Full adder carry-out: (A AND B) OR (Cin AND (A XOR B))
The carry-out of a full adder, (A AND B) OR (Cin AND (A XOR B)), has the same truth table as majority of three. The Karnaugh map finds the three-term majority form, which is shorter than the textbook formula.
Adding A, B and the carry-in Cin produces a carry-out when at least two of the three bits are 1. The textbook formula says it in adder terms: either both operand bits are 1, or exactly one is and the carry-in is 1. The engine's reading follows the same structure, an OR of A AND B and of Cin together with exactly one of A and B. On the eight-row table the output is true on four rows, Σm(3, 5, 6, 7), which is the majority function's column.
The checks find a contingent function: tautology violated with A = FALSE, B = FALSE, Cin = FALSE, satisfiable with the witness A = FALSE, B = TRUE, Cin = TRUE. The simplifier found no law that shortens the four-operator expression. The DNF converter, after XOR elimination and distribution, gives three terms and eight literals; the CNF is (A OR B) AND (A OR Cin) AND (B OR Cin). The Karnaugh map does better than both, with three two-cell groups and three prime implicants, deriving (A AND B) OR (A AND Cin) OR (B AND Cin). Gate counts for the original: 8 NAND, 23 NOR.
The carry chain decides the speed of a ripple-carry adder, which is why designers care which form they build: the majority form has no XOR on the carry path, while the textbook form can reuse the XOR already built for the sum. Both are correct; the table proves them equal on every row. In software the same function is two-out-of-three voting. The thing to watch is that “simplified” here came from the map, not from the simplifier, which found no local law to apply.
carry_out := (A AND B) OR (Cin AND (A XOR B)) · output
Exact checks
Truth table (8 rows)
| # | A | B | Cin | carry_out |
|---|---|---|---|---|
| 0 | FALSE | FALSE | FALSE | FALSE |
| 1 | FALSE | FALSE | TRUE | FALSE |
| 2 | FALSE | TRUE | FALSE | FALSE |
| 3 | FALSE | TRUE | TRUE | TRUE |
| 4 | TRUE | FALSE | FALSE | FALSE |
| 5 | TRUE | FALSE | TRUE | TRUE |
| 6 | TRUE | TRUE | FALSE | TRUE |
| 7 | TRUE | TRUE | TRUE | TRUE |
Plain-language reading
Beginnercarry_out is true exactly when at least one of the following holds (both may hold):
1. both A and B are true
2. both of the following hold:
2.1. Cin is true
2.2. exactly one of A and B is true (not both)
In every other case, carry_out is false.
About the operators:
- OR is inclusive: it is true when at least one part is true, including when several are.
- AND is true only when all of its parts are true.
- XOR (exclusive or) is true when exactly one of its two parts is true.
Intermediatecarry_out is true exactly when at least one of the following holds (both may hold):
1. both A and B are true
2. both of the following hold:
2.1. Cin is true
2.2. exactly one of A and B is true (not both)
In every other case, carry_out is false.
Advancedcarry_out is true exactly when at least one of the following holds (both may hold) [inclusive disjunction ∨]:
1. both A and B are true [conjunction ∧]
2. both of the following hold [conjunction ∧]:
2.1. Cin is true
2.2. exactly one of A and B is true (not both) [exclusive disjunction ⊕]
In every other case, carry_out is false.
Formal notation: (A ∧ B) ∨ (Cin ∧ (A ⊕ B))
Logic diagram
Simplification, step by step
The engine found no law that shortens this expression; it is already as short as its rewriting laws can make it.
(A AND B) OR (Cin AND (A XOR B))
Karnaugh map
- cells m6, m7
A AND B - cells m5, m7
A AND Cin - cells m3, m7
B AND Cin
Minterms and maxterms
carry_out(A, B, Cin) = Σm(3, 5, 6, 7)carry_out(A, B, Cin) = ΠM(0, 1, 2, 4)Variable order: A, B, Cin · 8 rows
Normal forms
Conjunctive normal form (CNF)
(A OR B) AND (A OR Cin) AND (B OR Cin)3 clauses, 6 literals · complete · XOR elimination ×1, Distribution ×1, Complement ×2, Idempotence ×1
Disjunctive normal form (DNF)
(A AND B) OR (A AND NOT B AND Cin) OR (NOT A AND B AND Cin)3 terms, 8 literals · complete · XOR elimination ×1, Distribution ×1
NAND-only and NOR-only
NAND-only form
NAND(NAND(A, B), NAND(Cin, NAND(NAND(A, NAND(B, B)), NAND(NAND(A, A), B))))8 gates · verified equivalent on every row
NOR-only form
NOR(NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(Cin, Cin), NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B))))), NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(Cin, Cin), NOR(NOR(NOR(A, A), B), NOR(A, NOR(B, B))))))23 gates · verified equivalent on every row
