Computer architecture

The Central Processing Unit (CPU) is often called the 'brain' of the computer. Its main job is to process all the data and instructions that make a computer work. It does this by continuously running a process called the Fetch-Decode-Execute cycle. This cycle is the fundamental operation of a CPU. It involves three main stages: 1. Fetch: The CPU retrieves the next instruction from memory (RAM). 2. Decode: The CPU's Control Unit interprets the instruction to understand what needs to be done. 3. Execute: The instruction is carried out. This might involve a calculation by the Arithmetic Logic Unit (ALU), moving data, or changing the state of the computer.

Most modern computers are based on the Von Neumann architecture. This design states that both program instructions and data are stored in the same memory space and are treated the same way. The CPU contains several key components to manage this. The Control Unit (CU) acts like a traffic cop, directing the flow of data and coordinating the actions of the other components. The Arithmetic Logic Unit (ALU) performs all mathematical calculations (add, subtract) and logical operations (AND, OR, NOT). Registers are small, extremely fast memory locations inside the CPU used for temporary storage. Key registers include: Program Counter (PC) which holds the address of the next instruction; Memory Address Register (MAR) which holds the address of the memory location to be accessed; Memory Data Register (MDR) which holds the data being transferred to or from memory; and the Accumulator (ACC) which stores the results of calculations from the ALU.

The performance of a CPU is not determined by a single factor, but by a combination of features. The three most important are Clock Speed, Number of Cores, and Cache Size. Clock Speed is the number of Fetch-Decode-Execute cycles the CPU can perform per second, measured in Gigahertz (GHz). A 3 GHz CPU can perform 3 billion cycles per second. A higher clock speed generally means faster processing. Cores are individual processing units within the CPU. A dual-core CPU has two processing units, a quad-core has four. More cores allow the CPU to perform multiple tasks simultaneously (parallel processing), which significantly improves performance in multitasking environments. Cache is a small amount of very fast, expensive memory located on the CPU itself. It stores frequently used instructions and data, so the CPU doesn't have to wait for them to be fetched from the much slower RAM. A larger cache size can lead to a big performance boost.

Primary memory is the main memory in a computer system that is directly accessible by the CPU. There are two main types: RAM and ROM. Random Access Memory (RAM) is used to store the operating system, currently running applications, and the data they are using. RAM is volatile, which means its contents are lost when the computer is powered off. It is also read/write, meaning data can be both read from it and written to it. The more RAM a computer has, the more applications it can run smoothly at the same time. Read-Only Memory (ROM) is non-volatile, meaning it retains its data even when the power is off. It is read-only, so its contents cannot be easily changed. ROM contains the essential startup instructions for the computer, known as the BIOS (Basic Input/Output System) or bootstrap loader. This program checks the hardware and loads the operating system from the hard drive into RAM when the computer is turned on.

Secondary storage is non-volatile, long-term storage for programs and data. It's where your files, applications, and operating system are saved when the power is off. It is not directly accessed by the CPU. Data must first be copied from secondary storage into RAM to be used. There are three main types: Magnetic storage, like a Hard Disk Drive (HDD), uses spinning magnetic platters and a read/write head to store data by magnetising spots on the disk's surface. Optical storage, like CDs, DVDs, and Blu-ray discs, uses a laser to read tiny indentations (pits) and flat areas (lands) on the surface of a spinning disc. Solid-State storage, like a Solid-State Drive (SSD) or USB flash drive, uses flash memory chips. It has no moving parts, making it much faster, more durable, and quieter than an HDD.

Sometimes, a computer's RAM becomes full. To handle this, the operating system can use Virtual Memory. This is a technique where a portion of the secondary storage (like the HDD or SSD) is set aside to be used as an extension of RAM. When RAM is full and a new program needs to be loaded, the OS moves the least-used data from RAM into this space on the hard drive (called the paging file or swap file). This frees up RAM for the new task. However, accessing the hard drive is much slower than accessing RAM, so heavy use of virtual memory can cause the computer to slow down noticeably, a phenomenon known as 'disk thrashing'.

Cloud Storage is a different concept. It involves storing your data on remote servers owned and maintained by a third-party company (like Google Drive or Dropbox), which you access over the internet. Advantages include being able to access your files from any device with an internet connection, easy sharing, and automatic backups. Disadvantages include needing a constant internet connection to access files, potential security and privacy risks, and ongoing subscription costs.

Not all computers are general-purpose machines like PCs or laptops. An embedded system is a combination of computer hardware and software designed to perform a specific, dedicated function within a larger mechanical or electrical system. They are all around us. For example, the computer inside a washing machine only runs the washing cycles; it cannot be used to browse the internet. The system inside a digital watch only tells the time and runs a stopwatch; it cannot run word processing software. Key characteristics of an embedded system are that they have a specific task, are not usually programmable by the end-user, and often need to respond to events in real-time. They typically use a microprocessor, which is a less complex version of a CPU, along with memory and input/output peripherals tailored to their task.