Instruction Format in Computer Architecture

Last Updated on July 20, 2023 by Mayank Dham

In the realm of computer architecture, instructions are the fundamental building blocks that govern the execution of programs on a processor. To efficiently and accurately carry out a wide range of operations, processors rely on specific formats to represent and decode instructions. These instruction formats play a crucial role in defining the structure and interpretation of machine instructions.

In this article, we will delve into the world of instruction formats in computer architecture, exploring their significance, types, and components. We will embark on a journey to understand how instruction formats enable processors to execute complex tasks by organizing and encoding instructions in a structured manner. Let’s discuss what is instruction format in computer architecture.

What is Instruction Format in Computer Architecture?

You must be conscious that a program written in a high-level language cannot be executed directly by the CPU. As a result, each software is first converted to binary format. The high-level program is translated by the compiler into its corresponding low-level instruction, which contains the numbers 0 and 1. These commands are computer-organized machine orders that the processor can carry out immediately.

The operating system loads the RAM with the machine commands. These instructions are fetched one by one by the CPU to begin the processing. The instruction registers contain these received instructions. The bits in machine instructions are categorized according to the format of the command. The CPU can decode a particular piece of information from each bit. Information required by the CPU contains the address of the data and the operation to be done.

A rectangular box that represents the instruction bits in memory words or a control register is used to represent the instruction structure. Three sections make up the collection of bits:

• Addressing Mode: The addressing mode indicates how the data is represented.
• Opcode: The opcode part indicates the operation type on the data.
• Operand: The operand part indicates either the data or the address of the data.

Let’s review and talk about a few key ideas linked to instruction format in computer architecture before continuing:

• Operational codes, operands, an opcode, and an addressing method are all included in an instruction.
• The most basic problem with the format design is the instruction length. The time required to receive an instruction will increase with its length.
• The range of the memory is directly proportional to the number of bits. In other words, the wider the range needed, the more number bits will be needed.
• The memory range that the instruction must target will be greater than the physical memory if a system supports virtual memory.
• The instruction duration should be the same as the data bus length or a multiple of it.

What is Instruction Format?

The forms for the instructions are a series of bits (0 and 1). When these pieces are combined, they form fields. The CPU receives precise information about the operation and location of the data from each field of the machine.

The bit configuration for an instruction is also specified by the instruction format. It may have a variety of locations and be of varying lengths. The address elements in the instruction format change depending on how the CPU’s registers are set up. The CPU’s supported file formats rely on the Instructions Set Architecture the processor has put in place.
Depending on the multiple address fields, the instruction is categorized as follows:

• Zero address instruction
• One address instructions
• Two address instruction
• Three address instruction

Data that is stored in memory or processor registers is used to carry out the tasks defined by a computer instruction. A CPU register’s operands are identified by an address. A binary integer with k bits called the registered address designates one of the 2k registers in the CPU. Therefore, a CPU with 16 CPUs will have a four-bit register address field and registers R0 through R15.

Example of Instruction format: The binary number 0011 will designate register R3.

Computer instructions can be any length and comprise any number of addresses. The internal layout of a computer’s registers determines how many address spaces it has. The majority of computers fit into one of three categories:

• Single accumulator organization.
• General register organization.
• Stack organization.

Single Accumulator Organization

An implied accumulator register is used in every action on a system. This type of computer utilizes one address field for the instruction format.

For instance, the assembly language command ‘ADD’ defines the instruction for arithmetic addition.
The action is produced by the ADD instruction, where X is the address of the operand.

AC ← AC + M[X].

AC is the accumulator register, and M[X] symbolizes the memory word located at address X.

General Register Organization

In their command format, general register-type computers use two or three address fields. Each address column identifies a memory or a processor register. The procedure R1 R + M [X] is specified by an instruction with the symbol ADD R1, X.
The memory address X and register R1 are the two address elements for this instruction.

Stack Organization

The PUSH and POP commands on a computer with a stack organization need an address field. As a result, the word at address X is pushed to the head of the stack by the instruction PUSH X. The stack pointer immediately updates. Since the operation is done on the top two items of the stack, stack-organized computers don’t need an address field for the operation type instructions.

Types of Instruction Format in Computer Architecture

Below are the types of instruction format in Computer Architecture.