The same rule as code
Python
# -*- coding: utf-8 -*-
# Generated by Tangibool (tangibool.python/1). Do not edit by hand; regenerate from the source revision.
# Model: not stated
# Source revision: reference
# Interpretation status: not stated
"""Tangibool evaluator (decision model).
Generated deterministically from the validated Boolean model; it evaluates exactly that
model. It does not establish that the model is a complete, legal, or safe statement of a
real-world rule, and evaluating examples does not verify the rule.
Source revision: reference
Interpretation status: not stated
Exporter: 1.0.0 (format tangibool.python/1; engine 1.0.0; semantics tangibool-semantics/1)
Logic digest: 2778956dbcdba3989c5c20463da4fd853619e93646c2c38e377f1c5fb09400a7
Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
Inputs (dict keys, declared order) and generated parameter names:
'A' -> input_A
'B' -> input_B
'Cin' -> input_Cin
Outputs (declared order) and their functions:
'carry_out' -> output_carry_out(inputs)
Assumptions (the declared scope, checked by in_scope(inputs)):
none; every complete assignment is in scope.
Input contract:
Every required input must be exactly True or False (type(value) is bool). Missing
inputs, None, strings, integers (including 0 and 1), and keys that are not declared
inputs raise InputError before anything is evaluated, including inputs that a
short-circuiting operator would never read. Results are always bool.
Scope:
Decision functions raise OutOfScopeError when the inputs violate an assumption; an
out-of-scope assignment is never turned into a False result. Pass check_scope=False
to evaluate without the scope check, and use in_scope(inputs) to test it separately.
Usage:
import tangibool_model as model
model.evaluate({'A': False, 'B': False, 'Cin': False})
# -> {'carry_out': False}
model.output_carry_out({'A': False, 'B': False, 'Cin': False})
# -> False
model.evaluate({'A': 1, 'B': False, 'Cin': False})
# raises InputError: 1 is an int, not a bool
"""
import collections.abc as _abc
from typing import Dict, Mapping, Tuple
__all__ = [
"MODEL_INFO",
"IDENTIFIER_MAP",
"INPUT_NAMES",
"OUTPUT_NAMES",
"ASSUMPTIONS",
"InputError",
"OutOfScopeError",
"minterm_index",
"in_scope",
"evaluate",
"output_carry_out",
]
MODEL_INFO = {
"format": "tangibool.python/1",
"mode": "decision",
"model": None,
"revision": "reference",
"interpretation": None,
"exporter": "1.0.0",
"engine_version": "1.0.0",
"semantics_version": "tangibool-semantics/1",
"logic_digest": "2778956dbcdba3989c5c20463da4fd853619e93646c2c38e377f1c5fb09400a7",
"assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
"primary_output": "carry_out",
"unused_inputs": (),
"subcircuits": (),
}
# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
"v_763f80cbcba7": {"kind": "input", "name": "A", "identifier": "input_A"},
"v_4149d8f0d349": {"kind": "input", "name": "B", "identifier": "input_B"},
"v_0033bc40418d": {"kind": "input", "name": "Cin", "identifier": "input_Cin"},
"f_5e68d8f570bb": {"kind": "output", "name": "carry_out", "identifier": "output_carry_out"},
}
INPUT_NAMES = ("A", "B", "Cin")
OUTPUT_NAMES = ("carry_out",)
ASSUMPTIONS = ()
_DECLARED_INPUTS = frozenset(INPUT_NAMES)
_SCOPE_INPUTS = ()
_ASSUMPTION_FORMULAS = {entry["name"]: entry["formula"] for entry in ASSUMPTIONS}
class InputError(ValueError):
"""Raised when inputs are missing, undeclared, or not exactly True/False.
Every required input must satisfy type(value) is bool. Missing inputs, None, strings,
integers (including 0 and 1), and keys that are not declared inputs are rejected before
anything is evaluated. A missing input is never treated as False. The individual problems
are listed in the problems attribute.
