The instruction unit (I-unit or IU), also called, e.g., instruction fetch unit (IFU), instruction issue unit (IIU), instruction sequencing unit (ISU), in a

central processing unit 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, an ...

(CPU) is responsible for organizing program instructions to be fetched from memory, and executed, in an appropriate order, and for forwarding them to an

execution unit In computer engineering, an execution unit (E-unit or EU) is a part of the central processing unit (CPU) that performs the operations and calculations as instructed by the computer program. It may have its own internal control sequence unit (not ...

(E-unit or EU). The I-unit may also do, e.g., address resolution, pre-fetching, prior to forwarding an instruction. It is a part of the

control unit The control unit (CU) is a component of a computer's central processing unit (CPU) that directs the operation of the processor. A CU typically uses a binary decoder to convert coded instructions into timing and control signals that direct the ope ...

, which in turn is part of the CPU. In the simplest style of

computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...

, the

instruction cycle The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch-execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instruction ...

is very rigid, and runs exactly as specified by the

programmer A computer programmer, sometimes referred to as a software developer, a software engineer, a programmer or a coder, is a person who creates computer programs — often for larger computer software. A programmer is someone who writes/creates ...

. In the instruction fetch part of the cycle, the value of the

instruction pointer The program counter (PC), commonly called the instruction pointer (IP) in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register (IAR), the instruction counter, or just part of the instruction sequencer, is ...

(IP) register is the address of the next instruction to be fetched. This value is placed on the

address bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...

and sent to the memory unit; the memory unit returns the instruction at that address, and it is latched into the

instruction register In computing, the instruction register (IR) or current instruction register (CIR) is the part of a CPU's control unit that holds the instruction currently being executed or decoded. In simple processors, each instruction to be executed is loaded ...

(IR); and the value of the IP is incremented or over-written by a new value (in the case of a jump or branch instruction), ready for the next instruction cycle. This becomes a lot more complicated, though, once performance-enhancing features are added, such as

instruction pipelining In computer engineering, instruction pipelining or ILP is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing inco ...

out-of-order execution In computer engineering, out-of-order execution (or more formally dynamic execution) is a paradigm used in most high-performance central processing units to make use of instruction cycles that would otherwise be wasted. In this paradigm, a proce ...

, and even just the introduction of a simple

instruction cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, which ...

John L. Hennessy John Leroy Hennessy (born September 22, 1952) is an American computer scientist, academician and businessman who serves as Chairman of Alphabet Inc. Hennessy is one of the founders of MIPS Computer Systems Inc. as well as Atheros and served as t ...

and David A. Patterson (1990), ''Computer Architecture: a quantitative approach'', Morgan Kaufmann Publishers, Palo Alto, USA,

References

Central processing unit {{microcompu-stub

See also

References