1 bsl 17) of
+ 0 -> sim_core:next_pc(Core1, Mem);
+ _ -> jump(Core1, Mem, EA)
+ end.
+
+%% Miscellaneous ===============================================================
+
+jump(Core, Mem, EA) ->
+ sim_core:run(Core#core{pc_offset = EA#ea.offset}, Mem).
diff --git a/erlang/apps/sim/src/sim_core.erl b/erlang/apps/sim/src/sim_core.erl
index ec812d3..0a76203 100644
--- a/erlang/apps/sim/src/sim_core.erl
+++ b/erlang/apps/sim/src/sim_core.erl
@@ -282,6 +282,8 @@ dispatch(Core, Mem, IR, EA) ->
8#217 -> sim_moves:handle_MOVMS(Core, Mem, IR, EA);
8#250 -> sim_moves:handle_EXCH(Core, Mem, IR, EA);
8#251 -> sim_moves:handle_BLT(Core, Mem, IR, EA);
+ 8#252 -> sim_arithmetic:handle_AOBJP(Core, Mem, IR, EA);
+ 8#253 -> sim_arithmetic:handle_AOBJN(Core, Mem, IR, EA);
8#260 -> sim_stack:handle_PUSHJ(Core, Mem, IR, EA);
8#261 -> sim_stack:handle_PUSH(Core, Mem, IR, EA);
8#262 -> sim_stack:handle_POP(Core, Mem, IR, EA);
diff --git a/erlang/apps/sim/test/sim_arithmetic_tests.erl b/erlang/apps/sim/test/sim_arithmetic_tests.erl
new file mode 100644
index 0000000..7cdb27a
--- /dev/null
+++ b/erlang/apps/sim/test/sim_arithmetic_tests.erl
@@ -0,0 +1,144 @@
+%%% -*- erlang-indent-level: 2 -*-
+%%%
+%%% simulator for pdp10-elf
+%%% Copyright (C) 2020 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 .
+%%%
+%%%=============================================================================
+%%%
+%%% Test cases for 2.6 Arithmetic Testing
+
+-module(sim_arithmetic_tests).
+
+-include("../src/sim_core.hrl").
+-include_lib("eunit/include/eunit.hrl").
+
+-define(DEFAULT_FLAGS, (1 bsl ?PDP10_PF_USER)).
+
+-define(LOW18(X), ((X) band ((1 bsl 18) - 1))).
+
+-define(INSN(OP, AC, I, X, Y),
+ (((OP) bsl (35 - 8)) bor
+ ((AC) bsl (35 - 12)) bor
+ ((I) bsl (35 - 13)) bor
+ ((X) bsl (35 - 17)) bor
+ ?LOW18(Y))).
+
+-define(COMMA2(LEFT, RIGHT), ((?LOW18(LEFT) bsl 18) bor ?LOW18(RIGHT))). % LEFT,,RIGHT in MACRO-10
+
+-define(EA(S, O), #ea{section = S, offset = O, islocal = false}).
+-define(AC(A), ?EA(1, A)).
+
+-define(INSN_INVALID, ?INSN(0, 0, 0, 0, 0)).
+
+-define(OP_MOVSI, 8#205).
+-define(OP_AOBJP, 8#252).
+-define(OP_AOBJN, 8#253).
+
+%% 2.6.1 Add One to Both Halves of AC and Jump =================================
+
+%% AOBJP - Add One to Both Halves of AC and Jump if Positive
+
+aobjp_test() ->
+ Prog1 =
+ [ {1, 8#100, ?INSN(?OP_MOVSI, 1, 0, 0, -2)} % 1,,100/ MOVSI 1,-2
+ , {1, 8#101, ?INSN(?OP_AOBJP, 1, 0, 0, 8#103)} % 1,,101/ AOBJP 1,103
+ , {1, 8#102, ?INSN_INVALID} % 1,,102/
+ , {1, 8#103, ?INSN_INVALID} % 1,,103/
+ ],
+ expect(Prog1, [], {1, 8#102}, ?DEFAULT_FLAGS, % no jump
+ [{?AC(1), ?COMMA2(-1, 1)} % AC1 = -1,,1
+ ]),
+ Prog2 =
+ [ {1, 8#100, ?INSN(?OP_MOVSI, 1, 0, 0, -1)} % 1,,100/ MOVSI 1,-1
+ , {1, 8#101, ?INSN(?OP_AOBJP, 1, 0, 0, 8#103)} % 1,,101/ AOBJP 1,103
+ , {1, 8#102, ?INSN_INVALID} % 1,,102/
+ , {1, 8#103, ?INSN_INVALID} % 1,,103/
+ ],
+ expect(Prog2, [], {1, 8#103}, ?DEFAULT_FLAGS, % jump
+ [{?AC(1), ?COMMA2(0, 1)} % AC1 = 0,,1
+ ]).