"""
def __init__(self, problems):
self.problems = tuple(problems)
super().__init__("Invalid inputs: " + "; ".join(self.problems))
class OutOfScopeError(ValueError):
"""Raised when the inputs violate the model's assumptions (its declared scope).
Outside its assumptions the rule has no decision: this error is never a False result.
The names of the violated assumptions are in the violated attribute.
"""
def __init__(self, violated):
self.violated = tuple(violated)
details = ", ".join(name + " (" + _ASSUMPTION_FORMULAS.get(name, "?") + ")" for name in self.violated)
super().__init__(
"Out of scope: the inputs violate "
+ ("assumption " if len(self.violated) == 1 else "assumptions ")
+ details
+ ". The rule has no decision outside its assumptions; this is not a False result."
)
def _short_repr(value: object) -> str:
text = repr(value)
return text if len(text) <= 60 else text[:57] + "..."
def _checked_inputs(inputs: Mapping[str, bool], required: Tuple[str, ...], check_scope: bool) -> Dict[str, bool]:
"""Validate inputs strictly before anything is evaluated and return a plain copy."""
if type(check_scope) is not bool:
raise TypeError("check_scope must be True or False")
if not isinstance(inputs, _abc.Mapping):
raise InputError(["inputs must be a mapping from input names to True/False, got " + type(inputs).__name__])
problems = []
for key in inputs:
if type(key) is not str or key not in _DECLARED_INPUTS:
declared = ", ".join(repr(name) for name in INPUT_NAMES) or "none"
problems.append("undeclared input " + _short_repr(key) + " (declared inputs: " + declared + ")")
needed = tuple(required) + (_SCOPE_INPUTS if check_scope else ())
for name in needed:
if name not in inputs:
problems.append("missing required input " + repr(name))
for key, value in inputs.items():
if type(key) is str and key in _DECLARED_INPUTS and type(value) is not bool:
problems.append(
"input " + repr(key) + " must be True or False (type bool), got "
+ type(value).__name__ + " " + _short_repr(value)
)
if problems:
raise InputError(list(dict.fromkeys(problems)))
return dict(inputs.items())
def _require_bool_args(function: str, pairs: Tuple[Tuple[str, object], ...]) -> None:
problems = [
"argument " + repr(name) + " of " + function + " must be True or False (type bool), got "
+ type(value).__name__ + " " + _short_repr(value)
for name, value in pairs
if type(value) is not bool
]
if problems:
raise InputError(problems)
def minterm_index(*values: bool) -> int:
"""Row index of an assignment over a declared variable order.
The first value is the most significant bit, so all-False is row 0. Used for indexed
minterm (sum) and maxterm (product) forms.
"""
index = 0
for value in values:
if type(value) is not bool:
raise InputError(["minterm_index() takes True/False values, got " + type(value).__name__ + " " + _short_repr(value)])
index = index * 2 + (1 if value else 0)
return index
def _scope_values(values: Dict[str, bool]) -> Tuple[Tuple[str, bool], ...]:
"""Value of every enabled, confirmed assumption for validated inputs, in declared order."""
return ()
def _require_in_scope(values: Dict[str, bool]) -> None:
violated = tuple(name for name, holds in _scope_values(values) if not holds)
if violated:
raise OutOfScopeError(violated)
def in_scope(inputs: Mapping[str, bool]) -> bool:
"""True when the inputs satisfy every assumption (the model's declared scope).
Requires the inputs the assumptions read; raises InputError for invalid inputs.
"""
values = _checked_inputs(inputs, (), True)
return all(holds for _, holds in _scope_values(values))
# ---- Formula bodies (inputs are validated by the public functions below) ----
def _compute_output_carry_out(input_A: bool, input_B: bool, input_Cin: bool) -> bool:
"""Decision output 'carry_out': (A AND B) OR (Cin AND (A XOR B))"""
return (input_A and input_B) or (input_Cin and (input_A != input_B))
# ---- Public API ----
def output_carry_out(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
"""Decision output 'carry_out' (stable ID f_5e68d8f570bb).
'carry_out' := (A AND B) OR (Cin AND (A XOR B))
Required inputs: 'A', 'B', 'Cin'.
Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
when check_scope is True and the inputs violate an assumption.
"""
values = _checked_inputs(inputs, ("A", "B", "Cin"), check_scope)
if check_scope:
_require_in_scope(values)
output_carry_out_value = _compute_output_carry_out(values["A"], values["B"], values["Cin"])
return output_carry_out_value
def evaluate(inputs: Mapping[str, bool], *, check_scope: bool = True) -> Dict[str, bool]:
"""Evaluate every output (declared order); each output is computed once.
