RISC-V extensions

The instruction set described previously is named RV32I, which stands for the RISC-V 32-bit integer instruction set. Although the RV32I ISA provides a complete and useful instruction set for many purposes, it lacks several functions and features available in other popular processors such as x86 and ARM.

The RISC-V extensions provide a mechanism for adding capabilities to the base instruction set in an incremental and compatible manner. Implementors of RISC-V processors can selectively include extensions in their design to optimize trade-offs between chip size, system capability, and performance.

These flexible design options are also available to developers of low-cost FPGA-based systems. We’ll see more about implementing a RISC-V processor in an FPGA later in this chapter. The major extensions we will cover now are named M, A, C, F, and D. We’ll also mention some other available extensions.

The M extension

The RISC-V M extension adds...