diff --git a/library/init/data/usize.lean b/library/init/data/usize.lean
new file mode 100644
index 0000000000..b24cda4556
--- /dev/null
+++ b/library/init/data/usize.lean
@@ -0,0 +1,30 @@
+/-
+Copyright (c) 2018 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Leonardo de Moura
+-/
+prelude
+import init.data.uint init.platform init.data.fin
+
+def usize_sz : nat := (2:nat) ^ system.platform.nbits
+def usize : Type := fin usize_sz
+
+instance : decidable_eq usize :=
+fin.decidable_eq usize_sz
+
+theorem usize_sz_pos : usize_sz > 0 :=
+show 0 < (2:nat) ^ system.platform.nbits, from
+nat.pos_pow_of_pos _ (nat.zero_lt_bit0 nat.one_ne_zero)
+
+def usize.of_nat (a : nat) : usize :=
+⟨a % usize_sz, nat.mod_lt _ usize_sz_pos⟩
+
+instance : has_zero usize := ⟨usize.of_nat 0⟩
+instance : has_one usize  := ⟨usize.of_nat 1⟩
+instance : has_add usize  := ⟨fin.add⟩
+instance : has_sub usize  := ⟨fin.sub⟩
+instance : has_mul usize  := ⟨fin.mul⟩
+instance : has_mod usize  := ⟨fin.mod⟩
+instance : has_div usize  := ⟨fin.div⟩
+instance : has_lt usize   := ⟨fin.lt⟩
+instance : has_le usize   := ⟨fin.le⟩
diff --git a/library/init/platform.lean b/library/init/platform.lean
new file mode 100644
index 0000000000..4d1ec3f2d2
--- /dev/null
+++ b/library/init/platform.lean
@@ -0,0 +1,18 @@
+/-
+Copyright (c) 2018 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Leonardo de Moura
+-/
+prelude
+import init.core
+
+namespace system
+
+structure platform_type :=
+(nbits : nat)
+
+-- TODO: mark as opaque
+def platform : platform_type :=
+{ nbits := 64 } -- TODO use `default nat` }
+
+end system