diff --git a/insns/insn_S.py b/insns/insn_S.py
new file mode 100644
index 0000000..b757fbf
--- /dev/null
+++ b/insns/insn_S.py
@@ -0,0 +1,32 @@
+from insn import *
+class rvfi_insn_S(rvfi_insn):
+    def __init__(self):
+        super(rvfi_insn_S, self).__init__()
+        self.insn_padding = Signal(32)
+        self.insn_imm = Signal(32)
+        self.insn_rs2 = Signal(5)
+        self.insn_rs1 = Signal(5)
+        self.insn_funct3 = Signal(3)
+        self.insn_opcode = Signal(7)
+        self.misa_ok = Signal(1)
+    def ports(self):
+        return super(rvfi_insn_S, self).ports()
+    def elaborate(self, platform):
+        m = super(rvfi_insn_S, self).elaborate(platform)
+
+        # S-type instruction format
+        m.d.comb += self.insn_padding.eq(self.rvfi_insn >> 32)
+        m.d.comb += self.insn_imm.eq(Value.as_signed(Cat(self.rvfi_insn[7:12], self.rvfi_insn[25:32])))
+        m.d.comb += self.insn_rs2.eq(self.rvfi_insn[20:25])
+        m.d.comb += self.insn_rs1.eq(self.rvfi_insn[15:20])
+        m.d.comb += self.insn_funct3.eq(self.rvfi_insn[12:15])
+        m.d.comb += self.insn_opcode.eq(self.rvfi_insn[:7])
+
+        m.d.comb += self.misa_ok.eq(1)
+
+        # default assignments
+        m.d.comb += self.spec_rd_addr.eq(0)
+        m.d.comb += self.spec_rd_wdata.eq(0)
+        m.d.comb += self.spec_mem_rmask.eq(0)
+
+        return m