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Instruction Set Architecture (ISA) refers to the set of instructions that a processor can execute. It defines the capabilities and functionalities of a processor, including the data types it can handle, the instructions it can execute, and the memory access methods it supports.

An ISA is typically designed by the processor manufacturer and is specific to a particular family or type of processor. It serves as an interface between the software and hardware, allowing the software to communicate with the processor to execute instructions and access memory.

The ISA defines the instruction set that a programmer can use to write programs for the processor. The instruction set includes the machine language instructions that the processor can execute, as well as the assembly language mnemonics that a programmer can use to write code.

There are three types of ISAs:

  1. Complex Instruction Set Computing (CISC): This is an ISA in which instructions can have variable length and perform multiple operations. CISC processors typically have a large number of instructions that can perform complex operations, which can make programming easier but also increase the complexity of the processor.
  2. Reduced Instruction Set Computing (RISC): This is an ISA in which instructions have a fixed length and perform only one operation. RISC processors typically have a smaller number of instructions that perform simple operations, which can make programming more difficult but also increase the speed and efficiency of the processor.
  3. Hybrid Instruction Set Computing (HISC): This is an ISA that combines features of both CISC and RISC architectures. HISC processors typically have a smaller number of instructions that can perform complex operations, but they also have a simpler instruction format and fewer addressing modes than CISC processors.

CISC processors have a large number of complex instructions, while RISC processors have a smaller number of simple instructions. The choice between CISC and RISC depends on the design goals of the processor and the applications it is intended for.

In addition to the instruction set, an ISA also specifies the registers available on the processor, the memory hierarchy, the I/O system, and other architectural features.

These details are crucial for the design of system software, compilers, and other tools that work with the processor.

2. ISA is a Bridge Between a Computer’s Software and Hardware

An Instruction Set Architecture (ISA) is a theoretical representation of a computer system in computer science. It is often referred to as simply “architecture” or “computer architecture”. The physical implementation of an ISA is carried out through a central processing unit (CPU). The ISA specifies how machine code will behave when executed on different implementations of that ISA, and this behavior remains the same across different microarchitectures, ensuring binary compatibility.

An ISA can be modified by adding instructions or features, or by adding support for larger data values or addresses. Even if an ISA is extended, an implementation of the original ISA will still be able to execute machine code written for earlier versions of the ISA without those extensions. However, machine code using the extensions will only be able to run on implementations that support those specific extensions.

an instruction set architecture (ISA)
an instruction set architecture (ISA)

We note that the Microarchitectural level lies just below the ISA level and hence is concerned with the implementation of the basic operations to be supported by the Computer as defined by the ISA.

To put it simply, an instruction set is a bridge between a computer’s software and hardware, consisting of a set of basic instructions that a processor can interpret. It defines the types of instructions to be supported by the processor, including operations, modes, and storage locations. The ISA level exists just below the microarchitectural level and is tied to the implementation of basic operations, enabling multiple processors based on the same ISA but with different microarchitectures to have different performance and efficiencies.

Multiple implementations of an ISA can have different performance, physical size, and monetary cost but are still able to run the same machine code, allowing lower-performance, lower-cost machines to be replaced with higher-performance, higher-cost machines without having to replace software. The progression of the microarchitectures of ISA implementations allows newer, higher-performance implementations to use software that runs on previous generations.

The ability to extend a processor with external hardware modules like FPGA or DSP cores allows for the acceleration of specific functions, achieving better performance, and the addition of custom instructions, including new algorithms or enhancing previous ones. Overall, an ISA allows the same program to run on many different microarchitecture processors by first defining the ISA and then designing different microarchitectures to comply with it.

3. MIPS ISA (Instruction Set Architecture)

MIPS ISA (Instruction Set Architecture) is a computer architecture developed by MIPS Technologies, Inc. that defines the set of instructions that a computer processor can execute. It is a Reduced Instruction Set Computer (RISC) architecture, meaning that it uses a small set of simple instructions that can be executed quickly.

MIPS is one of the most widely used ISAs in education due to its simplicity.

The MIPS ISA was first introduced in the 1980s and has been used in a variety of devices, including personal computers, workstations, embedded systems, and gaming consoles. It is known for its high performance, low power consumption, and simple instruction set.

