diff --git a/power_fv/utils.py b/power_fv/utils.py
new file mode 100644
index 0000000..b023013
--- /dev/null
+++ b/power_fv/utils.py
@@ -0,0 +1,13 @@
+from amaranth import *
+from amaranth.utils import bits_for
+
+
+__all__ = ["iea_mask"]
+
+
+def iea_mask(addr, msr_sf):
+    "Instruction effective address mask. In 32-bit mode (MSR.SF = 0), the upper 32 bits are cleared."
+    addr, msr_sf = Value.cast(addr), Value.cast(msr_sf)
+    assert len(msr_sf) == 1
+    mask = Cat(Repl(1, 32), Repl(msr_sf, 32))
+    return addr & mask