Required inputs: 'A', 'B', 'Cin'.
Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
when check_scope is True and the inputs violate an assumption.
"""
values = _checked_inputs(inputs, ("A", "B", "Cin"), check_scope)
if check_scope:
_require_in_scope(values)
output_carry_out_value = _compute_output_carry_out(values["A"], values["B"], values["Cin"])
return {
"carry_out": output_carry_out_value,
}
PostgreSQL
-- Tangibool PostgreSQL export (tangibool.postgresql/1)
-- Target: PostgreSQL 16 or later with typed boolean inputs; tested with PostgreSQL 18.3 (PGlite 0.5.8).
-- Not a claim of compatibility with other SQL dialects.
-- Model: not stated
-- Source revision: reference
-- Interpretation status: not stated
-- Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1)
-- Logic digest: 2778956dbcdba3989c5c20463da4fd853619e93646c2c38e377f1c5fb09400a7
-- Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
-- Mode: decision
--
-- Read-only: this file contains only WITH / VALUES / SELECT statements. No DDL, DML,
-- procedures, credentials, network access, or database changes. The expressions are pure
-- Boolean expressions, so their result does not depend on SQL evaluation order.
--
-- NON-NULL INPUT PRECONDITION: NULL is not a truth value of this rule. Every fragment below
-- is defined only when every input column it reads is a non-NULL boolean. The guard template
-- (section 2) marks any NULL input as 'invalid_input' and any row that violates the
-- assumptions as 'out_of_scope', and fills output columns only for 'ok' rows; a NULL in an
-- output column of another row is an envelope marker, not a third truth value. Never wrap
-- the fragments in COALESCE(..., FALSE) or otherwise choose a meaning for unknown inputs.
-- This evaluates the supplied model; it does not show that the model is a complete, legal,
-- or safe statement of a real-world rule.
--
-- Input columns (stable ID: name -> column):
-- v_763f80cbcba7: A -> "in_A"
-- v_4149d8f0d349: B -> "in_B"
-- v_0033bc40418d: Cin -> "in_Cin"
-- Output columns:
-- f_5e68d8f570bb: carry_out -> "out_carry_out"
-- Assumptions (scope K; exported separately and never folded into an output):
-- none; every complete input row is in scope.
-- ----------------------------------------------------------------------------------------------
-- 1. Expression fragments (parenthesised; non-NULL boolean inputs required)
-- ----------------------------------------------------------------------------------------------
--
-- [out_carry_out] output carry_out (stable ID f_5e68d8f570bb)
-- carry_out := (A AND B) OR (Cin AND (A XOR B))
-- PRECONDITION: "in_A", "in_B", "in_Cin" are non-NULL booleans.
-- (("in_A" AND "in_B") OR ("in_Cin" AND ("in_A" <> "in_B")))
--
-- ----------------------------------------------------------------------------------------------
-- 2. Guard template (read-only SELECT over an inputs list)
-- ----------------------------------------------------------------------------------------------
-- Replace the example rows between the markers with your own rows, or replace the inputs
-- CTE with a SELECT that provides the boolean columns listed in the header. status is
-- 'invalid_input' when any input is NULL, 'out_of_scope' when the assumptions are false, and
-- 'ok' otherwise; output columns are filled only for 'ok' rows (NULL elsewhere is an
-- envelope marker). Assumption columns are shown for rows with complete inputs.
