diff --git a/erlang/apps/lib/include/pdp10_opcodes.hrl b/erlang/apps/lib/include/pdp10_opcodes.hrl
index e727794..3d8f917 100644
--- a/erlang/apps/lib/include/pdp10_opcodes.hrl
+++ b/erlang/apps/lib/include/pdp10_opcodes.hrl
@@ -203,7 +203,6 @@
           %% the first word, and the A_OPCODE(00) EXTENDED format for the second word.
         , high13 :: 0..((1 bsl 13) - 1)
         , format :: pdp10_insn_format()
-        , a_unused = false :: boolean()
         , e_unused = false :: boolean()
         , extended = false :: boolean()
         , models :: pdp10_cpu_models()
diff --git a/erlang/apps/lib/src/pdp10_opcodes.erl b/erlang/apps/lib/src/pdp10_opcodes.erl
index 7340fd4..7ce6873 100644
--- a/erlang/apps/lib/src/pdp10_opcodes.erl
+++ b/erlang/apps/lib/src/pdp10_opcodes.erl
@@ -217,11 +217,13 @@ high13_keys(High13, Shift, Value, Keys) ->
 -define(A_OPCODE(OPCODE15), high13 = (OPCODE15) bsr 2, format = ?PDP10_INSN_A_OPCODE).
 -define(A_NONZERO(OPCODE9), high13 = (OPCODE9) bsl 4, format = ?PDP10_INSN_A_NONZERO).
 -define(IO(OPCODE15), high13 = (OPCODE15) bsr 2, format = ?PDP10_INSN_IO).
--define(A_UNUSED(OPCODE9), ?BASIC(OPCODE9), a_unused = true).
 -define(E_UNUSED(Opcode), Opcode, e_unused = true).
 -define(SECTION_ZERO(Opcode), Opcode, section0 = true).
 -define(SECTION_NONZERO(Opcode), Opcode, section0 = false).
 
+%% A_UNUSED is shorthand for A_OPCODE with the opcode bits in A set to zero.
+-define(A_UNUSED(OPCODE9), high13 = (OPCODE9) bsl 4, format = ?PDP10_INSN_A_OPCODE).
+
 %% The .priority field is used to disambiguate lookups when a key maps to
 %% multiple descriptors, and the models are not mutually exclusive.  This
 %% matters mostly for disassembly, i.e. insn_from_high13/4.