Some of the key features of the MIPS ISA include a load/store architecture, where data is loaded from memory into registers before being processed and then stored back in memory; a fixed-length instruction format, which simplifies instruction decoding and enables efficient pipelining; and a register file with 32 general-purpose registers.

The MIPS ISA has been widely used in academia for teaching computer architecture and operating systems. It has also influenced the design of other RISC architectures, including the ARM architecture.

3.1. the Main Types of Instructions in the MIPS ISA

The MIPS ISA has a relatively small and simple set of instructions, which can be classified into several categories based on their functionality.

Here are some of the main types of instructions in the MIPS ISA:

  1. Arithmetic and Logic Instructions: These instructions perform arithmetic and logical operations on data in the registers, such as addition, subtraction, bitwise AND/OR/XOR, and shift operations.
  2. Load and Store Instructions: These instructions transfer data between memory and registers. Load instructions load data from memory into a register, while store instructions store data from a register into memory.
  3. Branch and Jump Instructions: These instructions transfer control to a different part of the program. Branch instructions allow conditional or unconditional transfer of control to a nearby instruction, while jump instructions unconditionally transfer control to a distant instruction.
  4. Control Instructions: These instructions change the control flow of the program, such as setting a program counter, enabling or disabling interrupts, and returning from an exception.
  5. Floating-Point Instructions: These instructions perform arithmetic operations on floating-point numbers, which are represented in a special format in the registers.
  6. System Instructions: These instructions perform system-level operations, such as reading or writing to the console, accessing I/O devices, or performing privileged operations.

Each instruction in the MIPS ISA is represented by a unique binary code, which is decoded by the processor to perform the corresponding operation.

3.2 Instruction Format of MIPS ISA

The basic format of an instruction in the MIPS ISA is a 32-bit word, which is divided into several fields that specify different parts of the instruction.

Here are the main fields of an instruction in the MIPS ISA:

  1. Opcode: This field specifies the operation to be performed, such as arithmetic, load/store, branch, or jump.
  2. Registers: This field specifies the source and/or destination registers for the instruction. Most instructions in the MIPS ISA use three registers: two source registers and one destination register.
  3. Immediate Value: Some instructions in the MIPS ISA include an immediate value field, which provides a constant or offset value to be used in the operation.

The instruction format of the MIPS ISA can be divided into several different types, based on the fields that are used.

Here are some of the main instruction types in the MIPS ISA:

  1. R-Type Instructions: These instructions use only registers for their operands and do not include an immediate value field. Examples of R-type instructions include add, sub, and and.
  2. I-Type Instructions: These instructions use an immediate value field for their operands and include a destination register. Examples of I-type instructions include addi, lw, and sw.
  3. J-Type Instructions: These instructions are used for jumping to a specific address and include only an immediate value field. Examples of J-type instructions include j and jal.

The simple and regular instruction format of the MIPS ISA makes it relatively easy to decode and execute instructions quickly, which contributes to its performance and popularity in a variety of applications.

4.The Difference Between Instruction Set Architecture and Microarchitecture

Instruction Set Architecture (ISA) and microarchitecture are both important aspects of computer architecture, but they are different in their scope and level of detail.

ISA defines the set of instructions that a processor can execute, as well as the format of these instructions and their semantics. ISA specifies the interface between the software and hardware components of a computer system, and it provides a high-level abstraction of the processor’s behavior. Examples of ISA include MIPS, ARM, x86, and PowerPC.

On the other hand, microarchitecture refers to the internal implementation of a processor, including the specific circuitry and design choices used to implement the ISA. Microarchitecture details how the processor executes instructions, how it fetches and decodes instructions, how it accesses memory and registers, and how it handles interrupts and exceptions. Microarchitecture is typically determined by the design team that creates the processor, and it is optimized for performance, power efficiency, and other specific goals.

In summary, ISA is the external interface of a processor that specifies the instruction set and its behavior, while microarchitecture is the internal implementation of the processor that determines how the instructions are executed and how the processor behaves. The two concepts are related but distinct, and understanding both is important for understanding how computer systems work.


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