RISC-V vs ARM. Which One To Choose?

For quite a while, since the rise of smartphones in the late 2000s, the computer processors market has been dominated by ARM central processing units (CPUs) based on the reduced instruction set computer (RISC) architecture. Recently, however, a strong competitor has emerged with a considerably different approach towards the CPU architecture in microprocessors, mobile systems and microcontrollers. The name of this potential ARM killer is RISC-V (pronounced as “risk-five”). 

Over the last couple years, the debate regarding the competition between ARM and RISC-V has been getting more and more vibrant. 

Will RISC-V ultimately replace ARM as the top CPU specification or will both technologies coexist? Let’s take a closer look at these two computer processor architectures, their technical specifications and how they are different from each other. 

What is ARM? 

ARM (originally known as Acorn RISC Machine, ARM stands for Advanced RISC Machines) is a family of RISC instruction set architectures for computer processors, available for various computing devices and environments. 

The ARM CPU architecture is developed by the Arm Ltd company, which licenses the architectures to other companies, allowing them to design their own products that incorporate different components, including interfaces and memory. 

There have been a number of generations of ARM architecture. The original version, ARM1, was introduced in 1985, almost 40 years ago. First application for ARM processors was as an additional second processor for the BBC Micro, providing support to speed up the simulation software. ARM1 used 32-bit internal structure but also had 26-bit address space, limiting it to 64 MB of main memory. This limitation was removed in ARM 3.

ARM 8-A, released in 2011, received the support for 64-bit address space and 64-bit arithmetic. 

ARM processors quickly gained popularity due to their low power consumption, lower costs compared to available alternatives, and minimal heat generation. 

Even though ARM CPUs were widely used since the initial release of this architecture, they really came to power in the late 2000s, upon the release of the first smartphones. Being the best CPU choice for portable devices due to light weight and low power consumption, ARM processors are preferred by the manufacturers of smartphones, tablets and laptops. For the same reasons, ARMs are also widely used in embedded systems. 

According to the official data, more than 200 bln ARM chips have been produced around the world as of 2021. 

What is RISC? 

Since we already mentioned the RISC a number of times, a few words about it need to be said as well. 

RISC is a technology designed to simplify the individual instructions provided to the computer to perform certain tasks. The difference between RISC and CISC (a complex instruction set computer) is that RISC architecture typically requires more instructions provided to a computer in order to complete tasks as individual instructions in RISC are written in simpler code. 

One of the key concepts of RISC computers is that every instruction performs only one function during single CPU cycle. 

What is RISC-V?

RISC-V is basically the fifth generation of the RISC architecture, provided as an open standard instruction set architecture (ISA) based on the RISC standard principles. Unlike the majority of other ISA designs, it is provided under the open source license, so it’s free to use for all the computer chip producers.

The RISC-V specification defines both 32-bit and 64-bit address space options, and additionally includes a description of a 128-bit flat address space variant. 

The RISC-V is a load–store architecture, using IEEE 754 floating-point instructions. RISC-V ISA also includes instruction bit field locations as a way to simplify the use of multiplexers in CPUs. 

Started with a goal to create a practical open source ISA that will be easily deployable in various hardware and software designs, including embedded systems, the RISC-V ISA is a continuation of a long history of CPUs architecture design projects developed at the University of California, Berkeley, since the late 1980s.

History of the RISC-V specification development

The project to develop RISC-V specification was originally started in 2010 by the University of California experts with an intent to create a practicable instruction set that will be available for practical use in various CPUs manufacturing. 

Dr. Krste Asanović, a professor of computer science at UC Berkeley, was an author of the project to develop RISC-V. Eventually, Dr David Patterson, another UC Berkeley professor and one of the creators of the original RISC chips back in the early 1990s, joined the project.

As any ISA needs to be stable for commercial use, the RISC-V Foundation was formed in 2015 with a goal to develop, maintain and publish the intellectual property related to the RISC-V specification. The original authors of the project at UC Berkeley have transferred all the rights to this non-profit corporation controlled by its members.

Currently, the RISC-V Foundation comprises over 325 members, including representatives from companies such as Google, NVIDIA, Microsemi, Western Digital. The RISC-V Foundation members participate in the development of the RISC-V ISA specification and related projects. 

In 2019, due to the U.S. trade regulations concerns as the main reason, the RISC-V Foundation relocated to Switzerland. In 2020, the organization was renamed as RISC-V International, becoming a Switzerland-registered nonprofit business association.

Today, the RISC-V International publishes all the documentation and specifications related to RISC-V designs, which remains open source and available for everyone to use free of charge. However, only the members of RISC-V International can vote to approve any changes to RISC-V specifications. 

ARM vs RISC-V Comparison 

Here’s a table comparing technical specifications of ARM and RISC-V. 

Features	ARM	RISC-V
Architecture	Load-store	Load-store
Memory Addressing	64-bit Virtual	32 / 64-bit
Architecture size	64-bits	64-bits
License	Core / Architecture	Open source
Instruction Set	A64	None
Instruction Set Width	32-bit	32-bit
Instruction Set Compression	To 16-bit	To 16-bit
Endianness	Big	Little
Max speed	2.6GHz	3.0GHz
Pipeline length	12 stages	17 stages
Integer Registers	31	32 / 16
FP / SIMD units	2x 64 bits	2x 128 bits
Vector Registers	32	Add-On
Multiplication	Included	Add-On

Final thoughts. ARM vs RISC-V: Which one to choose? 

As you can probably tell from the comparison chart above, there is no simple answer to this question. 

In many ways, right now, ARM-based CPUs are still a better option, mainly due to much longer lifecycle and the fact that ARM Ltd has invested billions of dollars into this specification over the years. ARM processors have a huge market share, being used in the majority of smartphones, as well as laptops and even PCs that are choosing ARM instead of x86 architecture-based designs. 

We could say, however, that RISC-Vs are the future and a very strong contender to the throne of the most used computer processors architecture. RISC-V can provide better performance using a minimum amount of power. The fact that RISC-V is open source and free to use by any processor manufacturers is also a huge advantage.

Some manufacturers, such as Western Digital, for example, have already started implementing the RISC-Vs in their microcontrollers attached to RAMs and SSDs. 

RISC-V is also getting increasingly popular in IoT devices and embedded systems of various kinds, due to its highly scalable nature. But it will undoubtedly take several years for industry players to transition to using RISC-V instead of ARM-based designs. 

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