The Computational Core of Computer Processors
The Arithmetic Logic Unit (ALU) is the computational core of the CPU.
It is a combinational digital circuit designed to perform arithmetic and logical operations on binary data.
ALU takes inputs (operands + control signals), processes them, and produces an output (result + status flags).
The ALU is the "brain inside the brain" — the part of the processor that actually does the math and logic.
In Computer Organization and Architecture:
Directs the operation
Store temporary data
Performs calculations and decisions
The ALU can be divided into two main categories:
Binary addition with carry handling
Example: 0101 + 0011 = 1000 (5 + 3 = 8)
Using 2's complement method
Example: 0101 - 0011 = 0010 (5 - 3 = 2)
Adding or subtracting 1
In more advanced ALUs
Bitwise AND (both bits must be 1)
Example: 0101 AND 0011 = 0001
Bitwise OR (either bit is 1)
Example: 0101 OR 0011 = 0111
Bitwise inversion (flip 0 → 1, 1 → 0)
Example: NOT 0101 = 1010
Result = 1 only if inputs differ
Example: 0101 XOR 0011 = 0110
Equal, greater than, less than
1 if result = 0
1 if addition overflowed
1 if result is negative
1 if signed overflow occurs
Suppose ALU has two inputs:
Result = 1000 (binary 8)
Flags → Z=0, C=0, S=0, V=0
Result = 0010 (binary 2)
Flags → Z=0, C=0, S=0, V=0
Result = 0001
Result = 0110
In modern CPUs:
Every arithmetic or logic instruction (ADD, SUB, AND, OR) is carried out by ALU
Comparisons (like if A > B) rely on ALU's condition checking
ALU calculates effective memory addresses in load/store instructions
Based on ALU's flags, control unit decides next instruction (e.g., branch if zero)
The Arithmetic Logic Unit (ALU) is the computational engine of the CPU.
It executes arithmetic operations (like add, subtract, multiply, divide) and logical operations (like AND, OR, NOT, XOR).
It is a combinational circuit, meaning it gives results immediately based on inputs (no memory of past states).
Without ALU, a CPU cannot perform even the simplest computation.
e.g., 0101 + 0011 = 1000 (5 + 3 = 8)
e.g., 0101 - 0011 = 0010 (5 – 3 = 2)
e.g., 0011 × 0010 = 0110 (3 × 2 = 6)
e.g., 0110 ÷ 0010 = 0011 (6 ÷ 2 = 3)
bitwise AND (both bits must be 1)
Example: 0101 AND 0011 = 0001
bitwise OR (either bit is 1)
Example: 0101 OR 0011 = 0111
bitwise inversion (flip 0 → 1, 1 → 0)
Example: NOT 0101 = 1010
result = 1 only if inputs differ
Example: 0101 XOR 0011 = 0110
ALU's internal adder, subtractor, logic gates execute the operation
Flags are 1-bit indicators set/reset by ALU to describe result properties:
1 if result = 0
1 if there is a carry-out (in addition) or borrow (in subtraction)
1 if result is negative (MSB = 1)
1 if signed overflow occurs
ALU = the heart of the CPU, handling all arithmetic and logical work
Faster ALU = faster CPU = faster system performance
High-performance CPUs may have multiple ALUs for parallel processing
Handles basic arithmetic, comparisons, bit manipulation, address calculations, etc.
Extended to FPUs (Floating Point Units) for real numbers
Used in GPUs for graphics and parallel workloads
ALU must be accurate: even a single wrong calculation could crash a program
Errors here propagate through entire CPU
The ALU is the calculation powerhouse of the CPU.
Performs arithmetic, logic, and (sometimes) data transfer operations.
Works closely with registers and control unit.
Its speed, accuracy, and versatility directly impact CPU performance.
Extended into FPUs, GPUs, DSPs for specialized tasks.
The ALU is the foundation of all computations in digital systems — everything from adding two numbers, comparing conditions, to running AI or graphics pipelines, ultimately depends on this unit.