These include instructions for saturating arithmetic (arithmetic that give the greatest or lowest possible values, rather than wrapping around), 16 and 32 bit fixed point, fused multiply add instructions, and some SIMD instructions https://en.wikipedia.org/wiki/MIPS_architecture#MIPS_DSP

Basically your usual DSP fare.

ARM has a similar extension; see https://en.wikipedia.org/wiki/ARM_architecture#DSP_enhanceme... for a reference to it, or http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.... for details.

I don't think that RISC-V has a DSP extension yet; they are still working on nailing down their SIMD/vector extension story.