RISC-V chipmaker SiFive last week scored $175 million in a Series F funding round led by Coatue Management. The funding round brings the company's total raised to $350 million and puts it deep into double-unicorn territory with a valuation north of $2.5 billion.

SiFive develops chips based on a relatively new open source instruction set architecture (ISA) called RISC-V. The company was founded by many of the same architects behind the original ISA.

RISC-V is a reduced instruction set computer (RISC) architecture not dissimilar to Arm, Power, or Sun SPARC. But unlike previous permutations of the RISC architecture, RISC-V is entirely open source, freely available, and absolutely tiny.

With just 47 instructions, the architecture is just a fraction of the size of Arm at 200 instructions and minuscule compared to x86 at more than 1,500 instructions.

Why is this important? Instructions are the most basic calculations chips do. One instruction might be to add, while another might be to subtract or to multiply. The idea behind RISC-V is to keep the instruction set as small as possible so that implementors can add the instructions that matter to them using extensions.

This flexibility allows chipmakers to scale back their designs to behave more like ASICs or FPGAs or upscale them up to compete with Arm or x86-based systems. Chipmakers such as SiFive have taken advantage of this to achieve extremely power-efficient designs.

RISC-V was recently seen as little more than a curiosity with great potential, but one that would be hard pressed to compete against x86 and Arm. Today, RISC-V International counts more than two hundred chipmakers, software developers, and academic institutions that have announced support for the technology.

This includes Intel, which recently announced its foundry business would work with RISC-V International and several leading RISC-V chipmakers, including Andes Technology, Esperanto Technologies, Ventana Micro Systems, and SiFive, to offer customers a range of validated RISC-V IP cores.

The effort was backed by a $1 billion initiative to help customers accelerate their development pipelines around its foundry business, which included extending support beyond x86 to both Arm and RISC-V architectures.

“RISC-V is a great fit for our open ecosystem vision, which is why we are making investments and building partnerships that will empower the ecosystem and help drive further adoption of RISC-V,” Intel Foundry Services President Randhir Thakur wrote in a blog post.

Intel’s relationship with RISC-V, and more specifically SiFive, goes deeper than that. Intel early last summer reportedly made a $2 billion offer to acquire SiFive, according to a Bloomberg report.

Later that month, the chipmaker said it would license SiFive’s P550 processor tech.

“We are pleased to be a lead development partner with SiFive to showcase to mutual customers the impressive performance of their P550 on our 7-nanometer Horse Creek platform," Amber Huffman, CTO of IP engineering at Intel, said at the time. "By combining Intel's leading-edge interface IP such as DDR and PCIe with SiFive's highest performance processor, Horse Creek will provide a valuable and expandable development vehicle for cutting-edge RISC-V applications."

Intel reportedly abandoned acquisition talks with SiFive later that year after the two companies failed to agree on financial and integration terms, according to Bloomberg.