diff --git a/Clear/PrimOps.lean b/Clear/PrimOps.lean
index 12ff0d4..ef601b2 100644
--- a/Clear/PrimOps.lean
+++ b/Clear/PrimOps.lean
@@ -12,15 +12,20 @@ abbrev fromBool := Bool.toUInt256
 
 def evmAddMod (a b c : UInt256) : UInt256 :=
   if c = 0 then 0 else
-  Fin.mod (a + b) c
+  -- "All intermediate calculations of this operation are **not** subject to the 2^256 modulo."
+  Fin.mod (a.val + b.val) c
 
 def evmMulMod (a b c : UInt256) : UInt256 :=
   if c = 0 then 0 else
-  Fin.mod (a * b) c
+  -- "All intermediate calculations of this operation are **not** subject to the 2^256 modulo."
+  Fin.mod (a.val * b.val) c
 
 def evmExp (a b : UInt256) : UInt256 :=
   a ^ b.val
 
+def evmMod (x y : UInt256) : UInt256 :=
+  if y == 0 then 0 else x % y
+
 set_option linter.unusedVariables false in
 def primCall (s : State) : PrimOp → List Literal → State × List Literal
   | .Add, [a,b]               => (s, [a + b])
@@ -28,7 +33,7 @@ def primCall (s : State) : PrimOp → List Literal → State × List Literal
   | .Mul, [a,b]               => (s, [a * b])
   | .Div, [a,b]               => (s, [a / b])
   | .Sdiv, [a,b]              => (s, [UInt256.sdiv a b])
-  | .Mod, [a,b]               => (s, [Fin.mod a b])
+  | .Mod, [a,b]               => (s, [evmMod a b])
   | .Smod, [a,b]              => (s, [UInt256.smod a b])
   | .Addmod, [a,b,c]          => (s, [evmAddMod a b c])
   | .Mulmod, [a,b,c]          => (s, [evmMulMod a b c])
@@ -109,7 +114,7 @@ lemma EVMSub'            : primCall s .Sub [a,b]                     = (s, [a -
 lemma EVMMul'            : primCall s .Mul [a,b]                     = (s, [a * b]) := rfl
 lemma EVMDiv'            : primCall s .Div [a,b]                     = (s, [a / b]) := rfl
 lemma EVMSdiv'           : primCall s .Sdiv [a,b]                    = (s, [UInt256.sdiv a b]) := rfl
-lemma EVMMod'            : primCall s .Mod [a,b]                     = (s, [Fin.mod a b]) := rfl
+lemma EVMMod'            : primCall s .Mod [a,b]                     = (s, [evmMod a b]) := rfl
 lemma EVMSmod'           : primCall s .Smod [a,b]                    = (s, [UInt256.smod a b]) := rfl
 lemma EVMAddmod'         : primCall s .Addmod [a,b,c]                = (s, [evmAddMod a b c]) := rfl
 lemma EVMMulmod'         : primCall s .Mulmod [a,b,c]                = (s, [evmMulMod a b c]) := rfl
diff --git a/Clear/UInt256.lean b/Clear/UInt256.lean
index 390a6de..7a6126c 100644
--- a/Clear/UInt256.lean
+++ b/Clear/UInt256.lean
@@ -54,7 +54,7 @@ def eq0 (a : UInt256) : Bool := a = 0
 def lnot (a : UInt256) : UInt256 := (UInt256.size - 1) - a
 
 def byteAt (a b : UInt256) : UInt256 :=
-  b >>> (31 - a) * 8 <<< 248
+  b >>> (.ofNat ((31 - a.val) * 8)) &&& 0xFF
 
 def sgn (a : UInt256) : UInt256 :=
   if a ≥ 2 ^ 255 then -1
@@ -76,14 +76,15 @@ def sdiv (a b : UInt256) : UInt256 :=
     else a / b
 
 def smod (a b : UInt256) : UInt256 :=
-  if a ≥ 2 ^ 255 then
-    if b ≥ 2 ^ 255 then
-      Fin.mod (abs a) (abs b)
-    else (-1) * Fin.mod (abs a) b
+  if b == 0 then 0
   else
-    if b ≥ 2 ^ 255 then
-      (-1) * Fin.mod a (abs b)
-    else Fin.mod a b
+    let sgnA := if 2 ^ 255 <= a then -1 else 1
+    let sgnB := if 2 ^ 255 <= b then -1 else 1
+    let mask : UInt256 := .ofNat (2 ^ 256 - 1 : ℕ)
+    let absA := if sgnA == 1 then a else - (.xor a mask + 1)
+    let absB := if sgnB == 1 then b else - (.xor b mask + 1)
+    sgnA * (absA % absB)
+
 
 def slt (a b : UInt256) : Bool :=
   if a ≥ 2 ^ 255 then