diff --git a/insns/insn_lb.py b/insns/insn_lb.py
new file mode 100644
index 0000000..10d2561
--- /dev/null
+++ b/insns/insn_lb.py
@@ -0,0 +1,86 @@
+from nmigen import *
+
+class rvfi_insn_lb(Elaboratable):
+    def __init__(self, RISCV_FORMAL_ILEN=32, RISCV_FORMAL_XLEN=32):
+        self.RISCV_FORMAL_ILEN = RISCV_FORMAL_ILEN
+        self.RISCV_FORMAL_XLEN = RISCV_FORMAL_XLEN
+        self.rvfi_valid = Signal(1)
+        self.rvfi_insn = Signal(self.RISCV_FORMAL_ILEN)
+        self.rvfi_pc_rdata = Signal(self.RISCV_FORMAL_XLEN)
+        self.rvfi_rs1_rdata = Signal(self.RISCV_FORMAL_XLEN)
+        self.rvfi_rs2_rdata = Signal(self.RISCV_FORMAL_XLEN)
+        self.rvfi_mem_rdata = Signal(self.RISCV_FORMAL_XLEN)
+        self.spec_valid = Signal(1)
+        self.spec_trap = Signal(1)
+        self.spec_rs1_addr = Signal(5)
+        self.spec_rs2_addr = Signal(5)
+        self.spec_rd_addr = Signal(5)
+        self.spec_rd_wdata = Signal(self.RISCV_FORMAL_XLEN)
+        self.spec_pc_wdata = Signal(self.RISCV_FORMAL_XLEN)
+        self.spec_mem_addr = Signal(self.RISCV_FORMAL_XLEN)
+        self.spec_mem_rmask = Signal(int(self.RISCV_FORMAL_XLEN // 8))
+        self.spec_mem_wmask = Signal(int(self.RISCV_FORMAL_XLEN // 8))
+        self.spec_mem_wdata = Signal(self.RISCV_FORMAL_XLEN)
+    def ports(self):
+        input_ports = [
+            self.rvfi_valid,
+            self.rvfi_insn,
+            self.rvfi_pc_rdata,
+            self.rvfi_rs1_rdata,
+            self.rvfi_rs2_rdata,
+            self.rvfi_mem_rdata
+        ]
+        output_ports = [
+            self.spec_valid,
+            self.spec_trap,
+            self.spec_rs1_addr,
+            self.spec_rs2_addr,
+            self.spec_rd_addr,
+            self.spec_rd_wdata,
+            self.spec_pc_wdata,
+            self.spec_mem_addr,
+            self.spec_mem_rmask,
+            self.spec_mem_wmask,
+            self.spec_mem_wdata
+        ]
+        return input_ports + output_ports
+    def elaborate(self, platform):
+        m = Module()
+
+        # I-type instruction format
+        insn_padding = Signal(self.RISCV_FORMAL_ILEN)
+        m.d.comb += insn_padding.eq(self.rvfi_insn >> 32)
+        insn_imm = Signal(self.RISCV_FORMAL_XLEN)
+        m.d.comb += insn_imm.eq(Value.as_signed(self.rvfi_insn[20:32]))
+        insn_rs1 = Signal(5)
+        m.d.comb += insn_rs1.eq(self.rvfi_insn[15:20])
+        insn_funct3 = Signal(3)
+        m.d.comb += insn_funct3.eq(self.rvfi_insn[12:15])
+        insn_rd = Signal(5)
+        m.d.comb += insn_rd.eq(self.rvfi_insn[7:12])
+        insn_opcode = Signal(7)
+        m.d.comb += insn_opcode.eq(self.rvfi_insn[:7])
+
+        misa_ok = Signal(1)
+        m.d.comb += misa_ok.eq(1)
+
+        # LB instruction
+        addr = Signal(self.RISCV_FORMAL_XLEN)
+        m.d.comb += addr.eq(self.rvfi_rs1_rdata + insn_imm)
+        result = Signal(8)
+        m.d.comb += result.eq(self.rvfi_mem_rdata)
+        m.d.comb += self.spec_valid.eq(self.rvfi_valid & (insn_funct3 == 0b000) & (insn_opcode == 0b0000011))
+        m.d.comb += self.spec_rs1_addr.eq(insn_rs1)
+        m.d.comb += self.spec_rd_addr.eq(insn_rd)
+        m.d.comb += self.spec_mem_addr.eq(addr)
+        m.d.comb += self.spec_mem_rmask.eq((1 << 1) - 1)
+        m.d.comb += self.spec_rd_wdata.eq(Mux(self.spec_rd_addr, Value.as_signed(result), 0))
+        m.d.comb += self.spec_pc_wdata.eq(self.rvfi_pc_rdata + 4)
+        m.d.comb += self.spec_trap.eq(~misa_ok)
+
+        # default assignments
+        m.d.comb += self.spec_rs2_addr.eq(0)
+        m.d.comb += self.spec_mem_wmask.eq(0)
+        m.d.comb += self.spec_mem_wdata.eq(0)
+
+        return m