diff --git a/bin/sixtypical b/bin/sixtypical
index ff6d85f..7cdff71 100755
--- a/bin/sixtypical
+++ b/bin/sixtypical
@@ -58,5 +58,4 @@ if __name__ == '__main__':
 
         if options.execute:
             context = eval_program(program)
-            for key, value in sorted(context.iteritems()):
-                print "%s: %s" % (key, value)
+            print str(context)
diff --git a/src/sixtypical/analyzer.py b/src/sixtypical/analyzer.py
index e3d879b..d0feae5 100644
--- a/src/sixtypical/analyzer.py
+++ b/src/sixtypical/analyzer.py
@@ -158,7 +158,7 @@ def analyze_instr(instr, context, routines):
         context.assert_writeable(dest, FLAG_Z, FLAG_N)
         context.set_initialized(dest, FLAG_Z, FLAG_N)
     elif opcode in ('shl', 'shr'):
-        context.assert_initialized(dest)
+        context.assert_initialized(dest, FLAG_C)
         context.assert_writeable(dest, FLAG_Z, FLAG_N, FLAG_C)
         context.set_initialized(dest, FLAG_Z, FLAG_N, FLAG_C)
     elif opcode == 'call':
diff --git a/src/sixtypical/evaluator.py b/src/sixtypical/evaluator.py
index b3cf96a..07c108f 100644
--- a/src/sixtypical/evaluator.py
+++ b/src/sixtypical/evaluator.py
@@ -1,31 +1,38 @@
 # encoding: UTF-8
 
 from sixtypical.ast import Program, Routine, Block, Instr
-from sixtypical.model import ConstantRef, LocationRef
+from sixtypical.model import (
+    ConstantRef, LocationRef,
+    REG_A, REG_X, REG_Y, FLAG_Z, FLAG_N, FLAG_V, FLAG_C
+)
 
 
-# TODO: should not inherit from dict
-class Context(dict):
+class Context(object):
+    def __init__(self):
+        self._store = {}
+
+    def __str__(self):
+        return '\n'.join("%s: %s" % p for p in sorted(self._store.iteritems()))
+
     def get(self, ref):
         if isinstance(ref, ConstantRef):
             return ref.value
         elif isinstance(ref, LocationRef):
-            return self[ref.name]
+            return self._store[ref.name]
         else:
             raise ValueError(ref)
 
     def set(self, ref, value):
         assert isinstance(ref, LocationRef)
-        self[ref.name] = value
+        self._store[ref.name] = value
 
 
 def eval_program(program):
     assert isinstance(program, Program)
     routines = {r.name: r for r in program.routines}
-    context = Context({
-        'a': 0, 'x': 0, 'y': 0,
-        'c': 0, 'n': 0, 'z': 0, 'v': 0
-    })
+    context = Context()
+    for ref in (REG_A, REG_X, REG_Y, FLAG_Z, FLAG_N, FLAG_V, FLAG_C):
+        context.set(ref, 0)
     eval_routine(routines['main'], context, routines)
     return context
 
@@ -49,90 +56,90 @@ def eval_instr(instr, context, routines):
 
     if opcode == 'ld':
         result = context.get(src)
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'st':
         context.set(dest, context.get(src))
     elif opcode == 'add':
-        carry = context['c']
+        carry = context.get(FLAG_C)
         val = context.get(src)
         now = context.get(dest)
         result = now + val + carry
         if result > 255:
             result &= 255
-            context['c'] = 1
+            context.set(FLAG_C, 1)
         else:
-            context['c'] = 0
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+            context.set(FLAG_C, 0)
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'sub':
-        carry = context['c']
+        carry = context.get(FLAG_C)
         val = context.get(src)
         now = context.get(dest)
         result = now - val - carry
         if result < 0:
             result &= 255
-            context['c'] = 1
+            context.set(FLAG_C, 1)
         else:
-            context['c'] = 0
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+            context.set(FLAG_C, 0)
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'inc':
         val = context.get(dest)
         result = (val + 1) & 255
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'dec':
         val = context.get(dest)
         result = (val - 1) & 255
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'cmp':
         val = context.get(src)
         now = context.get(dest)
         result = now - val
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         if result < 0:
             result &= 255
-            context['c'] = 1
+            context.set(FLAG_C, 1)
         else:
-            context['c'] = 0
+            context.set(FLAG_C, 0)
     elif opcode == 'and':
         result = context.get(dest) & context.get(src)
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'or':
         result = context.get(dest) | context.get(src)
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'xor':
         result = context.get(dest) ^ context.get(src)
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'shl':
         val = context.get(dest)
-        carry = context['c']
-        context['c'] = 1 if val & 128 else 0
+        carry = context.get(FLAG_C)
+        context.set(FLAG_C, 1 if val & 128 else 0)
         result = ((val << 1) + carry) & 255
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'shr':
         val = context.get(dest)
-        carry = context['c']
-        context['c'] = 1 if val & 1 else 0
+        carry = context.get(FLAG_C)
+        context.set(FLAG_C, 1 if val & 1 else 0)
         result = (val >> 1) + (carry * 128)
-        context['z'] = 1 if result == 0 else 0
-        context['n'] = 1 if result & 128 else 0
+        context.set(FLAG_Z, 1 if result == 0 else 0)
+        context.set(FLAG_N, 1 if result & 128 else 0)
         context.set(dest, result)
     elif opcode == 'call':
         eval_routine(routines[instr.name], context, routines)
diff --git a/tests/SixtyPical Analysis.md b/tests/SixtyPical Analysis.md
index 7516668..069e238 100644
--- a/tests/SixtyPical Analysis.md	
+++ b/tests/SixtyPical Analysis.md	
@@ -349,27 +349,167 @@ Location must be initialized and writeable.
 
