CA/U1 Topic 3 Functions of different units of Computer
In order to carry out the operations mentioned in the previous section the computer allocates the task between its various functional units. The computer system is divided into three separate units for its operation. They are
1) Arithmetic logical unit
2) Control unit.
3) Central processing unit.
An arithmetic logic unit (ALU)
An arithmetic logic unit (ALU) is a major component of the central processing unit of a computer system. It does all processes related to arithmetic and logic operations that need to be done on instruction words. In some microprocessor architectures, the ALU is divided into the arithmetic unit (AU) and the logic unit (LU).
An ALU can be designed by engineers to calculate any operation. As the operations become more complex, the ALU also becomes more expensive, takes up more space in the CPU and dissipates more heat. That is why engineers make the ALU powerful enough to ensure that the CPU is also powerful and fast, but not so complex as to become prohibitive in terms of cost and other disadvantages.
The arithmetic logic unit is that part of the CPU that handles all the calculations the CPU may need. Most of these operations are logical in nature. Depending on how the ALU is designed, it can make the CPU more powerful, but it also consumes more energy and creates more heat. Therefore, there must be a balance between how powerful and complex the ALU is and how expensive the whole unit becomes. This is why faster CPUs are more expensive, consume more power and dissipate more heat.
The main functions of the ALU are to do arithmetic and logic operations, including bit shifting operations. These are essential processes that need to be done on almost any data that is being processed by the CPU.
ALUs routinely perform the following operations:
(i) Logical Operations: These include AND, OR, NOT, XOR, NOR, NAND, etc.
(ii) Bit-Shifting Operations: This pertains to shifting the positions of the bits by a certain number of places to the right or left, which is considered a multiplication operation.
(iii) Arithmetic Operations: This refers to bit addition and subtraction. Although multiplication and division are sometimes used, these operations are more expensive to make. Addition can be used to substitute for multiplication and subtraction for division.
An arithmetic logic unit is also known as an integer unit (IU).
Control Unit (CU)
A control unit (CU) handles all processor control signals. It directs all input and output flow, fetches code for instructions from microprograms and directs other units and models by providing control and timing signals. A CU component is considered the processor brain because it issues orders to just about everything and ensures correct instruction execution.
A CU takes its input from the instruction and status registers. Its rules of operation, or microprogram, are encoded in a programmable logic array (PLA), random logic or read-only memory (ROM).
CU functions are as follows:
- Controls sequential instruction execution
- Interprets instructions
- Guides data flow through different computer areas
- Regulates and controls processor timing
- Sends and receives control signals from other computer devices
- Handles multiple tasks, such as fetching, decoding, execution handling and storing results
CUs are designed in two ways:
(i) Hardwired control: Design is based on a fixed architecture. The CU is made up of flip-flops, logic gates, digital circuits and encoder and decoder circuits that are wired in a specific and fixed way. When instruction set changes are required, wiring and circuit changes must be made. This is preferred in a reduced instruction set computing (RISC) architecture, which only has a small number of instructions.
(ii) Microprogram control: Microprograms are stored in a special control memory and are based on flowcharts. They are replaceable and ideal because of their simplicity.
Central processing unit (CPU)
The central processing unit (CPU) is the unit which performs most of the processing inside a computer. To control instructions and data flow to and from other parts of the computer, the CPU relies heavily on a chipset, which is a group of microchips located on the motherboard.
The CPU has two components:
- Control Unit: extracts instructions from memory and decodes and executes them
- Arithmetic Logic Unit (ALU): handles arithmetic and logical operations
To function properly, the CPU relies on the system clock, memory, secondary storage, and data and address buses.
This term is also known as a central processor, microprocessor or chip.
The CPU is the heart and brain of a computer. It receives data input, executes instructions, and processes information. It communicates with input/output (I/O) devices, which send and receive data to and from the CPU. Additionally, the CPU has an internal bus for communication with the internal cache memory, called the backside bus. The main bus for data transfer to and from the CPU, memory, chipset, and AGP socket is called the front-side bus.
The CPU contains internal memory units, which are called registers. These registers contain data, instructions, counters and addresses used in the ALU’s information processing.
Some computers utilize two or more processors. These consist of separate physical CPUs located side by side on the same board or on separate boards. Each CPU has an independent interface, separate cache, and individual paths to the system front-side bus. Multiple processors are ideal for intensive parallel tasks requiring multitasking. Multicore CPUs are also common, in which a single chip contains multiple CPUs.