diff --git a/riscemu/CPU.py b/riscemu/CPU.py
index 5f43cf9..d0ab25e 100644
--- a/riscemu/CPU.py
+++ b/riscemu/CPU.py
@@ -143,7 +143,14 @@ class CPU:
         )
 
     def instruction_sub(self, ins: 'LoadedInstruction'):
-        INS_NOT_IMPLEMENTED(ins)
+        ASSERT_LEN(ins.args, 3)
+        dst = ins.get_reg(0)
+        src1 = ins.get_reg(1)
+        src2 = ins.get_reg(2)
+        self.regs.set(
+            dst,
+            self.regs.get(src1) - self.regs.get(src2)
+        )
 
     def instruction_lui(self, ins: 'LoadedInstruction'):
         INS_NOT_IMPLEMENTED(ins)
@@ -152,7 +159,14 @@ class CPU:
         INS_NOT_IMPLEMENTED(ins)
 
     def instruction_xor(self, ins: 'LoadedInstruction'):
-        INS_NOT_IMPLEMENTED(ins)
+        ASSERT_LEN(ins.args, 3)
+        dst = ins.get_reg(0)
+        src1 = ins.get_reg(1)
+        src2 = ins.get_reg(2)
+        self.regs.set(
+            dst,
+            self.regs.get(src1) ^ self.regs.get(src2)
+        )
 
     def instruction_xori(self, ins: 'LoadedInstruction'):
         INS_NOT_IMPLEMENTED(ins)
diff --git a/riscemu/helpers.py b/riscemu/helpers.py
index 14226ec..b5be871 100644
--- a/riscemu/helpers.py
+++ b/riscemu/helpers.py
@@ -44,13 +44,14 @@ def int_from_bytes(bytes, unsigned=False):
 
 def to_unsigned(num: int, bytes=4):
     if num < 0:
-        return 2**(bytes * 8) - num
+        return 2**(bytes * 8) + num
+    return num
 
 
 def to_signed(num: int, bytes=4):
     if num >> (bytes * 8 - 1):
         return num - 2 ** (8 * bytes)
-
+    return num
 
 # Colors