+
+%% AOBJN - Add One to Both Halves of AC and Jump if Negative
+
+aobjn_test() ->
+ Prog1 =
+ [ {1, 8#100, ?INSN(?OP_MOVSI, 1, 0, 0, -2)} % 1,,100/ MOVSI 1,-2
+ , {1, 8#101, ?INSN(?OP_AOBJN, 1, 0, 0, 8#103)} % 1,,101/ AOBJN 1,103
+ , {1, 8#102, ?INSN_INVALID} % 1,,102/
+ , {1, 8#103, ?INSN_INVALID} % 1,,103/
+ ],
+ expect(Prog1, [], {1, 8#103}, ?DEFAULT_FLAGS, % jump
+ [{?AC(1), ?COMMA2(-1, 1)} % AC1 = -1,,1
+ ]),
+ Prog2 =
+ [ {1, 8#100, ?INSN(?OP_MOVSI, 1, 0, 0, -1)} % 1,,100/ MOVSI 1,-1
+ , {1, 8#101, ?INSN(?OP_AOBJN, 1, 0, 0, 8#103)} % 1,,101/ AOBJN 1,103
+ , {1, 8#102, ?INSN_INVALID} % 1,,102/
+ , {1, 8#103, ?INSN_INVALID} % 1,,103/
+ ],
+ expect(Prog2, [], {1, 8#102}, ?DEFAULT_FLAGS, % no jump
+ [{?AC(1), ?COMMA2(0, 1)} % AC1 = 0,,1
+ ]).
+
+%% Common code to run short sequences ==========================================
+
+expect(Prog, ACs, ExpectedPC, ExpectedFlags, ExpectedEs) ->
+ {Core, Mem} = init(Prog, ACs),
+ {Core1, Mem1, {error, {sim_core, {dispatch, PC, _IR, _EA}}}} = sim_core:run(Core, Mem),
+ ActualPC = {PC bsr 18, PC band ((1 bsl 18) - 1)},
+ ?assertEqual(ExpectedPC, ActualPC),
+ ?assertEqual(ExpectedFlags, Core1#core.flags),
+ lists:foreach(fun({EA, ExpectedE}) ->
+ {ok, ActualE} = sim_core:c(Core1, Mem1, EA),
+ ?assertEqual(ExpectedE, ActualE)
+ end, ExpectedEs),
+ sim_mem:delete(Mem).
+
+init(Prog, ACs) ->
+ {PCSection, PCOffset} = prog_pc(Prog),
+ Mem = init_mem(Prog),
+ Core = init_core(PCSection, PCOffset, ACs),
+ {Core, Mem}.
+
+prog_pc([{Section, Offset, _Word} | _Rest]) -> {Section, Offset}.
+
+init_mem(Prog) -> init_mem(Prog, sim_mem:new()).
+
+init_mem([], Mem) -> Mem;
+init_mem([{Section, Offset, Word} | Rest], Mem) ->
+ init_word(Section, Offset, Word, Mem),
+ init_mem(Rest, Mem).
+
+init_word(Section, Offset, Word, Mem) ->
+ Address = (Section bsl 18) bor Offset,
+ PFN = Address bsr 9,
+ case sim_mem:mquery(Mem, PFN) of
+ false -> sim_mem:mmap(Mem, PFN, 4+2, core);
+ {_Prot, _What} -> ok
+ end,
+ ok = sim_mem:write_word(Mem, Address, Word).
+
+init_core(PCSection, PCOffset, ACs) ->
+ #core{ pc_section = PCSection
+ , pc_offset = PCOffset
+ , acs = init_acs(ACs, list_to_tuple(lists:duplicate(16, 0)))
+ , flags = ?DEFAULT_FLAGS
+ }.
+
+init_acs([], ACS) -> ACS;
+init_acs([{AC, Val} | Rest], ACS) -> init_acs(Rest, setelement(AC + 1, Val, ACS)).