diff --git a/README.md b/README.md
index 087f0d5..51d09a9 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ Formally verified implementation of the ARTIQ RTIO core in nMigen
 
 - Devise a suitable migration strategy for `artiq.gateware.rtio` from Migen to nMigen
 - [ ] Implement the core in nMigen
-- - [ ] `rtio.cri`
+- - [x] `rtio.cri` (`Interface` and `CRIDecoder` only)
 - - [ ] `rtio.sed.layouts`
 - - [ ] `rtio.sed.output_network`
 - - [ ] `rtio.sed.output_driver`
diff --git a/rtio/cri.py b/rtio/cri.py
index 8b13789..1d6bb0e 100644
--- a/rtio/cri.py
+++ b/rtio/cri.py
@@ -1 +1,124 @@
+"""Common RTIO Interface"""
 
+from nmigen import *
+from nmigen.utils import *
+from nmigen.hdl.rec import *
+
+# CRI write happens in 3 cycles:
+# 1. set timestamp and channel
+# 2. set other payload elements and issue write command
+# 3. check status
+
+commands = {
+    "nop": 0,
+    "write": 1,
+    # i_status should have the "wait for status" bit set until
+    # an event is available, or timestamp is reached.
+    "read": 2,
+    # targets must assert o_buffer_space_valid in response
+    # to this command
+    "get_buffer_space": 3
+}
+
+layout = [
+    ("cmd", 2, DIR_FANOUT),
+    # 8  MSBs of chan_sel = routing destination
+    # 16 LSBs of chan_sel = channel within the destination
+    ("chan_sel", 24, DIR_FANOUT),
+
+    ("o_timestamp", 64, DIR_FANOUT),
+    ("o_data", 512, DIR_FANOUT),
+    ("o_address", 8, DIR_FANOUT),
+    # o_status bits:
+    # <0:wait> <1:underflow> <2:destination unreachable>
+    ("o_status", 3, DIR_FANIN),
+
+    # pessimistic estimate of the number of outputs events that can be
+    # written without waiting.
+    # this feature may be omitted on systems without DRTIO.
+    ("o_buffer_space_valid", 1, DIR_FANIN),
+    ("o_buffer_space", 16, DIR_FANIN),
+
+    ("i_timeout", 64, DIR_FANOUT),
+    ("i_data", 32, DIR_FANIN),
+    ("i_timestamp", 64, DIR_FANIN),
+    # i_status bits:
+    # <0:wait for event (command timeout)> <1:overflow> <2:wait for status>
+    # <3:destination unreachable>
+    # <0> and <1> are mutually exclusive. <1> has higher priority.
+    ("i_status", 4, DIR_FANIN),
+]
+
+# We drop the support for keyword arguments here since
+# all instances of Interface in this file do not use them
+class Interface(Record):
+    def __init__(self):
+        super().__init__(layout)
+
+# Skip KernelInitiator for now (depends on AutoCSR)
+
+class CRIDecoder(Elaboratable):
+    def __init__(self, slaves=2, master=None, mode="async", enable_routing=False):
+        m = Module()
+        self.m = m
+        if isinstance(slaves, int):
+            slaves = [Interface() for _ in range(slaves)]
+        if master is None:
+            master = Interface()
+        self.slaves = slaves
+        self.master = master
+
+        # # #
+
+        # routing
+        if enable_routing:
+            destination_unreachable = Interface()
+            m.d.comb += destination_unreachable.o_status.eq(4)
+            m.d.comb += destination_unreachable.i_status.eq(8)
+            slaves = slaves[:]
+            slaves.append(destination_unreachable)
+            target_len = 2**(len(slaves) - 1).bit_length()
+            slaves += [destination_unreachable]*(target_len - len(slaves))
+
+        slave_bits = bits_for(len(slaves)-1)
+        selected = Signal(slave_bits)
+
+        if enable_routing:
+            routing_table = Memory(slave_bits, 256)
+
+            if mode == "async":
+                rtp_decoder = routing_table.read_port()
+            elif mode == "sync":
+                rtp_decoder = routing_table.read_port(clock_domain="rtio")
+            else:
+                raise ValueError
+            m.submodules += rtp_decoder
+            m.d.comb += rtp_decoder.addr.eq(self.master.chan_sel[16:])
+            m.d.comb += selected.eq(rtp_decoder.data)
+        else:
+            m.d.sync += selected.eq(self.master.chan_sel[16:])
+
+        # master -> slave
+        for n, slave in enumerate(slaves):
+            for name, size, direction in layout:
+                if direction == DIR_FANOUT and name != "cmd":
+                    m.d.comb += getattr(slave, name).eq(getattr(master, name))
+            with m.If(selected == n):
+                m.d.comb += slave.cmd.eq(master.cmd)
+
+        # slave -> master
+        with m.Switch(selected):
+            for n, slave in enumerate(slaves):
+                with m.Case(n):
+                    for name, size, direction in layout:
+                        if direction == DIR_FANIN:
+                            m.d.comb += getattr(master, name).eq(getattr(slave, name))
+
+    def elaborate(self, platform):
+        return self.m
+
+# Skip CRISwitch for now (depends on AutoCSR)
+
+# Skip CRIInterconnectShared for now (depends on CRISwitch)
+
+# Skip RoutingTableAccess for now (depends on AutoCSR)