A control register is a

processor register A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. ...

which changes or controls the general behavior of a

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and

coprocessor A coprocessor is a computer processor used to supplement the functions of the primary processor (the CPU). Operations performed by the coprocessor may be floating-point arithmetic, graphics, signal processing, string processing, cryptography o ...

control.

Control registers in x86 series

CR0

The CR0 register is 32 bits long on the

386 __NOTOC__ Year 386 ( CCCLXXXVI) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Honorius and Euodius (or, less frequently, year 113 ...

and higher processors. On x64 processors in

long mode In the x86-64 computer architecture, long mode is the mode where a 64-bit operating system can access 64-bit instructions and registers. 64-bit programs are run in a sub-mode called 64-bit mode, while 32-bit programs and 16-bit protected mode pr ...

, it (and the other control registers) is 64 bits long. CR0 has various control flags that modify the basic operation of the processor. Register CR0 is the 32 Bit version of the old

Machine Status Word A machine is a physical system using power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolecule ...

(MSW) register. The MSW register was expanded to the Control Register with the appearance of the i386 processor.

CR1

Reserved, the CPU will throw a #