<P> A power supply unit (PSU) converts alternating current (AC) electric power to low - voltage DC power for the internal components of the computer . Laptops are capable of running from a built - in battery, normally for a period of hours . </P> <P> The motherboard is the main component of a computer . It is a board with integrated circuitry that connects the other parts of the computer including the CPU, the RAM, the disk drives (CD, DVD, hard disk, or any others) as well as any peripherals connected via the ports or the expansion slots . </P> <P> Components directly attached to or to part of the motherboard include: </P> <Ul> <Li> The CPU (central processing unit), which performs most of the calculations which enable a computer to function, and is sometimes referred to as the brain of the computer . It is usually cooled by a heatsink and fan, or water - cooling system . Most newer CPUs include an on - die Graphics Processing Unit (GPU). The clock speed of CPUs governs how fast it executes instructions, and is measured in GHz; typical values lie between 1 GHz and 5 GHz . Many modern computers have the option to overclock the CPU which enhances performance at the expense of greater thermal output and thus a need for improved cooling . </Li> <Li> The chipset, which includes the north bridge, mediates communication between the CPU and the other components of the system, including main memory . </Li> <Li> Random - access memory (RAM), which stores the code and data that are being actively accessed by the CPU . For example, when a web browser is opened on the computer it takes up memory; this is stored in the RAM until the web browser is closed . RAM usually comes on DIMMs in the sizes 2GB, 4GB, and 8GB, but can be much larger . </Li> <Li> Read - only memory (ROM), which stores the BIOS that runs when the computer is powered on or otherwise begins execution, a process known as Bootstrapping, or "booting" or "booting up". The BIOS (Basic Input Output System) includes boot firmware and power management firmware . Newer motherboards use Unified Extensible Firmware Interface (UEFI) instead of BIOS . </Li> <Li> Buses that connect the CPU to various internal components and to expand cards for graphics and sound . </Li> <Li> The CMOS battery, which powers the memory for date and time in the BIOS chip . This battery is generally a watch battery . </Li> <Li> The video card (also known as the graphics card), which processes computer graphics . More powerful graphics cards are better suited to handle strenuous tasks, such as playing intensive video games . </Li> </Ul>

Which hardware component is used to control the operation of a computer system