diff --git a/tests/test_cpu_edge_cases.py b/tests/test_cpu_edge_cases.py
index 8937a07..b5dd270 100644
--- a/tests/test_cpu_edge_cases.py
+++ b/tests/test_cpu_edge_cases.py
@@ -51,6 +51,24 @@ def test_jmp_with_invalid_label(self):
         with pytest.raises(KeyError, match="Label nonexistent_label not found"):
             cpu.step()
 
+    def test_run_rebuilds_predecoded_when_missing_or_mismatched(self):
+        """Test fast-path rebuilds predecoded list when missing or length mismatched."""
+        cpu = CPU(trace_enabled=False)
+        cpu.load_program([("NOP", ())])
+        cpu._predecoded = []
+        cpu.run(max_steps=10, show_progress=False)
+        assert cpu.step_count == 1
+        assert len(cpu._predecoded) == 1
+
+    def test_run_fast_path_not_implemented_instruction(self):
+        """Test fast-path raises NotImplementedError for unknown instructions."""
+        cpu = CPU(trace_enabled=False)
+        cpu.load_program([("UNKNOWN_OPCODE", ())])
+        with pytest.raises(
+            NotImplementedError, match="Instruction UNKNOWN_OPCODE not implemented"
+        ):
+            cpu.run(max_steps=10, show_progress=False)
+
 
 class TestCPUProgramLoading:
     """Test CPU program loading and execution edge cases."""
diff --git a/tests/test_data_transfer.py b/tests/test_data_transfer.py
index 680c27c..40313b6 100644
--- a/tests/test_data_transfer.py
+++ b/tests/test_data_transfer.py
@@ -5,6 +5,8 @@
 
 import pytest
 
+from tiny8 import CPU
+
 
 class TestLDI:
     """Test LDI (Load Immediate) instruction."""
@@ -306,3 +308,21 @@ def test_16bit_pointer_pair(self, cpu_with_program, helper):
         """)
         helper.assert_register(cpu, 17, 99)
         helper.assert_memory(cpu, 0x0210, 99)
+
+
+class TestSER:
+    """Test SER (Set Register) instruction."""
+
+    def test_ser_basic(self, cpu_with_program, helper):
+        """Test setting register to $FF (255)."""
+        cpu = cpu_with_program("""
+            ser r16
+            ser r25
+        """)
+        helper.assert_registers(cpu, {16: 0xFF, 25: 0xFF})
+
+    def test_ser_direct_call(self):
+        """Test op_ser method directly."""
+        cpu = CPU()
+        cpu.op_ser(17)
+        assert cpu.read_reg(17) == 0xFF