Instruction cycle
Encyclopedia
An instruction cycle is the basic operation cycle of a computer. It is the process by which a computer retrieves a program
instruction from its memory
, determines what actions the instruction requires, and carries out those actions. This cycle is repeated continuously by the central processing unit
(CPU), from bootup to when the computer is shut down.
The time period during which one instruction is fetched from memory and executed when a computer is given an instruction in machine language. There are typically four stages of an instruction cycle that the CPU carries out: 1) Fetch the instruction from memory. 2) "Decode" the instruction. 3) "Read the effective address" from memory if the instruction has an indirect address. 4) "Execute" the instruction.
(PC), and stored in the Instruction register
(IR).
At the end of the fetch operation, the PC points to the next instruction that will be read at the next cycle.
Clock Pulse: T0-T1
During this cycle the instruction inside the IR (instruction register) gets decoded.
Clock Pulse: T2
If the instruction has an indirect address, 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 at this clock pulse.
If this is an I/O instruction or a Register instruction, the operation is performed (executed) at clock Pulse: T3.
Clock Pulse: T3
Clock Pulse: T3-T6 (Up to T6)
The result generated by the operation is stored in the main memory, or sent to an output device. Based on the condition of any feedback from the ALU, Program Counter may be updated to a different address from which the next instruction will be fetched.
The cycle is then repeated.
CPUs, for instance, the predefined PC value is
(ROM) which begin the process of loading the operating system
. (That loading process is called booting
.)
The fetch cycle processes the instruction from the instruction word which contains an opcode
.
If this is a I/O or Register instruction - the computer checks it's kind and execute the instruction.
The first step of the execute cycle is the Process-Memory. Data is transferred between the CPU and the I/O module. Next is the Data-Processing uses mathematical operations as well as logical operations in reference to data. Central alterations is the next step, is a sequence of operations, for example a jump operation. The last step is a combined operation from all the other steps.
:
(Increment the PC for next cycle at the same time)
The registers used above, besides the ones described earlier, are the Memory Address Register (MAR) and the Memory Data Register (MDR), which are used (at least conceptually) in the accessing of memory. Often, the MDR is expressed as the MBR (Memory Buffer Register).
Fetch and execute example (written in RTL - Register Transfer Language):
PC=0x5AF , AC=0x7EC3 , M[0x5AF]=0x932E , M[0x32E]=0x09AC , M[0x9AC]=0x8B9F.
T0 : AR = 0x5AF (PC)
T1 : IR = 0x932E (M[AR]) , PC=0x5BO
T2 : DECODE = ADD opCode 0x932E , AR=0x32E , I=1. (Indirect instruction)
T3 : AR = 0x9AC (M[AR])
T4 : DR = 0x8B9F
T5 : AC = 0x8B9F + 0x7EC3 = 0x0A62, E = 1 (carry out) , SC = 0
Summary: this example is for an ADD Instruction which made Indirect where:
T0-T1 is the Fetch operation.
T2 is the operation code Decode.
T3 Indirect Memory reference
T4-T5 Execute ADD operation
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
instruction from its memory
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
, determines what actions the instruction requires, and carries out those actions. This cycle is repeated continuously by the central processing unit
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
(CPU), from bootup to when the computer is shut down.
Circuits used
The circuits used in the CPU during the cycle are:- Program CounterProgram counterThe program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
(PC) - an incrementing counter that keeps track of the memory address of which instruction is to be executed next... - Memory Address RegisterMemory address registerThe Memory Address Register is a CPU register that either stores the memory address from which data will be fetched to the CPU or the address to which data will be sent and stored....
(MAR) - holds the address of a memory block to be read from or written to - Memory Data RegisterMemory data registerThe Memory Data Register is the register of a computer's control unit that contains the data to be stored in the computer storage , or the data after a fetch from the computer storage...
(MDR) - a two-way register that holds data fetched from memory (and ready for the CPU to process) or data waiting to be stored in memory - Instruction registerInstruction registerIn computing, an instruction register is the part of a CPU's control unit that stores the instruction currently being executed or decoded. In simple processors each instruction to be executed is loaded into the instruction register which holds it while it is decoded, prepared and ultimately...
(IR) - a temporary holding ground for the instruction that has just been fetched from memory - Control UnitControl unitA control unit in general is a central part of the machinery that controls its operation, provided that a piece of machinery is complex and organized enough to contain any such unit. One domain in which the term is specifically used is the area of computer design...
(CU) - decodes the program instruction in the IR, selecting machine resources such as a data source register and a particular arithmetic operation, and coordinates activation of those resources - Arithmetic logic unitArithmetic logic unitIn computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...
