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Arithmetic Logic Unit

Concept

An arithmetic logic unit (ALU) is a hardware block within a processor that performs arithmetic and logical operations. In conventional processor designs, the ALU is the execute-stage functional block that combines register values, immediate data from the instruction, and numeric constants to produce a result that is then consumed by memory access and write-back stages. Specialized implementations reported in the literature include reversible ALUs for quantum arithmetic and superconductor-based ALUs. Empirical fuzzing of processor RTL has additionally shown that incorrect ALU flag-generation logic is a non-trivial source of security-relevant defects.

First seen 5/25/2026
Last seen 7/16/2026
Evidence 4 chunks
Wiki v2

WIKI

Overview

An arithmetic logic unit (ALU) is a hardware block within a processor that performs arithmetic and logical operations. In a conventional processor pipeline, the ALU is the execute-stage functional block that combines register values, immediate data from the instruction, and numeric constants to produce a result that is then consumed by the memory and write-back stages. Specialized implementations reported in the literature include reversible ALUs for quantum arithmetic and superconductor-based ALUs. Empirical fuzzing of processor RTL has additionally shown that incorrect ALU flag-generation logic is a non-trivial source of security-relevant defects. [ALU overview]

Role in a conventional processor (Y86-64)

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NEIGHBORHOOD

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RELATIONSHIPS

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system element behavior ← mentions 93% 1e
Arithmetic logic units are listed as system elements whose behavior is described.

LINKED ENTITIES

2 links
Y86-64 used_in Y86-64 is the instruction set architecture whose SEQ and PIPE reference implementations both contain the ALU described in the evidence.
TheHuzz exemplified_by TheHuzz is the RTL instruction-fuzzing framework used to find ALU flag-generation defects in mor1kx.

CITATIONS

11 sources
11 citations — click to expand
[1] An arithmetic logic unit is a hardware block that performs arithmetic and logical operations, and in the SEQ implementation it is the execute-stage functional block with inputs ALU A and ALU B, an ALU fun. control input, condition-code output CC, and result output valE. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[2] The ALU operates on some combination of values read from the register file, immediate data from the instruction, and numeric constants. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[3] The ALU can perform either addition or the operation called for by an arithmetic or logical instruction. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[4] Within a SEQ clock cycle, the ALU is used after instruction fetch, PC increment, and register reads, and before optional memory access and write-back; some combination of the memory result and the ALU result is written back to the registers. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[5] PIPE is a five-stage pipelined implementation of the Y86-64 instruction set that partitions the computation into similar stages and uses the same set of functional blocks as SEQ (including the ALU), augmented with pipeline registers, data connections, and control logic to support up to five instructions in flight and resolve hazards. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[6] In mor1kx (TheHuzz Bug B5), the carry-flag logic for subtract instructions is implemented incorrectly, leading to a mismatch between RTL and the golden reference model and a risk of incorrect computations including in cryptographic functions; detected with 20 inputs and classified as a core-and-compute issue (CWE-1201). TheHuzz: Instruction Fuzzing of Processors Using Golden Reference Models
[7] In mor1kx (TheHuzz Bug B9), the overflow-bit update logic for multiply-and-subtract (MSB) and multiply-and-accumulate (MAC) instructions is incomplete, so software relying on overflow detection fails to detect overflow events; classified as CWE-1201 and detected with 3.35 × 10³ inputs. TheHuzz: Instruction Fuzzing of Processors Using Golden Reference Models
[8] In mor1kx (TheHuzz Bug B10), the overflow-flag logic for subtract instructions is implemented incorrectly, which can compromise security mechanisms that rely on the overflow flag; classified as CWE-1201 and detected with 2.21 × 10⁴ inputs. TheHuzz: Instruction Fuzzing of Processors Using Golden Reference Models
[9] A reversible arithmetic logic unit has been constructed for quantum arithmetic that effects basic arithmetic operations in a reversible way, and combined with a control unit can integrate classical arithmetic and logic operations into a reversible system; the work is presented as actual evidence that a reversible programmable logic device (RPLD) is realizable. Reversible arithmetic logic unit
[10] A 4-bit superconductor ALU implemented in a bit-parallel architecture operates above 30 GHz, consumes 4.75 mW including applied reverse current (static and dynamic), contains more than 9000 Josephson junctions with about 7000 used for wiring/delay lines/path balancing, has more than 18% bias margin, operates at 4 K in a pulse-tube cryocooler, and is designed as a co-processor interfacing to external CMOS memory and processors. Efficient Superconductor Arithmetic Logic Unit for Ultra-Fast Computing
[11] Digital and analog simulations and experimental verification of the fabricated superconductor ALU confirmed successful operation of both the arithmetic and logic units. Efficient Superconductor Arithmetic Logic Unit for Ultra-Fast Computing