Zero Address Instruction

The location of the operands is tacitly represented because this instruction lacks an operand field. These commands are supported by the stack-organized computer system. It is necessary to translate the arithmetic expression into a reverse polish notation in order to assess it.

Example of Zero address instruction: Consider the actions below, which demonstrate how the expression X = (A + B) (C + D) will be formatted for a stack-organized computer.

TOS: Top of the Stack
PUSH        A       TOS ← A
PUSH        B       TOS ← B
ADD                     TOS ← (A + B)
PUSH        C       TOS ← C
PUSH        D       TOS ← D
ADD             TOS ← (C + D)
MUL             TOS ← (C + D) ∗ (A + B)
POP     X           M [X] ← TOS

One Address Instruction

This instruction performs data manipulation tasks using an implied accumulator. A register that the CPU uses to carry out logical processes is called an accumulator. The accumulator is inferred in one address instruction, so it doesn’t need an explicit reference. A second register is required for addition and subtraction. Instead, we’ll ignore the second register in this case and presume that the accumulator already holds the outcomes of all the operations.

Example of One address instruction: The program to evaluate X = (A + B) ∗ (C + D) is as follows:

LOAD        A       AC ← M [A]
ADD     B       AC ← A [C] + M [B] 
STORE       T       M [T] ← AC 
LOAD        C       AC ← M [C] 
ADD     D       AC ← AC + M [D] 
MUL     T       AC ← AC ∗ M [T] 
STORE       X       M [X] ← AC

All actions involve a memory operand and the accumulator (AC) register.
Any memory address is M[].
M[T] points to a temporary memory spot where the interim outcome is kept.

There is only one operand space in this instruction format. To transfer data, this address field employs two unique instructions, namely:

• LOAD: This is used to transfer the data to the accumulator.
• STORE: This is used to move the data from the accumulator to the memory.

Two Address Instructions

The majority of commercial computers use this command. There are three operand fields in this address command format. Registers or memory addresses can be used in the two address sections.

Example of Two address instruction: The program to evaluate X = (A + B) ∗ (C + D) is as follows:

MOV     R1, A       R1 ← M [A]
ADD     R1, B       R1 ← R1 + M [B]
MOV     R2, C       R2 ← M [C]
ADD     R2, D       R2 ← R2 + M [D]
MUL     R1, R2      R1 ← R1∗R2
MOV     X, R1       M [X] ← R1

The MOV command moves the operands from the processor registers to the memory. sensors R1, R2.

Three Address Instruction

A three-address command must have three operand elements in its format. These three fields could either be registers or memory locations.

Example of Three address instruction: The assembly language program X = (A + B) * (C + D) Take a look at the instructions that follow, which describe the register transfer procedure for each instruction.

ADD     R1, A, B        R1 ← M [A] + M [B] 
ADD     R2, C, D        R2 ← M [C] + M [D] 
MUL     X, R1, R2       M [X] ← R1 ∗ R2

R1 and R2 are the two CPU registers.
The operand at the memory location represented by A is indicated by the symbol M [A]. The data or location that the CPU will use is contained in operands 1 and 2. The address of the output is in operand 3.

What is Instruction Pipeline in Computer Architecture?

In addition to the data stream, the command stream can also undergo pipeline processing.
In order to perform tasks like fetching, decoding, and executing instructions, the majority of digital processors with complicated instructions need an instruction pipeline.
Each command must generally be processed by the machine in the order listed below.

• Fetch instructions from memory.
• Decode the instruction.
• Calculate the effective address.
• Fetch the operands from memory.
• Execute the instruction.
• Store the result in the proper place.

Each phase is carried out in its own segment, and occasionally various segments may require varying amounts of time to process the incoming data. Additionally, there are instances when two or more segments may need to access memory simultaneously, necessitating one section to wait until the other is done. If the instruction cycle is split into equal-length parts, the structure of an instruction pipeline will be more effective. A four-segment instruction pipeline is among the most prevalent instances of this form of organization.
In a four-segment instruction chain, two or more distinct segments are combined to form a single segment. For instance, the decoding of the command and determining the effective location can both be done in one section.

Block Diagram of Instruction Pipeline in Computer Architecture:

The block diagram that follows depicts a standard four-segment instruction flow. Four sections make up the teaching sequence.

