diff --git a/vvp/opcodes.txt b/vvp/opcodes.txt
index b6b6a4c90..f8275e9a8 100644
--- a/vvp/opcodes.txt
+++ b/vvp/opcodes.txt
@@ -353,8 +353,7 @@ thread bit vector.
 
 This instruction adds, subtracts, or multiplies an immediate value to
 the addressed index register. The index register holds numeric values,
-so the <value> is a number. The <idx> value selects the index register,
-and may be 0, 1, 2 or 3.
+so the <value> is a number. The <idx> value selects the index register.
 
 
 * %jmp <code-label>
diff --git a/vvp/vthread.cc b/vvp/vthread.cc
index d8bde41d0..5915a71bf 100644
--- a/vvp/vthread.cc
+++ b/vvp/vthread.cc
@@ -1840,19 +1840,19 @@ bool of_INV(vthread_t thr, vvp_code_t cp)
 
 bool of_IX_ADD(vthread_t thr, vvp_code_t cp)
 {
-  thr->words[cp->bit_idx[0] & 3].w_int += cp->number;
+  thr->words[cp->bit_idx[0]].w_int += cp->number;
   return true;
 }
 
 bool of_IX_SUB(vthread_t thr, vvp_code_t cp)
 {
-  thr->words[cp->bit_idx[0] & 3].w_int -= cp->number;
+  thr->words[cp->bit_idx[0]].w_int -= cp->number;
   return true;
 }
 
 bool of_IX_MUL(vthread_t thr, vvp_code_t cp)
 {
-  thr->words[cp->bit_idx[0] & 3].w_int *= cp->number;
+  thr->words[cp->bit_idx[0]].w_int *= cp->number;
   return true;
 }