Instruction Set Architecture (ISA)

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Arjit Sharma
01 Min

The Instruction Set Architecture (ISA) is the interface between CPU’s hardware and software it runs. In this article we will try to understand about ISA using x86 (ISA used in many CPUs)

What Is an ISA?

An ISA is the bridge between software and hardware, defining the instructions a CPU can execute. Think of x86 as the specific dialect spoken by Intel and AMD processors. It specifies -

  • Instructions: Commands like “add two numbers” or “move data.”
  • Format: How instructions are written in binary.
  • Resources: Which registers and memory the CPU uses.

How x86 Instructions Work

Let’s take a simple x86 instruction -

MOV EAX, 42

This tells the CPU to move the number 42 into a register called EAX. MOV is a data movement instruction. EAX is one of x86’s registers. Immediate addressing mode is used.

Compatibility

When you write a program (say, in C), the compiler translates it into x86 machine code. This code runs only on x86 CPUs and not on ARM or RISC-V, because each ISA has a unique language. ARM might have a different instruction or register names.

Real-World Example

Suppose in your code somewhere you do score = score + 10. The compiler might generate x86 code like -

MOV EAX, [score] // Load score from memory to EAX
ADD EAX, 10 // Add 10 to EAX
MOV [score], EAX // Store result back to memory

Conclusion

ISA defines the instructions a CPU can execute and how they’re structured. It forms the interface between software and processor hardware.

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