diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..eb2c507
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,32 @@
+name: Run Tests
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install dependencies
+        run: pip install pytest pytest-cov
+
+      - name: Run base calculator tests
+        run: pytest test_calculator.py -v
+
+      - name: Run binary module tests
+        run: pytest tests/testbinary.py -v
+
+      - name: Coverage report
+        run: pytest tests/testbinary.py --cov=modules --cov-report=term-missing
\ No newline at end of file
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..9fb5e81
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,23 @@
+# Virtual environment
+.venv/
+env/
+venv/
+
+# Python cache
+__pycache__/
+*.pyc
+*.pyo
+*.pyd
+
+# Pytest cache
+.pytest_cache/
+.coverage
+htmlcov/
+
+# IDE
+.vscode/
+.idea/
+
+# OS
+.DS_Store
+Thumbs.db
\ No newline at end of file
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..c586371
--- /dev/null
+++ b/README.md
@@ -0,0 +1,16 @@
+# SE Calculator — Feature 5.6: Binary Number System
+
+Group 6 | Branch: FeatureA_6 | Process: Classical Waterfall
+
+## Features
+- Binary ↔ Decimal conversion
+- Binary arithmetic (add, sub, mul, div)
+- 1's and 2's complement
+- Input validation with custom exceptions
+
+## Running Tests
+    pytest tests/testbinary.py -v
+
+## Input Format
+- Binary: B'1010
+- Decimal: D'10
diff --git a/calculator.py b/calculator.py
index 5daac9a..e3b8c90 100644
--- a/calculator.py
+++ b/calculator.py
@@ -1,11 +1,57 @@
+import re
+from modules.binary import (
+    binary_to_decimal, decimal_to_binary,
+    binary_add, binary_subtract,
+    binary_multiply, binary_divide,
+    ones_complement, twos_complement,
+)
+
+
 class Calculator:
-    def add(self, a, b):
-        return a + b
-    def subtract(self, a, b):
-        return a - b
-    def multiply(self, a, b):
-        return a * b
+    def add(self, a, b):        return a + b
+    def subtract(self, a, b):   return a - b
+    def multiply(self, a, b):   return a * b
     def divide(self, a, b):
         if b == 0:
             raise ValueError("Division by zero")
         return a / b
+
+    def evaluate(self, expression: str) -> str:
+        """
+        Route string expressions to the correct module.
+        Examples:
+            "bin(10)"          -> "B'1010"
+            "dec(B'1010)"      -> "D'10"
+            "1s(B'1010)"       -> "B'0101"
+            "2s(B'0111)"       -> "B'1001"
+            "B'011 + B'010"    -> "B'101"
+        """
+        expr = expression.strip()
+
+        if re.match(r"^bin\(", expr, re.I):
+            val = re.search(r"\((.+)\)", expr).group(1).strip()
+            return decimal_to_binary(f"D'{val}" if not val.upper().startswith("D'") else val)
+
+        if re.match(r"^dec\(", expr, re.I):
+            val = re.search(r"\((.+)\)", expr).group(1).strip()
+            return binary_to_decimal(val)
+
+        if re.match(r"^1s\(", expr, re.I):
+            val = re.search(r"\((.+)\)", expr).group(1).strip()
+            return ones_complement(val)
+
+        if re.match(r"^2s\(", expr, re.I):
+            val = re.search(r"\((.+)\)", expr).group(1).strip()
+            return twos_complement(val)
+
+        # Binary arithmetic: "B'011 + B'010"
+        m = re.match(
+            r"(B'[01]+)\s*([\+\-\*\/])\s*(B'[01]+)", expr, re.I
+        )
+        if m:
+            a, op, b = m.group(1), m.group(2), m.group(3)
+            ops = {'+': binary_add, '-': binary_subtract,
+                   '*': binary_multiply, '/': binary_divide}
+            return ops[op](a, b)
+
+        raise ValueError(f"Unrecognised expression: {expression}")
\ No newline at end of file
diff --git a/modules/__init__.py b/modules/__init__.py
new file mode 100644
index 0000000..eefee6b
--- /dev/null
+++ b/modules/__init__.py
@@ -0,0 +1,10 @@
+from .binary import (
+    binary_to_decimal,
+    decimal_to_binary,
+    binary_add,
+    binary_subtract,
+    binary_multiply,
+    binary_divide,
+    ones_complement,
+    twos_complement,
+)
\ No newline at end of file
diff --git a/modules/binary.py b/modules/binary.py
new file mode 100644
index 0000000..063df4e
--- /dev/null
+++ b/modules/binary.py
@@ -0,0 +1,116 @@
+# -- Pritam 038 -------------------------------------------------------
+
+from .exceptions import InvalidBinaryInputError, NegativeBinaryResultError
+
+
+def _strip(b: str) -> str:
+    """Strip B' prefix and whitespace."""