 ### cmp ###
 
-TODO: write some tests
+Some rudimentary tests for cmp.
+
+    | routine main
+    |   inputs a
+    |   trashes z, c, n
+    | {
+    |     cmp a, 4
+    | }
+    = ok
+
+    | routine main
+    |   inputs a
+    |   trashes z, n
+    | {
+    |     cmp a, 4
+    | }
+    ? IllegalWriteError: c
+
+    | routine main
+    |   trashes z, c, n
+    | {
+    |     cmp a, 4
+    | }
+    ? UninitializedAccessError: a
 
 ### and ###
 
-TODO: write some tests
+Some rudimentary tests for and.
+
+    | routine main
+    |   inputs a
+    |   outputs a, z, n
+    | {
+    |     and a, 4
+    | }
+    = ok
+
+    | routine main
+    |   inputs a
+    |   trashes z, n
+    | {
+    |     and a, 4
+    | }
+    ? IllegalWriteError: a
+
+    | routine main
+    |   trashes z, n
+    | {
+    |     and a, 4
+    | }
+    ? UninitializedAccessError: a
 
 ### or ###
 
-TODO: write some tests
+Writing unit tests on a train.  Wow.
+
+    | routine main
+    |   inputs a
+    |   outputs a, z, n
+    | {
+    |     or a, 4
+    | }
+    = ok
+
+    | routine main
+    |   inputs a
+    |   trashes z, n
+    | {
+    |     or a, 4
+    | }
+    ? IllegalWriteError: a
+
+    | routine main
+    |   trashes z, n
+    | {
+    |     or a, 4
+    | }
+    ? UninitializedAccessError: a
 
 ### xor ###
 
-TODO: write some tests
+Writing unit tests on a train.  Wow.
+
+    | routine main
+    |   inputs a
+    |   outputs a, z, n
+    | {
+    |     xor a, 4
+    | }
+    = ok
+
+    | routine main
+    |   inputs a
+    |   trashes z, n
+    | {
+    |     xor a, 4
+    | }
+    ? IllegalWriteError: a
+
+    | routine main
+    |   trashes z, n
+    | {
+    |     xor a, 4
+    | }
+    ? UninitializedAccessError: a
 
 ### shl ###
 
-TODO: write some tests
+Some rudimentary tests for shl.
+
+    | routine main
+    |   inputs a, c
+    |   outputs a, c, z, n
+    | {
+    |     shl a
+    | }
+    = ok
+
+    | routine main
+    |   inputs a, c
+    |   outputs c, z, n
+    | {
+    |     shl a
+    | }
+    ? IllegalWriteError: a
+
+    | routine main
+    |   inputs a
+    |   outputs a, c, z, n
+    | {
+    |     shl a
+    | }
+    ? UninitializedAccessError: c
 
 ### shr ###
 
-TODO: write some tests
+Some rudimentary tests for shr.
+
+    | routine main
+    |   inputs a, c
+    |   outputs a, c, z, n
+    | {
+    |     shr a
+    | }
+    = ok
+
+    | routine main
+    |   inputs a, c
+    |   outputs c, z, n
+    | {
+    |     shr a
+    | }
+    ? IllegalWriteError: a
+
+    | routine main
+    |   inputs a
+    |   outputs a, c, z, n
+    | {
+    |     shr a
+    | }
+    ? UninitializedAccessError: c
 
 ### call ###