RISC-V is a modular instruction set architecture (ISA) with great customization capabilities that enable innovation and differentiation without fragmentation. On top of the baseline modules from ratified/standard ISA extensions, such as integer instructions or floating-point instructions, designers can add custom instructions: pure design freedom! And the reasons for adding instructions are many: better performance, smaller memory footprint, lower power consumption, or anything in between.

This means one important thing: the software (the final application(s)) is compiled for the particular RISC-V ISA. The software development kit (SDK) must know which ISA modules the RISC-V processor implements, so it can automatically leverage them. This includes both standard instructions and custom instructions.