+    b = b.strip().upper()
+    if b.startswith("B'"):
+        b = b[2:]
+    return b
+
+
+def _validate(b: str) -> None:
+    if not b or not all(c in '01' for c in b):
+        raise InvalidBinaryInputError(b)
+
+
+def binary_to_decimal(b: str) -> str:
+    """
+    Convert binary string to decimal.
+    e.g. binary_to_decimal("B'1010") -> "D'10"
+    """
+    raw = _strip(b)
+    _validate(raw)
+    return f"D'{int(raw, 2)}"
+
+
+def decimal_to_binary(d: str) -> str:
+    """
+    Convert decimal string to binary.
+    e.g. decimal_to_binary("D'10") -> "B'1010"
+    """
+    d = d.strip().upper()
+    if d.startswith("D'"):
+        d = d[2:]
+    n = int(d)
+    if n < 0:
+        raise ValueError("Negative decimal not supported in basic mode")
+    return f"B'{bin(n)[2:]}"
+
+  
+
+# ── Saiyam 037: Arithmetic ──────────────────────────────────────────────────────
+
+def binary_add(a: str, b: str) -> str:
+    """
+    Add two binary numbers.
+    e.g. binary_add("B'011", "B'010") -> "B'101"
+    """
+    da = int(binary_to_decimal(a).split("'")[1])
+    db = int(binary_to_decimal(b).split("'")[1])
+    return decimal_to_binary(f"D'{da + db}")
+
+
+def binary_subtract(a: str, b: str) -> str:
+    """
+    Subtract two binary numbers.
+    e.g. binary_subtract("B'101", "B'010") -> "B'011"
+    """
+    da = int(binary_to_decimal(a).split("'")[1])
+    db = int(binary_to_decimal(b).split("'")[1])
+    if da < db:
+        raise NegativeBinaryResultError()
+    bit_len = max(len(_strip(a)), len(_strip(b)))
+    result = bin(da - db)[2:].zfill(bit_len)
+    return f"B'{result}"
+
+
+def binary_multiply(a: str, b: str) -> str:
+    """
+    Multiply two binary numbers.
+    e.g. binary_multiply("B'011", "B'010") -> "B'110"
+    """
+    da = int(binary_to_decimal(a).split("'")[1])
+    db = int(binary_to_decimal(b).split("'")[1])
+    return decimal_to_binary(f"D'{da * db}")
+
+
+def binary_divide(a: str, b: str) -> str:
+    """
+    Integer divide two binary numbers.
+    e.g. binary_divide("B'110", "B'010") -> "B'11"
+    """
+    da = int(binary_to_decimal(a).split("'")[1])
+    db = int(binary_to_decimal(b).split("'")[1])
+    if db == 0:
+        raise ValueError("Binary division by zero")
+    return decimal_to_binary(f"D'{da // db}")
+
+
+
+#-----------suhani 034: complement-------------------------------
+
+
+def ones_complement(b: str) -> str:
+    """
+    Flip all bits.
+    e.g. ones_complement("B'1010") -> "B'0101"
+    """
+    raw = _strip(b)
+    _validate(raw)
+    return "B'" + ''.join('1' if bit == '0' else '0' for bit in raw)
+
+
+def twos_complement(b: str) -> str:
+    """
+    Add 1 to the one's complement.
