拡張性（かくちょうせい）とは、機械やソフトウェアなどが本来持つ機能に加えて、追加の機能を組み込んだり、性能を後から向上させることができるような設計上の特徴を指します。
例えば、コンピュータハードウェアの場合、拡張性は重要な要素の一つです。一般的なコンピュータは入力、制御、演算、記憶、出力などの要素で構成されますが、特に汎用コンピュータやパーソナルコンピュータでは、あらかじめ厳密な機器構成を設定することが難しいため、後からシステムを拡張できるような柔軟な設計が求められます。
例えば、パーソナルコンピュータでは、入力デバイスとしてキーボードやマウスが一般的ですが、拡張性を持たせるためにはPS/2コネクタやUSBなどの外部インターフェースを提供し、様々な入力デバイスを接続できるようにします。同様に、制御部分や演算装置も拡張性を持たせ、外部の拡張カードやアップグレードを容易に取り入れることができます。
また、記憶領域や出力装置に関しても拡張性が重要です。ハードディスクの容量が不足する場合や、ディスプレイを大型なものに変更したい場合に、拡張カードや外部インターフェースを利用して容易に拡張できるようになっています。
コンピュータの拡張性を実現するためには、汎用拡張バス（PCI、ISA、Cバス、NuBusなど）や外部インターフェース（USB、IEEE 1394）などが用意されており、これらを活用してハードウェアを柔軟に構築できます。筐体自体にも拡張カードやドライブを内蔵できるスペースを確保するなど、物理的な拡張を可能にする工夫もされています。
Scalability, in the context described, refers to a design feature where machines or software can have additional functionalities added beyond their inherent capabilities. This includes the ability to incorporate added features or improve performance at a later stage.
For computer hardware, the components typically include input, control, processing, storage, and output. When constructing a computer system for a specific purpose, the requirements for each element are determined, and they are combined to build a single computer device. For devices like calculators or embedded computers in household appliances, there is usually no need to modify them later, and therefore, there is no necessity to design them with scalability in mind.
However, for general-purpose computers such as personal computers or home gaming consoles, which are expected to have a broader range of uses during their operational phase, it is not feasible to precisely define the requirements for the device's configuration beforehand. In such cases, a design approach is taken to determine the overall functionality and performance based on the common denominator of user needs. The individual hardware devices needed for specific purposes are then integrated into the system by system integrators or users after the intended use is decided. The degree of freedom in configuring the system ultimately determines the scalability of computer hardware.
Taking the example of personal computers, input is typically done using a keyboard, mouse, or trackpad. Modern personal computers, including laptops, commonly support the expansion of input methods through PS/2 connectors or USB, allowing the connection of devices like trackballs, graphics tablets, scanners, digital cameras, and various sensors for scientific and technical applications. The need for expanding external interfaces arises when connecting different input and output devices, and this is where the concept of extending control mechanisms, such as bus controllers, comes into play.
Concerning the expansion of processing units, upgrades like replacing the CPU or adding coprocessors (e.g., floating-point units or graphic accelerators) are common practices to enhance computational capabilities. This flexibility in upgrading CPUs is an example of designing for scalability.
In cases where storage capacity becomes a limitation, as in running out of hard disk space, a scalable design allows for solutions like adding or replacing hard disk drives. Similarly, when facing memory constraints that hinder launching applications or cause slowdowns, a design considering scalability permits addressing these issues by adding more main memory.
For output devices, scalability involves options such as replacing or adding larger and higher-resolution displays, adding printers, or upgrading speakers.
To facilitate these expansions, personal computers conventionally provide various means of extension. Universal expansion buses like PCI, ISA, C Bus, NuBus, as well as external interfaces like USB and IEEE 1394, play crucial roles in ensuring scalability. Specific interfaces for individual devices, such as disk drive interfaces like ATA, also contribute to the overall scalability. Additionally, the computer chassis itself is designed to accommodate expansion cards and drive devices, underscoring the importance of physical expansion space in ensuring scalability.