Segment 1:
First in, first out (FIFO) buffers can be used to perform the instruction fetch section.

Segment 2:
The memory-fetched command is decoded in the second section before the effective location is ultimately determined in a different arithmetic circuit.

Segment 3:
In the third section, an input is retrieved from memory.

Segment 4:
In the concluding section of the pipeline organization, the directions are eventually carried out.

Advantages of Instruction Format in Computer Architecture:

• Efficient Execution: Well-designed instruction formats allow processors to efficiently execute instructions by providing a clear and structured representation of operations. By organizing instructions in a format that the processor can easily decode and execute, instruction formats contribute to improved performance and reduced execution time.
• Code Density: Instruction formats play a crucial role in code density, which refers to the amount of code required to perform a specific task. Compact instruction formats, such as fixed-length formats, can lead to denser code, reducing memory requirements and improving cache utilization. This is particularly advantageous in embedded systems or applications with limited memory resources.
• Simplified Instruction Decoding: A well-defined instruction format simplifies the decoding process for the processor. When instructions are organized in a consistent and predictable manner, the decoder can efficiently extract the necessary information, such as the opcode and operands, for instruction execution. Simplified instruction decoding contributes to faster and more efficient processing.
• Flexibility and Expandability: Instruction formats can be designed to provide flexibility and expandability in the instruction set architecture (ISA). Variable-length or hybrid formats allow for a wider range of instructions to be supported, including complex operations or specialized instructions. This flexibility enables the processor to handle diverse tasks and adapt to evolving computing needs.
• Improved Compiler Optimization: Clear and well-structured instruction formats provide opportunities for compilers to optimize code generation. Compilers can leverage the knowledge of instruction formats to generate code that takes advantage of specific instructions or encoding patterns. This optimization can lead to improved execution speed and efficiency.
• Efficient Operand Handling: Instruction formats define how operands are represented and accessed. By using appropriate addressing modes and operand formats, instruction formats enable efficient operand handling, minimizing memory accesses and reducing data movement. This contributes to improved performance and reduced latency in data operations.
• Simplicity in Hardware Design: Instruction formats that follow a consistent structure simplify the design and implementation of hardware components, such as instruction decoders and execution units. Clear and straightforward instruction formats allow for more efficient hardware implementation, reducing complexity and potential design errors.
• Portability and Compatibility: Well-defined instruction formats enhance portability and compatibility across different architectures and implementations. Standardized instruction formats ensure that software written for one processor can be easily ported or executed on another processor that follows the same instruction format, facilitating code reusability and software development.

Disadvantages of Instruction Format in Computer Architecture:

• Limited Instruction Set Expressiveness: Instruction formats impose constraints on the representation and organization of instructions. While this enables efficient execution, it can also limit the expressiveness and range of operations that can be performed. Certain complex or specialized operations may be challenging to represent within the constraints of the instruction format, potentially requiring additional instructions or workarounds.
• Increased Instruction Decoding Complexity: Instruction formats that allow for variable-length or complex instructions can increase the complexity of instruction decoding. Decoding variable-length instructions may require more intricate logic and introduce potential decoding errors or inefficiencies. Complex instruction formats may also require more hardware resources, leading to increased cost and complexity in the design of the processor.
• Code Generation Challenges: Instruction formats may pose challenges for compilers and code generation. Compilers need to generate code that adheres to the specific constraints and semantics of the instruction format. Complex instruction formats may require more sophisticated analysis and optimization techniques in code generation, increasing the complexity of the compiler design and compilation process.
• Backward Compatibility Concerns: Introducing changes or updates to the instruction format can create compatibility concerns for existing software and systems. Alterations in the instruction format may render previously written code incompatible or require modifications for proper execution. Ensuring backward compatibility while introducing new instruction formats can be a challenging task.
• Trade-off Between Code Density and Decoding Efficiency: Instruction formats that prioritize code density, such as variable-length formats, can introduce challenges in decoding efficiency. Variable-length instructions may require more time and resources to decode, impacting overall processor performance. Achieving a balance between code density and decoding efficiency becomes a crucial consideration in the design of instruction formats.
• Increased Complexity in Hardware Design: Instruction formats that deviate from simple fixed-length formats can lead to increased complexity in hardware design. Variable-length instructions or formats with complex addressing modes and operand representations require more intricate circuitry, leading to increased hardware design complexity, potential performance bottlenecks, and increased production costs.
• Instruction Fetch Overhead: Instruction formats, especially those with variable-length instructions, can introduce instruction fetch overhead. Fetching and aligning variable-length instructions from memory may require additional time and resources compared to fixed-length instructions. This overhead can impact the overall execution time and performance of the processor.
• Increased Programmer Effort: Non-standard or complex instruction formats may require programmers to invest more effort in understanding and utilizing them effectively. Learning and adhering to specific instruction format conventions and constraints can introduce additional complexity for software development and maintenance.

