diff --git a/pdpasmhelper.py b/pdpasmhelper.py
index 581e8cf..3793e25 100644
--- a/pdpasmhelper.py
+++ b/pdpasmhelper.py
@@ -238,6 +238,9 @@ class PDP11InstructionAssembler:
     def dec(self, dst):
         return self._1op(0o005300, dst)
 
+    def decb(self, dst):
+        return self._1op(0o105300, dst)
+
     def neg(self, dst):
         return self._1op(0o005400, dst)
 
@@ -247,6 +250,9 @@ class PDP11InstructionAssembler:
     def tst(self, dst):
         return self._1op(0o005700, dst)
 
+    def tstb(self, dst):
+        return self._1op(0o105700, dst)
+
     def swab(self, dst):
         return self._1op(0o000300, dst)