Why RISC-V Is Better for Custom Silicon

May 15, 2025

hardware VLSI RISC-V ISA open-source cores custom extensions performance benchmarks ecosystem toolchain SiFive FPGA prototyping embedded SoCs academic research future trends security modularity

RISC-V is not automatically better than ARM or x86 for every product. It is better when you need architectural control: choose only the ISA features you need, verify a smaller design, and avoid ISA licensing constraints.

ISA first: what RISC-V actually defines

An ISA is the software-hardware contract: instructions, registers, privilege model, memory ordering, and exception behavior. Microarchitecture is separate. Pipeline depth, out-of-order logic, branch predictors, and cache policy are implementation choices.

RISC-V keeps this contract explicit and modular:

Base integer ISAs: RV32I, RV64I, RV128I
Embedded base: RV32E and RV64E (reduced register set)
Privileged architecture: machine, supervisor, user modes and CSR behavior
Virtual memory options: Sv39, Sv48, Sv57 for high-end address spaces

This separation lets a tiny microcontroller core and a Linux-capable application core share the same architecture family without forcing the same hardware complexity.

Extensions are the real reason RISC-V scales

RISC-V is built around optional standard extensions. You start from I and add what your software needs.

Common extensions:

M: integer multiply and divide
A: atomic memory operations for locks, futexes, and SMP correctness
F and D: IEEE754 single and double precision floating point
C: compressed 16-bit encodings to reduce code size and fetch bandwidth
V: vector ISA for data-parallel kernels
B and Zb*: bit-manip instructions for crypto, parsing, DSP-style integer code
Zicsr and Zifencei: control/status register and instruction fetch fencing support

Naming convention matters:

Z* means standardized, focused extensions
X* means vendor-defined custom extensions

That model gives you standard software compatibility where needed and freedom where it helps PPA.

RV64GC: why it is the practical Linux target

If you care about running a mainstream Linux userspace, RV64GC is the profile most teams target first.

RV64GC usually implies:

RV64I base integer ISA
M, A, F, D extensions
C compressed extension
Zicsr and Zifencei (included through the G profile in toolchains)

In practice, many distros and toolchains assume this baseline with:

-march=rv64gc -mabi=lp64d

Why this works well for Linux:

64-bit address space and integer model for modern kernels
atomics for scheduler and synchronization primitives
floating-point ABI compatibility with common userland builds
compressed instructions for better I-cache behavior and smaller binaries

What is required to run Linux on RISC-V

RV64GC alone is necessary but not sufficient. A Linux-capable SoC also needs platform features around the core.

Minimum practical ingredients:

Privilege support for M-mode, S-mode, U-mode
MMU support, usually Sv39 on many current systems
Timer and interrupt infrastructure for preemption and device handling
UART or equivalent console for early boot and diagnostics
DRAM controller and basic storage path
Device tree description for kernel hardware discovery

Boot flow in most systems:

ROM code initializes minimal hardware.
OpenSBI runs in M-mode and exposes SBI services.
A bootloader such as U-Boot loads kernel, DTB, and initramfs.
Linux boots in S-mode and schedules user processes in U-mode.

SBI is important because Linux does not need direct M-mode ownership for every machine-level action. It calls into firmware through a standard interface.

Variants and where they fit

Variant	Typical target	Technical reason
`RV32IMC`	MCU and deeply embedded control	Low area, low power, good code density
`RV64GC`	Linux application processors	Strong software compatibility baseline
`RV64GCV`	ML and DSP acceleration	Vector throughput for parallel kernels
`RV64I + Xcustom`	Domain-specific SoCs	Tailored instructions for workload hot spots

Why this helps VLSI teams in real projects

Advantages are concrete and measurable:

Area and power control: do not implement instruction blocks you do not need
Smaller verification scope: fewer ISA features reduce corner-state explosion
Easier HW/SW co-design: add custom ops while keeping a standard compiler front-end
Open ecosystem: GCC, LLVM, QEMU, Spike, compliance suites, formal collateral
No ISA royalty cost: useful for startups, university tapeouts, and cost-sensitive products

This is why RISC-V is popular in accelerators, edge SoCs, SSD controllers, and custom control planes.

Limits and trade-offs

RISC-V still has trade-offs:

Peak performance still depends on microarchitecture quality, not ISA branding
Linux enablement effort is platform-dependent and can be non-trivial
Vendor ecosystem maturity varies by segment, especially for profiling and debug tools

So the right question is not “Is RISC-V always better?”. The right question is “Does ISA modularity and customization improve this product’s PPA and schedule?”

Bottom line

RISC-V is strongest where architectural flexibility matters. RV64GC is a practical baseline for Linux-capable SoCs, and the extension model lets you scale from small embedded cores to application processors and domain-specific accelerators inside one ISA family.