When you write a program in C or C++, how does the processor know what to do?
Your source code might describe a calculation, a loop, or a function. But the processor does not directly read those statements. It executes machine instructions.
Different processors can support different sets of instructions. This is where names such as x86, Arm, and RISC-V enter the picture.
RISC-V, pronounced “risk-five,” is an instruction set architecture. It defines instructions and behavior that software can rely on when running on a compatible processor.
But what does that actually mean? And how does it connect to registers, memory, programming languages, and operating systems?
Let’s build the explanation one step at a time.
1. A CPU Needs Clearly Defined Instructions
A CPU executes instructions that tell it to perform specific operations.
An instruction might add two values, read data from memory, write a result back, or change which instruction executes next.
These operations are much smaller than the tasks we describe in everyday language. A request such as “sort these names” may require many instructions working together.
A machine instruction is represented by a pattern of bits. A bit is a binary digit: either zero or one.
The processor’s circuits interpret that pattern and perform the specified operation.
For this to work, software and hardware must agree on what each instruction means. A particular bit pattern cannot mean addition to the software but subtraction to the processor.
That agreement is part of the instruction set architecture.
2. What Is an Instruction Set Architecture?
An instruction set architecture, or ISA, defines the interface between machine-level software and a processor.
It specifies which instructions are available, how they are represented, and what behavior they must produce. It also defines features that instructions use, such as registers.
A register is a small storage location inside the CPU that instructions can use to hold values.
The ISA describes what software can expect. It does not prescribe every detail of the circuits that make it happen.
Think of an instruction such as “add these two values.” The architecture defines the required result. Processor designers decide how to build hardware that produces that result.
RISC-V is one such architecture. Its specifications are publicly available as an open standard. RISC-V International
3. RISC-V Is an Architecture, Not One Particular Chip
A company can design a processor that implements RISC-V. Another company can create a different processor that implements it too.
Those processors may have different speeds, power requirements, and internal designs.
The internal organization of a processor is called its microarchitecture. This includes how its execution units are arranged and how it moves instructions through the hardware.
An execution unit is a part of the processor that performs a type of operation, such as arithmetic.
Two processors can follow the same architectural rules while using different microarchitectures.
One might prioritize low power consumption. Another might use more hardware to improve performance.
This means the name RISC-V alone does not tell you how fast a processor is. Performance also depends on the actual implementation, the surrounding system, and the program being executed.
4. What Does “Open” Mean?
RISC-V provides an open, royalty-free instruction set architecture.
People and organizations can use that architecture to develop processors without paying royalties for using the ISA itself.
However, an open architecture does not mean every processor built around it has a publicly available design.
A company may create a proprietary RISC-V processor and keep its internal implementation private. Others may publish their processor designs for people to study or modify. Both approaches can use the same open architecture. RISC-V International
Designing, testing, manufacturing, and supporting a chip still requires resources.
The key distinction is between access to the architectural specification and access to a particular hardware implementation.
5. Start Small: The RV32I Base
RISC-V is organized around base instruction sets, with extensions that add capabilities.
An extension is an additional set of architectural features or instructions.
For a first example, we can use RV32I, the 32-bit base integer instruction set.
Here, “32” means its integer registers are 32 bits wide. “I” identifies the base integer instruction set. An integer is a whole-number value.
RV32I has 32 integer registers, named x0 through x31. Register x0 always supplies the value zero.
Consider this assembly instruction:
add x5, x6, x7
It means: add the values in x6 and x7, and place the result in x5.
If x6 holds 10 and x7 holds 20, x5 receives 30.
The destination comes first, followed by the two source registers. RISC-V RV32I Specification
6. From an Instruction to Hardware Activity
The text “add x5, x6, x7” is assembly language: a human-readable way to represent a machine instruction.
An assembler is a tool that converts assembly instructions into their binary encodings.
The CPU executes that binary encoding.
At a simplified level, the processor fetches the instruction, decodes it, obtains the source values, performs the addition, and writes the result to the destination.
To decode an instruction means to interpret its encoding so the processor can determine the operation and the values it needs.
The addition can be performed by an arithmetic logic unit, or ALU. This is circuitry that handles operations such as addition and logical comparisons.
Modern processors may overlap work from multiple instructions, but this simplified example shows how an architectural instruction connects to hardware activity.
7. How Does the Processor Use Memory?
Registers hold a limited amount of data. Programs also need memory to store larger collections of information.
A memory address is a number that identifies a location in memory.
In the basic RISC-V model, ordinary arithmetic instructions work on registers. Separate load and store instructions transfer data between registers and memory.
A load reads a value from memory into a register. A store writes a register value to memory. RISC-V RV32I Specification
A calculation might therefore involve reading values, adding them, and writing the result.
However, a program does not need to access memory before and after every addition. If the required values are already in registers, the processor can use them there.
Keeping useful values in registers can reduce the amount of memory traffic a program requires.
8. Where Do C and C++ Fit In?
You do not need to write every RISC-V program in assembly language.
You can write in a higher-level language such as C or C++ and use a compiler to translate the source code for the target architecture.
The target is the environment where the resulting program is intended to run.
For example, a compiler can translate a C++ calculation into instructions for a RISC-V processor.
There is no fixed rule that one line of C++ becomes one machine instruction. A statement may require several instructions, or it may disappear during optimization.
Optimization means transforming a program to improve properties such as speed or code size while preserving the behavior required by the language.
If a calculation has a result the compiler can determine in advance, it may place that result directly into the generated code.
This is why studying assembly can be useful: it shows what the compiler actually produced.
9. The Operating System Still Has a Job
Being able to execute instructions is only part of running an application.
A program may also need files, memory allocation, network connections, or access to devices.
On a system running Linux, the operating system provides these services and manages the resources that programs share.
A system call is a controlled request from a program for a service provided by the kernel. The kernel is the central part of the operating system that manages resources and enforces important protection boundaries.
For example, a program can calculate a result using CPU instructions, then request that the system write that result to a file.
These are connected activities, but they involve different responsibilities.
The processor executes instructions. The operating system coordinates access to the larger machine.
A compiled application therefore needs more than a matching CPU architecture. It must also match its intended operating environment and have the software support it depends on.
10. Conclusion
RISC-V gives us a concrete way to connect software with processor behavior.
The instruction set architecture defines operations that software can use. A processor implements those operations with hardware. Compilers translate higher-level programs into instructions for that processor.
Registers hold working values. Arithmetic instructions operate on them. Loads and stores connect those values to memory.
Above this hardware interface, the operating system manages resources and provides services to applications.
You do not need to memorize a complete instruction set to understand this relationship. One addition instruction is enough to begin seeing how the layers connect.
With that foundation in place, we can return to operating systems and examine an essential question: how does a computer separate ordinary application code from the privileged code that manages the system?