+    e.g. twos_complement("B'0111") -> "B'1001"
+    """
+    raw = _strip(b)
+    _validate(raw)
+    ones = ones_complement(f"B'{raw}")[2:]
+    val = int(ones, 2) + 1                         # no modulo
+    result = bin(val)[2:].zfill(len(ones))         # zfill won't truncate carry
+    return f"B'{result}"
\ No newline at end of file
diff --git a/modules/exceptions.py b/modules/exceptions.py
new file mode 100644
index 0000000..56b13f2
--- /dev/null
+++ b/modules/exceptions.py
@@ -0,0 +1,9 @@
+
+# ---- Harsh 035: Exceptions ----------
+class InvalidBinaryInputError(ValueError):
+    def __init__(self, value: str):
+        super().__init__(f"'{value}' is not a valid binary string")
+
+class NegativeBinaryResultError(ValueError):
+    def __init__(self):
+        super().__init__("Binary subtraction result cannot be negative")
diff --git a/test_calculator.py b/test_calculator.py
index 7cb79e9..5bb6c42 100644
--- a/test_calculator.py
+++ b/test_calculator.py
@@ -15,13 +15,13 @@ def test_sub(self):
     def test_multiply(self):
         self.assertEqual(self.calc.multiply(2, 3), 6)
 
-    def test_divide(self):
+    def test_divide_positive(self):
         self.assertEqual(self.calc.divide(2, 4), 0.5)
 
-    def test_divide(self):
+    def test_divide_negative(self):
         self.assertEqual(self.calc.divide(4, -2), -2)
-    
-    def test_divide_fail(self): # this will fail
+
+    def test_divide_fail(self):
         self.assertNotEqual(self.calc.divide(4, -2), 2)
 
     def test_divide_by_zero(self):
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..9d76c9a
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+from .testbinary import *
\ No newline at end of file
diff --git a/tests/testbinary.py b/tests/testbinary.py
new file mode 100644
index 0000000..702c3e5
--- /dev/null
+++ b/tests/testbinary.py
@@ -0,0 +1,85 @@
+
+# ── deepjit pal (036): Exceptions─────────────────────────────────────────────────────
+
+
+import unittest
+from modules.binary import (
+    binary_to_decimal, decimal_to_binary,
+    binary_add, binary_subtract,
+    binary_multiply, binary_divide,
+    ones_complement, twos_complement,
+)
+from modules.exceptions import InvalidBinaryInputError, NegativeBinaryResultError
+
+
+class TestBinaryConversions(unittest.TestCase):
+    def test_bin_to_dec_basic(self):
+        self.assertEqual(binary_to_decimal("B'1010"), "D'10")
+
+    def test_bin_to_dec_zero(self):
+        self.assertEqual(binary_to_decimal("B'0"), "D'0")
+
+    def test_bin_to_dec_one(self):
+        self.assertEqual(binary_to_decimal("B'1"), "D'1")
+
+    def test_dec_to_bin_basic(self):
+        self.assertEqual(decimal_to_binary("D'10"), "B'1010")
+
+    def test_dec_to_bin_zero(self):
+        self.assertEqual(decimal_to_binary("D'0"), "B'0")
+
+
+class TestBinaryArithmetic(unittest.TestCase):
+    def test_add(self):
+        self.assertEqual(binary_add("B'011", "B'010"), "B'101")
+
+    def test_subtract(self):
+        self.assertEqual(binary_subtract("B'101", "B'010"), "B'011")
+
+    def test_subtract_negative_raises(self):
+        with self.assertRaises(NegativeBinaryResultError):
+            binary_subtract("B'001", "B'010")
+
+    def test_multiply(self):
+        self.assertEqual(binary_multiply("B'011", "B'010"), "B'110")
+
+    def test_divide(self):
+        self.assertEqual(binary_divide("B'110", "B'010"), "B'11")
+
+    def test_divide_by_zero(self):
+        with self.assertRaises(ValueError):
+            binary_divide("B'110", "B'0")
+
+
+class TestComplements(unittest.TestCase):
+    def test_ones_complement(self):
+        self.assertEqual(ones_complement("B'1010"), "B'0101")
+
+    def test_ones_all_zeros(self):
+        self.assertEqual(ones_complement("B'0000"), "B'1111")
+
+    def test_twos_complement(self):
+        self.assertEqual(twos_complement("B'0111"), "B'1001")
+
+
+    def test_twos_complement_zero(self):
+        self.assertEqual(twos_complement("B'0000"), "B'10000")
+
+
+class TestInvalidInput(unittest.TestCase):
+    def test_invalid_char(self):
+        with self.assertRaises(InvalidBinaryInputError):
+            binary_to_decimal("B'1021")
+
+    def test_empty_string(self):
+        with self.assertRaises(InvalidBinaryInputError):
+            binary_to_decimal("B'")
+
+    def test_missing_prefix(self):
+        with self.assertRaises(InvalidBinaryInputError):
+            binary_to_decimal("1021")
+
+
+if __name__ == '__main__':
+    unittest.main()
+