ARM vs RISC-V: A Comprehensive Architecture Comparison

For decades, the landscape of processor architectures has been defined by a few dominant players. Among them, ARM has established an unassailable throne in mobile computing, while RISC-V has emerged as a disruptive, open-source alternative promising unprecedented flexibility. Understanding the differences between these two Reduced Instruction Set Computer (RISC) architectures is crucial for engineers, investors, and technology enthusiasts alike.

This article dissects the technical, economic, and philosophical differences between ARM and RISC-V, exploring their historical roots, licensing models, performance characteristics, and future trajectories.

Historical Context

The story of ARM begins in 1983 at Acorn Computers in the UK. Designed by Sophie Wilson and Steve Furber, the ARM (originally Acorn RISC Machine) architecture prioritized low power consumption and simplicity. After Acorn spun off the division, it became ARM Limited, and later SoftBank, before being acquired by Nvidia in 2024. Today, ARM licenses its designs to hundreds of semiconductor companies, powering over 90% of smartphones globally.

Conversely, RISC-V was born in 2010 at the University of California, Berkeley, led by Professor Krste Asanović and David Patterson. Frustrated by the legal complexities and proprietary nature of existing ISAs, the team sought to create a truly open standard. The "V" denotes the fifth iteration of the Berkeley RISC projects. In 2015, the RISC-V International foundation was established to steward the specification, ensuring it remains free and open forever.

Licensing & Philosophy

The most fundamental difference lies not in transistors, but in intellectual property.

• ARM's Model: ARM operates on a proprietary licensing model. Companies pay upfront fees and ongoing royalties based on chip sales. While this ensures a consistent revenue stream for ARM to fund R&D, it creates financial barriers for startups and locks users into a specific ecosystem.
• RISC-V's Model: RISC-V is released under the ISC License, a MIT-style open-source license. There are no royalties, no upfront fees, and no mandatory compliance checks. This democratizes chip design, allowing anyone—from a garage startup to a national supercomputer lab—to build processors without fear of patent litigation.

Architectural Differences

Both architectures are RISC-based, emphasizing a load-store architecture where arithmetic operations only occur on registers, not memory. However, key distinctions exist:

Base Instruction Set

RISC-V uses a fixed 32-bit base instruction format (RV32I and RV64I) with clear extensions (M for multiplication, A for atomicity, F/D for floating-point). This modularity allows designers to build only what they need. ARM's instruction set has evolved through multiple generations (ARMv7, ARMv8/AArch64) with more complexity baked in, particularly with its Thumb-2 mixed 16/32-bit instruction set designed to balance code density and performance.

Registers

Both provide 32 general-purpose 64-bit registers in their standard configurations. However, RISC-V's calling conventions and register allocation strategies differ, impacting compiler optimization paths.

Ecosystem & Maturity

ARM boasts an unparalleled ecosystem. Every major OS (Android, iOS, Windows on ARM, Linux) has deeply optimized toolchains, drivers, and binaries for ARM. The availability of highly optimized IP cores (Cortex-A, Cortex-R, Cortex-M) allows silicon companies to time-to-market with minimal risk.

RISC-V is catching up rapidly. Linux kernel support is now mainline and robust. Glibc, musl, and major compilers (GCC, Clang/LLVM) support RISC-V. However, the ecosystem is fragmented. Because anyone can modify the ISA, ensuring binary compatibility across different RISC-V implementations remains an ongoing challenge. Core IP availability is also less mature than ARM's, though companies like SiFive, Ventana, and Andes Technology are rapidly filling the gap.

Performance & Power

Performance is highly dependent on microarchitecture, not just the ISA. High-end ARM cores (Cortex-X4, Cortex-A720) dominate mobile performance-per-watt metrics. RISC-V cores are currently competitive in the embedded and mid-range server spaces. Projects like the SiFive Performance Core Series and open-source efforts like CVA6 demonstrate that RISC-V can match ARM Cortex-A55 level performance.

For specialized workloads (AI accelerators, cryptographic engines), RISC-V's custom instruction extension capability offers a distinct advantage. Engineers can tailor the instruction set directly to the algorithm, reducing cycles and power consumption significantly.

Comparison Table

Assembly Example

Below is a simple example adding two integers and storing the result. Note RISC-V's consistent register naming x0-x31 vs ARM's w0-w30 (32-bit) or x0-x30 (64-bit).

add x0, x1, x2    @ x0 = x1 + x2
str x0, [x3]      @ Store result to memory

add x0, x1, x2    # x0 = x1 + x2
sw  x0, 0(x3)     # Store word to memory address in x3

Future Outlook

The competition between ARM and RISC-V is not necessarily a zero-sum game. ARM will likely continue to dominate general-purpose mobile and high-performance computing where its mature IP and ecosystem provide immense value. RISC-V is poised to capture the embedded, IoT, and specialized accelerator markets, where customization and zero-cost licensing are paramount.

We are entering a hybrid era. Expect to see RISC-V cores handling background tasks and sensors, while ARM handles UI and heavy processing, or RISC-V being used as coprocessors alongside x86/ARM main processors.

Conclusion

ARM and RISC-V represent two different philosophies of computing: one prioritizing a polished, high-performance commercial standard, the other championing open innovation and modularity. For the foreseeable future, both will coexist, driving innovation from different angles. For engineers, understanding both architectures is becoming an essential skill in the modern semiconductor landscape.

References: RISC-V International Specification v20250301; ARM Architecture Reference Manual AArch64; IEEE Computer Society (2024). "The Rise of Open Source Hardware."