What an architecture actually defines
An instruction set architecture defines:
• the instructions a CPU understands
• register layout
• privilege levels
• memory ordering rules
• how software talks to hardware
It does not define:
• pipeline depth
• cache design
• clock speed
• manufacturing process
Two RISC-V CPUs can run the same program and still be completely different internally.
Why RISC exists
RISC means Reduced Instruction Set Computing. The idea is to keep instructions simple and predictable so compilers and hardware can be efficient.
Compared to this:
• x86 is complex and full of legacy behavior
• ARM is RISC but controlled and licensed
RISC-V keeps the RISC philosophy but removes ownership .
The base instruction set
Every RISC-V CPU starts with a base ISA.
RV32I and RV64I are the foundations.
RV32I is a 32-bit integer instruction set.
RV64I is a 64-bit integer instruction set.
The base includes:
• integer arithmetic
• control flow
• memory load and store
• 32 general-purpose registers
There is no multiplication, floating point, or atomics in the base. Those are added only if needed.
This is why RISC-V works well for tiny chips.
Extensions are how RISC-V scales
RISC-V is modular. Features are added using extensions.
Common extensions:
• M for multiply and divide
• A for atomic instructions
• F and D for floating point
• C for compressed instructions
• V for vector processing
A Linux-capable system usually supports:
RV64IMAFDC
This means the CPU includes only what it needs. No wasted complexity.
Registers and calling conventions
RISC-V has 32 integer registers.
Some examples:
• x0 is always zero
• x1 is the return address
• x2 is the stack pointer
Nothing is hidden. Nothing is special without documentation. This makes compilers, debuggers, and OS kernels simpler.
Privilege modes and operating systems
RISC-V defines three privilege levels:
• machine mode for firmware
• supervisor mode for the kernel
• user mode for applications
This matches modern OS design perfectly.
Because of this, Linux support was straightforward.
Linux runs natively on RISC-V with full kernel support.
Memory model
RISC-V uses a relaxed memory model. Instructions may be reordered unless explicitly prevented.
This improves performance and scalability but requires correct synchronization.
The design is similar to ARM, not x86.
Microarchitecture freedom
RISC-V does not force one CPU design.
Implementations can be:
• in-order or out-of-order
• single-core or many-core
• tiny microcontrollers or server CPUs
The same software can run on all of them.
Operating systems that support RISC-V
Fully supported operating systems:
• Linux
• FreeBSD
• Zephyr
• RTEMS
Experimental or partial support:
• Android
• research Windows ports
Linux support is the most important signal of maturity.
Real RISC-V processors available today
Embedded and microcontroller level:
• ESP32-C3
• ESP32-C6
• Microchip PolarFire SoC
Linux-capable processors:
• SiFive Performance series
• Alibaba T-Head Xuantie
These are real chips that boot Linux and run real workloads.
Who should use RISC-V
RISC-V is ideal for:
• custom silicon designers
• embedded and IoT devices
• governments and public infrastructure
• research and education
• long-term platforms avoiding vendor lock-in
It is not ideal yet for:
• high-end consumer desktops
• gaming systems
• proprietary driver-heavy environments
Why RISC-V is succeeding where others failed
Unlike Intel Itanium, RISC-V does not try to replace everything at once.
It grows gradually.
It coexists.
It does not force migration.
That is why it is working.
Final thoughts
RISC-V is not exciting because it is fast today.
It is exciting because it is free, simple, and stable.
It is an architecture designed to last decades.
That alone makes it one of the most important CPU designs of our time.
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