- Arithmetic logic unit (ALU) is defined as a circuit board embedded within a computer’s central processing unit (CPU), which performs mathematical and logical operations using gateways made of electrical transistors that can convey signals in 0s and 1s.
- This article explains its meaning, the various components of ALU, and its functionalities.
Table of Contents
What Is an Arithmetic Logic Unit (ALU)?
Arithmetic logic unit (ALU) is a circuit board embedded within a computer’s central processing unit (CPU), which performs mathematical and logical operations using gateways made of electrical transistors that can convey signals in 0s and 1s.
Illustration of ALU
Source: WebeduclickOpens a new window
At its core, the ALU is a digital combinational circuit that executes arithmetic and bitwise functions on integer binary variables. In contrast, a floating-point unit (FPU) handles floating-point values (i.e., numerical operations on a probability-led rendition of real values).
If the CPU is the computer’s brain, then the ALU is its calculator. It is where the circuitry to perform arithmetic computations on information stored in registers occurs. It also contains the circuitry for comparing values, such as determining if the values in registers are identical.
Everything is ultimately coordinated by the CU (control unit), which employs the ALU, memory unit, registers, and other CPU components as required to fetch and decode instructions, read data from storage, execute the commands, and possibly write registers back.
While this overarching function remains the same, arithmetic logic units (ALUs) can be of the following types:
- Integer-based: An integer ALU is intended to execute logical and arithmetic functions on integers that are non-decimal whole numbers. Typically, this ALU is used to execute elementary mathematical operations such as additions, subtractions, multiplications, and divisions, as is frequently used in computers.
- ALUs with a bit-slice structure: A bit-slice ALU composes many smaller ALUs, each responsible for executing operations on a distinct collection of bits. This permits the ALU to operate on larger values by decomposing them into smaller bits.
- Hybrid arithmetic logic units: A hybrid ALU is a mixture of many kinds of ALUs and is intended to execute a wide variety of logical and arithmetic functions. This ALU is often seen in more advanced and powerful computer processors.
How were ALUs developed?
In 1945, the mathematician John von Neumann presented the ALU idea in a study on the EDVAC computer’s underpinnings.
Throughout the early stages of the information era, electronic circuitry was fairly expensive, large, and energy-consuming. Therefore, most early computers, such as the PDP-8, featured a rudimentary ALU that simultaneously worked on a single data bit while presenting programmers with a larger word size (the greatest quantity of bits a CPU can compute at once).
The 1948 Whirlwind I was one of the early computers with several ALU circuits and the ability to process 16-bit words.
Fairchild created the very first ALU constructed as an integrated circuit in 1967 with the Fairchild 3800, which consisted of an 8-bit ALU with only an accumulator. Other integrated ALUs followed quickly. Typically, these devices were “bit slice†capable, which meant that they enabled the use of many linked ALU chips to construct an ALU with a larger word size.
The next phase in the evolution of ALUs was when microprocessors began to appear in the 1970s. Although transistors were reduced in size frequently, there was insufficient room for a full-word-width-complete ALU. Consequently, several early microprocessors had a limited ALU that required many cycles per instruction.
Following Moore’s law, transistor geometries shrunk further over time, making constructing bigger ALUs on microprocessors possible.
Modern integrated circuit (IC) circuits are exponentially thinner than early microprocessors, allowing very complicated ALUs to be accommodated on ICs. Numerous ALUs of the present day have broad word widths and features like barrel shifters or binary multipliers. This enables them to execute (in just one clock cycle) operations that require numerous clock cycles on prior ALUs.
ALUs may be implemented as mechanical, electro-mechanical, or entirely electronic circuits, but their core operations and conceptualization remain unchanged.
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How does an arithmetic logic unit work?
The ALU is equipped with an assortment of electrical output and input connections, resulting in the transmission of digital signals between both the ALU and the external electronics. External circuits supply data to the ALU inputs, and the ALU transmits information directly to the external electronics, indicating the mathematical program’s output.
