The design of RISC-V instruction sets is modular. Rather than take the approach of a large and complex monolith, a modular design enables flexible implementations that suit specific applications.
RISC-V defines base user-level integer instruction sets. Additional capability to these are specified as optional extensions, thus giving implementations flexibility to pick and choose what they want for their applications. The specifications of the base ISA has been frozen since 2014. Some of the extensions are also frozen while many others are being defined.
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