What is a microprocessor?

Like CPUs, microprocessors are commonly thought of as the “brain” of the computer. Unlike traditional CPUs, microprocessors integrate the arithmetic, logic and control circuits of more traditional CPUs into a single multipurpose, clock-driven and register-based digital circuit.

While the terms CPU and microprocessor are often used interchangeably, a microprocessor is essentially an all-in-one CPU on a single chip. This single-chip design reduces the number of vulnerable electrical connections, improving reliability by reducing potential points of failure. As such, microprocessors have almost completely replaced traditional, multicomponent CPUs.

Before microprocessors, computers used racks of ICs to accomplish the computer’s main computing functions. In 1971, a significant shift occurred in computing technology with the introduction of the Intel 4004, the first commercially available microprocessor. This groundbreaking device, designed by Federico Faggin, was not just a response to a request for 12 custom microchips for a calculator from Japanese electronics maker Busicom. It was a landmark moment that revolutionized the way we think about computing, as Intel, instead of producing 12 individual chips, created a general-purpose logic device.

Underpowered by today’s standards, the Intel 4004’s architecture operated at a clock speed of 740 kHz and could only process data streams with a maximum of 4 bits, also known as a “nibble.” While the 4004 is weak compared to modern 8-bit to 64-bit microprocessors, it is universally considered to be a revolutionary breakthrough in circuitry. As predicted by Moore’s law, rapid semiconductor technology development led to major processing power improvements. Manufactured by leading suppliers, such as Intel and Pentium, and used by all major tech companies, such as IBM®, Microsoft and Apple, microprocessors power thousands of systems and applications from TV remote controls to the International Space Station.

Modern microprocessors combine millions of small transistors, resistors and diodes assembled on a semiconductor material to create the key components of a CPU. These components are arranged into various types of unique computer architecture to perform computations and run instructions. An average microprocessor's functions can be broken down into four main steps.

1. Fetch: The microprocessor retrieves (or "fetches") instructions from computer memory. The fetch process can be initiated by automatic or manual input.
2. Decode: The microprocessor "decodes" the instructions, essentially interpreting the input or command into a request and instigating a specific process or computation.
3. Execute: Simply put, the microprocessor performs the required or requested operation.
4. Store: The result of the execution is committed to the computer’s memory.

Microprocessors can complete these processes by combining the main components of a CPU into a singular circuit. The key components of a microprocessor are the following:

• Arithmetic logic unit (ALU): The main logic unit of the CPU, this component performs logical operations, including mathematical calculations and data comparisons.
  
  
• Control unit (CU): The CU circuit interprets instructions and initiates their execution, directing the processor's basic operations.
  
  
• Registers: Registers provide small, fast memory storage used by a CPU to temporarily hold data and instructions during computational processes.
  
  
• Cache memory: Microprocessors and CPUs use cache memory, a high-speed form of memory located close to the CPU, to store frequently accessed data to accelerate performance.
  
  
• Busses and bus interfaces: Bus interfaces provide entry and exit points for data to travel across various groups of wires (referred to as busses), such as the address bus or data bus. Busses and interfaces physically connect different internal components, enabling and facilitating communication within the CPU and other peripherals like input/output (I/O) units.
  
  
• Transistors: One of the main building blocks of ICs, transistors are small semiconductors that regulate, amplify and generate electrical currents and signals. They can also act as simple switches or be combined to form logic gates. The number of transistors is a common indicator of microprocessor power.
  
  
• Processor cores: Individual processing units within microprocessors are known as cores. Modern processors frequently incorporate multiple cores (dual-core, quad-core) allowing for parallel processing by enabling the performance of multiple tasks simultaneously.
  
  
• Clock: Although not all microprocessors contain an internal clock, they are all clock-driven. Some rely on external clock chips, which are known for improved accuracy. Whether internal or external, a microprocessor's clock cycle determines the frequency at which it will carry out commands. Modern clock speeds are measured in megahertz (MHz) and gigahertz (GHz).

The architecture of a microprocessor refers to various design and organization methodologies of the processor’s various CPU components. These are the key architectural elements of a microprocessor:

• Instruction Set Architecture (ISA): The microprocessor’s ISA defines the instruction set that the processor can perform. ISAs like the Reduced Instruction Set Computer (RISC) and Complex Instruction Set Computer (CISC) architectures provide various methods for data processing, offering varying levels of performance, reliability and speed suitable for different types of applications.
  
  
• Data path: A microprocessor’s data path dictates the order in which data moves through the microprocessor’s components (buses, ALU, registers), influencing overall performance.
  
  
• Control path: Similar to the data path, the control path element of a microprocessor’s architecture instructs the sequence of operations and manages data transmission within the CPU.
  
  
• Memory hierarchy: The memory hierarchy is a critical component of the processor’s architecture, providing a structure for different levels of memory (cache, registers, RAM) to optimize for efficient data access and retrieval speed.

Microprocessors range in power, performance, architecture methodologies, size, energy consumption and many other variables and are designed for all sorts of applications. General-purpose microprocessors are common in personal computers and mobile devices, while specialized high-performance units like Digital Signal Processors (DSPs) are designed for demanding tasks like audio, speech, video and image processing.

The following are some of the main types of microprocessors.