What is instruction cycle?

A sequence of instructions makes up a program stored in a computer's memory unit. The CPU executes these instructions by going through a cycle for each instruction. The instruction cycle in a simple computer includes the steps listed below:

• Get the instruction from memory.
• Decode the instruction.
• From memory, read the effective address.
• Execute the instruction.

The instruction cycle is divided into four phases: fetching an instruction from memory, decoding the fetched instruction, reading the address from memory, and finally, instruction execution. The computer processor carries it out.

Role of Component

In a computer, there are a lot of instructions; these instructions are called Instruction Set of Architecture that is ISA. An execute instruction is a machine language instruction that runs data as a machine instruction.
The program to be run consists of instructions stored in memory.

The central processing unit (CPU) executes the program's instructions to finish the task. The CPU is primarily responsible for the execution of instructions. In the CPU, instructions are executed in CPU registers. Now we see some CPU and their registers.

• Central Processing Unit (CPU): A computer's central processing unit (CPU) is regarded as its brain. The CPU is an electrical circuitry that executes instructions from a computer program. In a computer, the Processor operates instructions by completing fundamental arithmetic, logic, control, and I/O operations as necessary. A computer's CPU is mainly responsible for its speed and efficiency. Only the CPU in a computer can execute instructions. However, because the program might be quite lengthy, the CPU loads it into the main memory (RAM) and then fetches and executes instructions one by one from memory.
• Memory Address Register (MAR): It indicates the memory address from which data or instructions are read or stored.
• Program Counter (PC): The PC keeps track of the next instruction to be executed once an ongoing instruction has been completed.
• Instruction Register (IR): Holds the last instruction fetched.

The processor reduces the processing of instructions to a two-step process. The CPU reads (fetches) instructions (codes) one at a time from memory and executes or performs the operation provided. The reading of an instruction from a memory address to a CPU register is known as instruction fetch. Depending on the type of instruction, the execution of this instruction may include multiple operations.

The instruction cycle refers to the processing produced by a single instruction (fetch and execution). The fetch cycle and the execute cycle make up the instruction cycle. If the electric power source is turned off, an unrecoverable fault occurs, or if the program itself quits, the program is terminated.

A program counter is utilized for the fetch cycle of a standard CPU. The program counter keeps track of the next instruction to be fetched. As programs are run in order, the next instruction in the sequence is usually fetched first.

An instruction register is loaded with the fetched instruction in the CPU and binary code. The CPU decodes the command and performs the necessary action.

Steps for Executing of Instruction by CPU

There are six steps to the execution of instructions.

• Fetch instruction: The fetching instructions from the main memory begins the execution cycle. The current program counter will acquire the instruction placed in the instruction register (IR).
• Decode Instruction: During this cycle, the decoder interprets the encoded instruction stored in the IR (instruction register).
• Perform ALU Operation: Two operands in the instruction will be operated on a specified operator in the instructions in an ALU (Arithmetic Logic Unit). For example, if the instruction were to add two numbers, the addition would occur here. The outcome of the process is output by the ALU, which takes two values and outputs one.
• Access Memory: There are only two memory access instructions: LOAD and STORE. A value from memory is copied to a register by LOAD, and a register value is copied to memory by STORE. This step is skipped if you follow any other instructions.
• Update Register File: The ALU's output/result is written back to the register file in this phase to update the register file. It's also possible that the result is due to a LOAD from memory. Some instructions do not provide results that can save. The BRANCH and JUMP instructions, for example, have no outcomes to save.
• Update Program Counter(PC): At the completion of the current instruction's execution, we must update the program counter (PC) to the address of the next instruction so that we may return to step 1 and have the CPU acquire the next instruction. If the instruction was BRANCH or JUMP, the program counter might need to be set to a different memory address than the following one.

Why Instruction Cycle is required?

• An instruction cycle for a computer system is required to properly comprehend the flow of instructions and the execution of an instruction in a computer processor.
• It deals with the entire flow of instructions from the time the computer system boots up to when it shuts down. The internal flow of the central processing unit can be better understood via the instruction cycle, allowing any problems to be quickly remedied.
• It deals with the fundamental functions of a computer processor, and it necessitates a thorough understanding of the many phases involved.
• All sorts of instructions for the computer processing system follow the fetch-decode-execute cycle.

Significance of Instruction of Cycle

• The instructions are the basic actions executed in the central processing unit's main memory, which is why they are crucial for the processor system.
• It is a series of phases that aid in comprehending the flow of instruction. The instruction cycle allows the computer processor to visualize instruction flow from beginning to end.
• It is usual for all instruction sets to require a thorough understanding to perform all operations efficiently.
• The processing time may be easily calculated using the instruction cycle, which aids in determining the Processor's speed.
• The Processor's speed determines how many instructions may be performed in the central processing unit at the same time.

Common Mistake

• Using a value as a reference or vice versa.
• Uninitialized variables' default values are misunderstood.
• We are using unspecified or incorrect string comparison methods.
• Extension approaches are confusing or tricking/ faked.
• Allowing compiler warnings to develop is a bad idea.

Related Concept

Conditional / Branch Instruction

Direct, indirect, and relative are the three types of Conditional instruction. It indicates the instruction contains the target address, specifies where to find the target address (e.g., a register or memory location), or specifies the difference between the current and target addresses. The target address is derived in one of four categories by a branch instruction:

• The sum of a constant and the address of the branch instruction itself is the destination address.
• The absolute address given as an operand to the instruction is the destination address.
• The address in the Link Register is the target address.
• The address in Count Register is the target address.

The target address can be computed far enough ahead of the branch to pre-fetch instructions along the target path using the first two approaches.

Pre-fetching instructions along the branch path using the third and fourth techniques is also possible if the Link Register or Count Register is filled sufficiently ahead of the branch instruction.

Practice Problems

1. ______________ is the brain of computer.

1. Monitor
2. Keyboard
3. CPU
4. None of these

Answer: C

Explanation: The Processor operates instructions by completing fundamental arithmetic, logic, control, and I/O operations, and also CPU executes the program. So it is the brain of the Computer.

2._____________________ indicates the memory address from which data or instructions are to be read  or write.

1. MAR
2. IR
3. PC
4. CPU

Answer: A

Explanation: In a processor, every register has its own function. It is the function of MAR.

3. A value from memory is copied to a ________________ by LOAD, and a register value is copied to __________ by STORE.

1. MAR and PC
2. Register and Memory
3. IR and MAR
4. None of these

Answer: B

Explanation: In a processor, every register has its own function. It is the function of MAR.

4. ______________ keeps track of the next instruction to be executed once an ongoing instruction has been completed.

1. MAR
2. IR
3. PC
4. ALU

Answer: C

Explanation: In a processor, every register has its own function. It is the function of PC.

5. If the instruction is _____________ or _____________, the program counter to be set to a different memory address than the following one.

1. LOAD or STORE
2. BRANCH or JUMP
3. None of these
4. Both A and B

Answer: B

Explanation: In a processor, every register has its own function and their memory location.