- Data flow into the ALU: Generally, the ALU has direct access to the CPU controllers, primary memory (random access memory and permanent storage in a personal computer), and input/output devices. The inputs go from memory to the ALU along a bus-like electrical route.
- Understanding the input: The input comprises an instruction word, also known as a machine instruction word, which comprises an operation code or “opcode,†operands (inputs), and a format code. The operation code instructs the ALU on which operation to execute while the operands are utilized throughout the operation. For instance, two operands may be combined or logically compared.
- Applying functions: The ALU employs its internal components to conduct binary computations for various tasks. Various ALU capabilities and designs can manage varying degrees of complexity.
- Output generation: The output comprises a result stored in a memory register with settings indicating whether the operation succeeded. If not, a status will be recorded in a permanent location, commonly referred to as a machine status word.
- Temporary storage: The ALU typically has storage locations for input operands, adding operands, cumulative results (stored in an accumulator), and shifting results. Gated circuits control the movement of bits and the operations executed on them in the ALU’s subunits.
- Programmable algorithms: A sequence logic unit controls the gates in ALU circuits using a specific algorithm or sequencing for every operation code. Multiplications and divisions are performed via a sequence of addition or subtraction, or shifting operations inside the arithmetic unit, but engineers may program the ALU for additional operations as well.
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Where is the ALU used?
Arithmetic logic units are integral to nearly every computing system capable of processing data. It is found in all advanced processors, and its core uses include the following:
- PCs and laptops: In standard computers, the ALU performs the computations and comparisons necessary to run a wide variety of software programs. This includes word processing, spreadsheet, and graphics applications.
- Mainframes and servers: The ALU is used by mainframes and servers to conduct the necessary computations and comparisons to process client requests and provide the appropriate responses.
- Embedded systems and IoT: The ALU is employed in embedded devices intended to execute a single function inside a broader device or ecosystem. Examples include airplane control mechanisms, medical devices, automobile systems, and Internet of Things (IoT) enabled technology.
- Mobile devices: An ALU in mobile devices like smartphones and tablets performs computations and comparisons, similar to what it does on personal computers. This allows individuals to browse the internet and execute programs.
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ALU Components
An ALU has two main components — the arithmetic unit and the logic unit, along with some additional components. Here is a brief overview of each.
ALU Components
1. Arithmetic unit
The arithmetic unit performs all numerical operations in a computer, like addition and subtraction. It also performs other basic operations, such as the increment operation. At its most fundamental level, the ALU component combines two binary digits. Two input signals, X and Y, may yield an output consisting of the summation of two digits. The output, X and Y, are all single bits.
Since X and Y can be 0 or 1, the first three possibilities are 0+0=0, 0+1=1, and 1+0=1 (1 is equal to true, and 0 is equal to false). However, 1+1 in binary language equals (10)2, which has more than one bit. The arithmetic unit ALU has a special circuit called the half adder to accommodate this. In addition to half adders, the ALU has full and 8-bit adders, which are essentially circuit architectures.
2. Logic unit
The logic unit is the second half of the ALU. Instead of mathematical operations, the logic unit performs logical operations such as AND, OR, and XOR. In addition, it conducts numerical tests. For instance, it determines whether or not the integer is negative. It also regulates whether the ALU’s output is zero. In addition, it supports a variety of flags, including overflow, zero, and negative. These flags assist us by identifying if the output is 0 or not and which input is greater than another.
In addition to these core components, the ALU contains:
3. Built-in storage
The ALU typically has storage locations for input operands, adding operands, the cumulative result (saved in an accumulator), and shifting results. Gated circuits control the movement of bits and the tasks performed on them within the ALU’s subunits. These gates are regulated by a sequential logic unit that employs a specific algorithm or sequencing for every operation code. In the arithmetic logic unit, division and multiplication are accomplished using a sequence of additions, subtractions, and shifts.
Let us now take a closer look at the accumulator component.
4. Accumulator
In a computer’s ALU, the accumulator is a buffer that stores intermediary arithmetic logic unit outcomes. During the processing cycles, it stores data or the findings of an operation. During every ALU operation, the accumulator may contain any of the operands. For instance, if we wanted to add two bytes, one would be placed in the accumulator and the second in memory or a general-purpose register.
