Instruction Cycle
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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 A central processing unit (CPU), also called a central processor, main processor or just Processor (computing), processor, is the electronic circuitry that executes Instruction (computing), instructions comprising a computer program. The CPU per ...
(CPU) follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage. In simpler CPUs, the instruction cycle is executed sequentially, each instruction being processed before the next one is started. In most modern CPUs, the instruction cycles are instead executed concurrently, and often in
parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster of ...
, through an
instruction pipeline 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 ...
: the next instruction starts being processed before the previous instruction has finished, which is possible because the cycle is broken up into separate steps.


Role of components

The program counter (PC) is a special
register Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), th ...
that holds the memory address of the next instruction to be executed. During the fetch stage, the address stored in the PC is copied into the memory address register (MAR) and then the PC is incremented in order to "point" to the memory address of the next instruction to be executed. The CPU then takes the instruction at the memory address described by the MAR and copies it into the memory data register (MDR). The MDR also acts as a two-way register that holds data fetched from memory or data waiting to be stored in memory (it is also known as the memory buffer register (MBR) because of this). Eventually, the instruction in the MDR is copied into the current instruction register (CIR) which acts as a temporary holding ground for the instruction that has just been fetched from memory. During the decode stage, the control unit (CU) will decode the instruction in the CIR. The CU then sends signals to other components within the CPU, such as the arithmetic logic unit (ALU) and the floating point unit (FPU). The ALU performs arithmetic operations such as addition and subtraction and also multiplication via repeated addition and division via repeated subtraction. It also performs logic operations such as
AND or AND may refer to: Logic, grammar, and computing * Conjunction (grammar), connecting two words, phrases, or clauses * Logical conjunction in mathematical logic, notated as "∧", "⋅", "&", or simple juxtaposition * Bitwise AND, a boolea ...
, OR, NOT, and binary shifts as well. The FPU is reserved for performing floating-point operations.


Summary of stages

Each computer's CPU can have different cycles based on different instruction sets, but will be similar to the following cycle: # Fetch stage: The next instruction is fetched from the memory address that is currently stored in the program counter and stored into the instruction register. At the end of the fetch operation, the PC points to the next instruction that will be read at the next cycle. # Decode stage: During this stage, the encoded instruction presented in the instruction register is interpreted by the decoder. #*Read the effective address: In the case of a memory instruction (direct or indirect), the execution phase will be during the next clock pulse. If the instruction has an
indirect address Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how the machine language instructions i ...
, the effective address is read from main memory, and any required data is fetched from main memory to be processed and then placed into data registers (clock pulse: T3). If the instruction is direct, nothing is done during this clock pulse. If this is an I/O instruction or a register instruction, the operation is performed during the clock pulse. # Execute stage: The control unit of the CPU passes the decoded information as a sequence of control signals to the relevant functional units of the CPU to perform the actions required by the instruction, such as reading values from registers, passing them to the ALU to perform mathematical or logic functions on them, and writing the result back to a register. If the ALU is involved, it sends a condition signal back to the CU. The result generated by the operation is stored in the main memory or sent to an output device. Based on the feedback from the ALU, the PC may be updated to a different address from which the next instruction will be fetched. #Repeat cycle In addition, on most processors
interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
s can occur. This will cause the CPU to jump to an interrupt service routine, execute that and then return. In some cases an instruction can be interrupted in the middle, the instruction will have no effect, but will be re-executed after return from the interrupt.


Initiation

The cycle begins as soon as power is applied to the system, with an initial PC value that is predefined by the system's architecture (for instance, in Intel
IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation o ...
CPUs, the predefined PC value is 0xfffffff0). Typically, this address points to a set of instructions in
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(ROM), which begins the process of loading (or '' booting'') the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
.


Fetch stage

The fetch step is the same for each instruction: # The CPU sends the contents of the PC to the MAR and sends a read command on the control bus # In response to the read command (with address equal to PC), the memory returns the data stored at the memory location indicated by the PC on the data bus # The CPU copies the data from the data bus into its MDR (also known as MBR; see
Role of components A role (also rôle or social role) is a set of connected behaviors, rights, obligations, beliefs, and norms as conceptualized by people in a social situation. It is an expected or free or continuously changing behavior and may have a given indivi ...
section above) # A fraction of a second later, the CPU copies the data from the MDR to the instruction register for instruction decoding # The PC is incremented so that it points to the next instruction. This step prepares the CPU for the next cycle. The control unit fetches the instruction's address from the memory unit.


Decode stage

The decoding process allows the CPU to determine what instruction is to be performed so that the CPU can tell how many operands it needs to fetch in order to perform the instruction. The opcode fetched from the memory is decoded for the next steps and moved to the appropriate registers. The decoding is typically performed by
binary decoder In digital electronics, a binary decoder is a combinational logic circuit that converts binary information from the n coded inputs to a maximum of 2n unique outputs. They are used in a wide variety of applications, including instruction decoding ...
s in the CPU's
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 op ...
.


Reading the effective address

This step evaluates which type of operation is to be performed. If it is a memory operation, the computer checks whether it's a direct or indirect memory operation: * Direct memory operation - Nothing is done. * Indirect memory operation - The effective address is read from memory. If it is an I/O or register instruction, the computer checks its type and executes the instruction.


Execute stage

The CPU sends the decoded instruction as a set of control signals to the corresponding computer components. If the instruction involves arithmetic or logic, the ALU is utilized. This is the only stage of the instruction cycle that is useful from the perspective of the end-user. Everything else is overhead required to make the execute step happen.


See also

*
Time slice In computing, preemption is the act of temporarily interrupting an executing task, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task. This preempt ...
, unit of operating system scheduling *
Classic RISC pipeline In the history of computer hardware, some early reduced instruction set computer central processing units (RISC CPUs) used a very similar architectural solution, now called a classic RISC pipeline. Those CPUs were: MIPS, SPARC, Motorola 88000, ...
*
Cycles per instruction In computer architecture, cycles per instruction (aka clock cycles per instruction, clocks per instruction, or CPI) is one aspect of a processor's performance: the average number of clock cycles per instruction for a program or program fragment. ...


References

{{DEFAULTSORT:Instruction cycle Instruction processing