(ALU) - performs mathematical and logical operations
The time period during which one instruction is fetched from memory and executed when a computer is given an instruction in machine language. There are typically four stages of an instruction cycle that the CPU carries out: 1) Fetch the instruction from memory. 2) "Decode" the instruction. 3) "Read the effective address" from memory if the instruction has an indirect address. 4) "Execute" the instruction.
Instruction cycle
Each computer's CPU can have different cycles based on different instruction sets, but will be similar to the following cycle:1. Fetch the instruction
The next instruction is fetched from the memory address that is currently stored in the Program CounterProgram counter
The program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
(PC), and stored in the Instruction register
Instruction register
In computing, an instruction register is the part of a CPU's control unit that stores the instruction currently being executed or decoded. In simple processors each instruction to be executed is loaded into the instruction register which holds it while it is decoded, prepared and ultimately...
(IR).
At the end of the fetch operation, the PC points to the next instruction that will be read at the next cycle.
Clock Pulse: T0-T1
2. Decode the instruction
The decoder interprets the instruction.During this cycle the instruction inside the IR (instruction register) gets decoded.
Clock Pulse: T2
3. Read the effective address
In case of a memory instruction (direct or indirect) the execution phase will be in the next clock pulse.If the instruction has an indirect address, 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 at this clock pulse.
If this is an I/O instruction or a Register instruction, the operation is performed (executed) at clock Pulse: T3.
Clock Pulse: T3
4. Execute the instruction
The Control Unit of CPU passes the decoded information as a sequence of control signals to the relevant function 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.Clock Pulse: T3-T6 (Up to T6)
The result generated by the operation is stored in the main memory, or sent to an output device. Based on the condition of any feedback from the ALU, Program Counter may be updated to a different address from which the next instruction will be fetched.
The cycle is then repeated.
Initiating the cycle
The cycle starts immediately when power is applied to the system using an initial PC value that is predefined for the system architecture (in Intel IA-32IA-32
IA-32 , also known as x86-32, i386 or x86, is the CISC instruction-set architecture of Intel's most commercially successful microprocessors, and was first implemented in the Intel 80386 as a 32-bit extension of x86 architecture...
CPUs, for instance, the predefined PC value is
0xfffffff0). Typically this address points to instructions in a read-only memoryRead-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
(ROM) which begin the process of loading the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
. (That loading process is called booting
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
.)
Fetch cycle
Step 1 of the Instruction Cycle is called the Fetch Cycle. These steps are the same for each instruction.The fetch cycle processes the instruction from the instruction word which contains an opcode
Opcode
In computer science engineering, an opcode is the portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question...
.
Decode
Step 2 of the instruction Cycle is called the decode. The opcode fetched from the memory is being decoded for the next steps and moved to the appropriate registers.Read the effective address
Step 3 is deciding which operation it is. If this is a Memory operation - in this step the computer checks if it's a direct or indirect memory operation:- Direct memory instruction - Nothing is being done.
- Indirect memory instruction - The effective address is being read from the memory.
If this is a I/O or Register instruction - the computer checks it's kind and execute the instruction.
Execute cycle
Step 4 of the Instruction Cycle is the Execute Cycle. These steps will change with each instruction.The first step of the execute cycle is the Process-Memory. Data is transferred between the CPU and the I/O module. Next is the Data-Processing uses mathematical operations as well as logical operations in reference to data. Central alterations is the next step, is a sequence of operations, for example a jump operation. The last step is a combined operation from all the other steps.
The Fetch-Execute cycle in Transfer Notation
Expressed in register transfer notationRegister transfer notation
Register Transfer Notation is a way of specifying the behavior of a digital synchronous circuit. An example of high-level RTN is Verilog, and a low-level example is Register Transfer Language....
:
(Increment the PC for next cycle at the same time)The registers used above, besides the ones described earlier, are the Memory Address Register (MAR) and the Memory Data Register (MDR), which are used (at least conceptually) in the accessing of memory. Often, the MDR is expressed as the MBR (Memory Buffer Register).
Fetch and execute example (written in RTL - Register Transfer Language):
PC=0x5AF , AC=0x7EC3 , M[0x5AF]=0x932E , M[0x32E]=0x09AC , M[0x9AC]=0x8B9F.
T0 : AR = 0x5AF (PC)
T1 : IR = 0x932E (M[AR]) , PC=0x5BO
T2 : DECODE = ADD opCode 0x932E , AR=0x32E , I=1. (Indirect instruction)
T3 : AR = 0x9AC (M[AR])
T4 : DR = 0x8B9F
T5 : AC = 0x8B9F + 0x7EC3 = 0x0A62, E = 1 (carry out) , SC = 0
Summary: this example is for an ADD Instruction which made Indirect where:
T0-T1 is the Fetch operation.
T2 is the operation code Decode.
T3 Indirect Memory reference
T4-T5 Execute ADD operation