5. General-purpose register
A register functions as a fast memory for receiving, storing, and transmitting information and instructions, which the ALU (and, therefore, the CPU) will require immediately. General purpose registers comprise additional CPU registers employed whenever data or a memory address is necessary. These registers serve as storage for operands and pointers. Notably, a numerical value used as input for an action is called an operand, and a pointer indicates instructions.
6. Data buses
A data bus is utilized to transport data from the memory to the registers and from the registers to the memory. A fundamental ALU consists of three parallel data buses containing two entry operands (X and Y) and one output. Every data bus is a collection of signals carrying a single binary integer. Generally, the total number of signals composing each bus and the exterior circuit encircling the ALU are similar (such as the enclosing CPU).
7. Circuit operation
The circuit is the last and most important component of the ALU. The ALU is a combined logic circuit, meaning its outputs respond asynchronously to input changes. In normal functioning, the ALU inputs receive steady signals. When sufficient time has elapsed (called the “propagation delayâ€), the outcome of the operation is transferred to the output register.
External circuitry attached to the ALU is accountable for maintaining input signal stability. In addition, adequate time is allowed for the signal to propagate before the findings are evaluated.
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Functions of an ALU
The arithmetic logic unit performs numerous functions. Let’s understand a few.
ALU Functions
1. Basic mathematical operations
It executes arithmetic operations such as addition, subtraction, and multiplication. It may include X and Y as carry-in or carry-out options, which occurs when the result cannot fit in a 1-bit binary structure. Similarly, it can subtract B from A or vice versa with the difference at Y and carry-in or carry-out.
2. Advanced mathematical operations
The ALU can also perform advanced operations like increments and decrements. When X or Y is raised by 1, and the output reflects the new value, this is an increment. Similarly, decrement occurs when X or Y is lowered by 1, with the output representing the new value.
3. Logic operations
It executes logical operations such as AND, OR, X-OR, NOT, etc. These are essentially instructions or rules that help the ALU determine whether something is true or false given a set of conditions. AND and OR functions allow the arithmetic logic unit to understand how two conditions work together. It also plays an important role in logic calculations, such as fuzzy logic.
4. Bit shift operations
This function relates to ALU shift operations, whereby one can shift the X (or Y) operand left or right and output the shifted operand. Typically, simple ALUs can only move the operand by a one-bit position. Complex ALUs use barrel shifters, which enable processors to move the operand by a definite number of bits in a single operation.
5. Data checks through special value
ALUs may also provide distinct outputs. They create different signals that are frequently used to regulate program branching. Traditional ones include Z (zero), C (carry), N (negative), or V. (signed carry). These values may provide a wealth of information. For instance, if the result of subtracting two integers is 0, both must be identical. This is equally applicable to larger than and less than calculations.
6. Transferring data to and from registers
The arithmetic logic unit accepts the accumulator or temporary register’s inputs. Additionally, the result is stored in the accumulator (the temporary register). Lastly, it delivers outcome states to the signal register.
7. Specially programmed operations
Engineers can also program the ALU to perform any operation of their choice, such as in the case of supercomputers. However, as the complexity of the operations increases, the ALU becomes much more expensive as it generates more heat and occupies more space on the CPU. Therefore, there needs to be a balance between the complexity and strength of ALU and its cost. The fundamental reason faster CPUs are much more expensive is that their ALUs use more energy and generate more heat.
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Takeaway
Since the birth of computing, arithmetic logic units have been at the center of mathematical calculations and data-driven performance. Today, ALUs may take a backseat in personal computing but are integral to business operations like data analytics, forecasting, financial analysis, etc.
Companies are constantly working to develop new and more advanced ALUs. For example, GBT Technologies recently filed a patent for a new ALU architecture that uses proprietary circuitry and parallelization to speed up processes. Enterprises must stay abreast of new ALU developments and trends to use the most sophisticated computing technology available today.
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