CPU modes in the context of Central processing units

A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary processor in a given computer. Its electronic circuitry executes instructions of a computer program, such as arithmetic, logic, controlling, and input/output (I/O) operations. This role contrasts with that of external components, such as main memory and I/O circuitry, and specialized coprocessors such as graphics processing units (GPUs).

The form, design, and implementation of CPUs have changed over time, but their fundamental operation remains almost unchanged. Principal components of a CPU include the arithmetic–logic unit (ALU) that performs arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching (from memory), decoding and execution (of instructions) by directing the coordinated operations of the ALU, registers, and other components. Modern CPUs devote a lot of semiconductor area to caches and instruction-level parallelism to increase performance and to CPU modes to support operating systems and virtualization.

In computing, a fatal exception error or fatal error is an error that causes a program to abort (ABEND) and may therefore return the user to the operating system. When this happens, data that the program was processing may be lost. A fatal error is usually distinguished from a fatal system error (colloquially referred to in the MS Windows operating systems by the error message it produces as a "blue screen of death"). A fatal error occurs typically in any of the following cases:

• An illegal instruction has been attempted
• Invalid data or code has been accessed
• An operation is not allowed in the current ring or CPU mode
• A program attempts to divide by zero (only for integers; with the IEEE floating point standard, this creates an infinity instead).

In some systems, such as macOS and Microsoft Windows, a fatal error causes the operating system to create a log entry or to save an image (core dump) of the process.