Arithmetic logic unit (ALU) - what is it?

As you know, the computer processor consists of four basic components: arithmetic logic unit, I / O module, as well as memory and management units. This architecture was defined in the last century and, despite the fact that a lot of time has passed, the classical structure of von Neumann remains relevant.

What is ALU?

Arithmetic logic device is one of the components of the processor, which is necessary for performing logical and arithmetic type transformations, starting elementary and ending with complex expressions. The bit size of the operands used is usually considered the length of a word, or the size.

The main task of the ALU is to process the data stored in the computer's RAM. In addition, the arithmetic logic device is capable of producing control signals that direct the computer to the choice of the correct path for performing the necessary computational process depending on the final data types. All operations involve electronic circuits, each of which is structurally divided into thousands of elements. Such boards are usually high-speed and are characterized by high density.

Depending on the signals that are input, the ALU performs different types of operations with two numbers. Any arithmetic logic device of the computer provides for the implementation of four basic actions, shifts, and logical transformations. The set of operations of ALU is its main characteristic.

The components of the arithmetic logic unit are four main groups of nodes that correspond to the processes of control, transmission, storage and transformation of incoming data.

ALU Storage Units

This category includes:

• The triggers that store the auxiliary bits and the different signs of the results;
• Registers responsible for the integrity of the operands, intermediate and final results.

Sometimes registers of an arithmetic logic device can be combined into a specialized memory block, and triggers can form a single state register.

Transmission nodes ALU

This category includes:

• The buses connecting blocks of the device among themselves;
• Multiplexers and valves responsible for selecting the correct direction of operations.

Conversion nodes ALU

These include:

• Summers that perform micro-operations;
• Logic execution schemes;
• Shifters;
• Correctors for decimal arithmetic;
• Code converters used to obtain inverse or additional data;
• Counters for counting the number of completed cycles and for implementing auxiliary transformations.

Control nodes ALU

This category of objects includes:

• The control unit;
• Signal decoder;
• Logic conversion schemes necessary to form branches for executing firmware.

Action of the processor control unit

This block is responsible for generating a sequence of functional signals necessary for the correct execution of the specified command. As a rule, such transformations are realized for several cycles.

The control device provides automatic execution of the program. At the same time, the necessary coordinated branches of other components of the machine are involved.

The basic principle of microprogramming, which has a clear number of characteristics, is responsible for the operation of the control device.

Classification of ALU

Arithmetic logic devices by the way of operating variables are divided into parallel and sequential ones. The main difference between these ALUs lies in the way in which operands are represented and operations performed.

By the nature of use arithmetic logic devices are divided into multifunctional and block. In the ALU of the first type, the same schemes are used to perform operations with different forms of representing numbers, which adapt to the requested mode of working with data. In block devices, all operations are performed through the distribution by data types. For operations with decimal numbers, numeric and alphabetic fields, digits with a floating or fixed point, different schemes are used. In this case, the arithmetic logic device works much faster due to the parallel performance of the given tasks. But they also have a disadvantage - increased costs for equipment support.

An arithmetic logic device can be used for:

• Decimal numbers;
• Floating-point numbers;
• Numbers with a fixed point.

Device Operations

The ALU structure presupposes the execution of actions through logical functions, which are divided into such groups:

• Decimal arithmetic;
• Binary arithmetic for numbers with a clearly marked point;
• Hexadecimal arithmetic for expressions with a floating delimiter;
• Modification of command addresses;
• Logical type operations;
• Transformation of alphanumeric fields;
• Special arithmetic.

Modern electronic computers are able to implement all the above-mentioned types of activity, and microcomputers do not have such a basic functionality, so the most complex procedures are performed through the connection of small subroutines.

Arithmetic operations and logical procedures

All actions of ALU can be conditionally divided into several groups.

Arithmetic operations include division, multiplication, subtraction of modules, ordinary subtraction and addition.

The logical "and" and "or", that is, the conjunction and disjunction, as well as the comparison of data for equality, are classed as logical transformations. Such procedures, as a rule, are carried out over binary words consisting of multiple digits.

Special arithmetic operations include normalization, logical and arithmetic shifts. There is a significant difference between these transformations. If the arithmetic shift at a location is changed only by digital digits, then at a logical sign the digit is attached to the motion.

Each operation that occurs through the use of an arithmetic logic device can be called a sequence of functions of a logical type, which are described by the multi-bit logic for electronic computers. For example, binary computers use binary logic and so on, up to the decimal system.

Absolutely all arithmetic-logical transformations have their own operands, and the output results are treated as bit strings with sixteen digits. The only exceptions are the primitives of the sign division of DIVS. A variety of flags allow you to treat the data at the output as numbers with a minus or plus sign for overflow. The logic of the bit conversion is built on modulo arithmetic. The flag is placed if unpredictable changes occur with a sign. For example, adding two positive numbers, you should get the result with a "+" sign. But if a carry occurs in the sign bit that sets the unit, and the result is negative, then the overflow flag is set.

The transfer bit logic is based on unsigned arithmetic. This flag is set by the system if the generated transfer from the higher bit can not be written as a result. This ALU bit is very effective when using transformations with multi-word representations.

Conclusion

Arithmetic logic device is used to perform logical and arithmetic transformations over necessary operands, in which roles commands or codes of numbers often appear. After the action is completed, the result is returned to the memory for use in the following calculations.