Working of the CPU

You must be conscious that a program written in a high-level language cannot be executed directly by the CPU. As a result, each software is first converted to binary format. The high-level program is translated by the compiler into its corresponding low-level instruction, which contains the numbers 0 and 1. These commands are computer-organized machine orders that the processor can carry out immediately.

The operating system loads the RAM with the machine commands. These instructions are fetched one by one by the CPU to begin the processing. The instruction registers contain these received instructions. The bits in machine instructions are categorized according to the format of the command. The CPU can decode a particular piece of information from each bit. Information required by the CPU contains the address of the data and the operation to be done.
A rectangular box that represents the instruction bits in memory words or a control register is used to represent the instruction structure. Three sections make up the collection of bits:

Addressing Mode: The addressing mode indicates how the data is represented.
Opcode: The opcode part indicates the operation type on the data.
Operand: The operand part indicates either the data or the address of the data.

Let’s review and talk about a few key ideas linked to instruction format in computer architecture before continuing:

• Operational codes, operands, an opcode, and an addressing method are all included in an instruction.
• The most basic problem with the format design is the instruction length. The time required to receive an instruction will increase with its length.
• The range of the memory is directly proportional to the number of bits. In other words, the wider the range needed, the more number bits will be needed.
• The memory range that the instruction must target will be greater than the physical memory if a system supports virtual memory.
• The instruction duration should be the same as the data bus length or a multiple of it.

Conclusion
In this article, we explored the significance of instruction format in computer architecture, their types, components, and the impact they have on processor performance. We learned that instruction formats play a vital role in organizing and encoding instructions, allowing processors to interpret and execute them accurately and efficiently. Different types of instruction formats, such as fixed-length, variable-length, and hybrid formats, offer varying advantages and trade-offs in terms of code density, instruction decoding complexity, and flexibility.

By understanding the components of instruction formats, including opcodes, operands, addressing modes, and control bits, developers gain insights into how instructions are represented and how processors perform operations on data. Instruction formats have a direct impact on instruction decoding, execution time, and overall processor performance.

FAQs related to Instruction Format in Computer Architecture

1. What is the 3 address instruction format?
A sort of machine-specific command is the three-address instruction. It has one opcode and three address elements. Two address entries are used—one for the source and one for the destination—to identify the location. For instance, X Equals (A + B).

2. What is an example of an instruction format?
This sort of computer utilizes one address field for the command format. For instance, the assembly language command ADD defines the arithmetic addition instruction. where X is the operand’s location. In this instance, the ADD command yields the action AC AC + M[X].

3. What is the instruction format of 8085?
The 8085 instruction set has a 1-byte opcode. We can create 256 distinct binary algorithms using 8-bit binary code. For opcodes, 246 numbers have been used in this. Instructions for the 8085 architecture can be 1, 2, or 3 bytes in size.

4. What is the 1-address instruction format?
For data processing, this uses an inferred ACCUMULATOR register. The accumulator holds one input, and the register or memory address holds the other. Implied indicates that there is no need to explicitly state that one operand is in the accumulator because the Processor already knows this. 3.

5. What is RISC vs CISC?
CISC stands for Complicated Instruction Set Computer, whereas RISC stands for Reduced Instruction Set Computer. Out of these, RISC computers have the fewest addressing nodes and a relatively smaller collection of commands.

6. What is a 32-bit instruction format?
Six elements make up the 32-bit instruction: cond, op, funct, Rn, Rd, and Src2. The fields marked in blue, op (also known as the opcode or operation code) and funct (also known as the function code), encode the action that the command executes.