WITH inputs ("row_id", "in_A", "in_B", "in_Cin") AS (
-- tangibool:example-rows:begin
VALUES
(1, FALSE, FALSE, FALSE),
(2, FALSE, FALSE, TRUE),
(3, FALSE, TRUE, FALSE),
(4, FALSE, TRUE, TRUE),
(5, TRUE, FALSE, FALSE),
(6, TRUE, FALSE, TRUE),
(7, TRUE, TRUE, FALSE),
(8, TRUE, TRUE, TRUE),
(9, NULL::boolean, FALSE, FALSE)
-- tangibool:example-rows:end
),
checked AS (
SELECT
inputs.*,
CASE
WHEN "in_A" IS NULL OR "in_B" IS NULL OR "in_Cin" IS NULL THEN 'invalid_input'
ELSE 'ok'
END AS "status"
FROM inputs
)
SELECT
"row_id",
"in_A",
"in_B",
"in_Cin",
"status",
CASE WHEN "status" = 'ok' THEN (("in_A" AND "in_B") OR ("in_Cin" AND ("in_A" <> "in_B"))) END AS "out_carry_out"
FROM checked
ORDER BY "row_id";
SMT-LIB
; Tangibool SMT-LIB export (tangibool.smtlib/1) ; Profile: SMT-LIB 2.6 Boolean subset (QF_UF): Bool constants, define-fun, let, push/pop, assert, check-sat, echo. ; Tested with Z3 5.1.0 (z3-solver 5.2.0, WebAssembly build). ; Model: not stated ; Source revision: reference ; Interpretation status: not stated ; Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1) ; Logic digest: 2778956dbcdba3989c5c20463da4fd853619e93646c2c38e377f1c5fb09400a7 ; Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78 ; Mode: decision ; ; Reading results: each query block states what sat and unsat mean for its question. A sat ; answer comes with a witness or counterexample (ask for it with (get-model) after sat only); ; unsat settles that scoped query and is solver-checked, not an independently checked proof ; certificate; unknown settles nothing. Blocks are scoped with push/pop, so assertions never ; accumulate across questions. This checks the supplied model, not whether it is a complete, ; legal, or safe statement of a real-world rule. ; ; Symbols (stable ID: name -> symbol): ; v_763f80cbcba7: A -> in_A ; v_4149d8f0d349: B -> in_B ; v_0033bc40418d: Cin -> in_Cin ; f_5e68d8f570bb: carry_out -> out_carry_out (set-option :produce-models true) (set-info :smt-lib-version 2.6) (set-logic QF_UF) ; ---- Inputs (declared order) (declare-const in_A Bool) ; A (declare-const in_B Bool) ; B (declare-const in_Cin Bool) ; Cin ; ---- Outputs (dependencies first) ; output carry_out (stable ID f_5e68d8f570bb): (A AND B) OR (Cin AND (A XOR B)) (define-fun out_carry_out () Bool (or (and in_A in_B) (and in_Cin (xor in_A in_B)))) ; ---- Assumptions (the scope K; never folded into an output) ; K: conjunction of the enabled, confirmed assumptions (true when there are none) (define-fun K () Bool true) ; ---- Definitions only: nothing is asserted and no check is run. ; Add (assert ...) and (check-sat) inside (push 1) ... (pop 1) to ask a question.
LaTeX
carry_out = \left(A \cdot B\right) + \left(\mathit{Cin} \cdot \left(A \oplus B\right)\right)The common slip
Assuming the simplifier will find the majority form. It applies named laws one at a time, and none of them rewrites Cin AND (A XOR B) into two majority terms, so it reports the expression unchanged. The Karnaugh map, working from the table rather than the syntax, finds the three-term form directly.
Questions
Which form should go in the circuit, the textbook one or the majority one?
They compute the same column, Σm(3, 5, 6, 7), so the choice is about sharing: the textbook form reuses the A XOR B already built for the sum, the majority form keeps XOR off the carry path. The page gives the gate counts for the textbook form only.
Related
Guides
Operators and gates
- AND gate: truth table for 2, 3 and 4 inputs, symbol and expression
- OR gate: truth table for 2, 3 and 4 inputs, inclusive or explained
- XOR gate: truth table for 2, 3 and 4 inputs (odd parity, not “exactly one”)
Tools
- Truth table generator: any Boolean expression, up to 12 variables, with intermediate columns
- Boolean algebra calculator and simplifier with steps (every law named and verified)
- Karnaugh map solver: 2 to 4 variables, groups, SOP and POS, don’t-cares from assumptions
See also
- Full adder sum: A XOR B XOR Cin
- Majority of three (2-out-of-3 voting): truth table, K-map and NAND form
- Half adder carry: A AND B
- Karnaugh map solver: 2 to 4 variables, groups, SOP and POS, don’t-cares from assumptions
- “Simplified” is not “minimal”: what a simplifier can promise
- Karnaugh map grouping rule: adjacent cells drop the variable that changes
Worked examples
Generated from the Tangibool engine, semantics tangibool-semantics/1, engine 1.0.0, build . Every table, map, simplification and verdict on these pages was computed by the Tangibool engine when the site was built, and every claim was re-checked by the same exact checker the app uses. Limits are stated where they apply. © 2026 Foreranger LTD.