One architecture I’ve been keeping an eye on for the past few years is the revolutionary, open-source RISC-V ISA. A number of companies have announced low-level RISC-V CPUs, though these are often used as embedded controllers. Companies like Alibaba Group, Western Digital, and SiFive have all been working to design new RISC-V chips that can hit higher levels of performance.
SiFive has announced its new Hive Unmatched platform, built around the SiFive FU740 SoC. This is a five-core, heterogeneous processor with four SiFive U74 cores and one SiFive S7 core. This low-level core is a bit like Nvidia’s old Companion Core in Tegra 4 — it’s an embedded core for running a real-time operating system (RTOS) or something similar. According to WikiChip, the U74 has an eight-stage pipeline (up from 5-7), 10 percent additional frequency headroom, and a 2-cycle SRAM latency (down from 5 cycles).
The U74 is a high-performance core only within the confines of the RISC-V ecosystem and addressable market — it’s not going to be competing against Ryzen or Core any time soon — but the CPU design has still advanced in several ways. The dual-issue, in-order CPU design can handle up to two instructions being issued at once, compared with just a single instruction for previous designs.
Architecturally, the U74 sounds a bit like the old Cortex-A8 in terms of certain capabilities, though I want to make it clear I’m basing that on features like the dual-issue support and a longer pipeline compared with past designs. We don’t actually have enough information about the SiFive U74 to make a proper comparison on that point.
SiFive’s Hive Unmatched platform is a mini-ITX motherboard with features like ATX power supply connectors, a PCIe slot, gigabit ethernet, and onboard USB ports. It includes 8GB of RAM, 32MB of QSPI flash, and a microSD card for storage. If you want debugging and monitoring capabilities, you can plug into the micro USB type-B connector.
There are two M.2 slots on the board, one for Wi-Fi and one for storage, and four USB 3.2 Gen 1 Type A ports on the back. The system ships with a bootable SD card that includes Linux, and the board will sell for $655. It’s a significant improvement over the company’s last dev board, the Hive Unleashed, which had much weaker cores, no PCIe support, and no USB 3.2 Gen 1 ports.
This is, rather obviously, a board meant for developers to build software with as opposed to a consumer product, but the fact that we’re seeing more and more interest in RISC-V is a healthy sign of long-term interest in the architecture. Nvidia’s next moves re: ARM will impact just how quickly RISC-V takes off. While the architecture is expected to grow and win market share in certain segments no matter what, if Nvidia begins making changes to the classic ARM licensing model that its customers feel benefit Nvidia more than themselves, RISC-V could quickly become a must-ramp solution. It’s historically pretty rare for a company to lose its market in this way, and Nvidia is surely aware of the RISCs risks, but the rising interest in RISC-V is one of the reasons the CPU industry is more exciting now than at any time in decades.
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