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RISC-V Instruction Set Manual, Volume I: RISC-V User-Level ISA , isa-449cd0c 2023/04/18

5 RV32E and RV64E Base Integer Instruction Sets, Version 2.0

This chapter describes a proposal for the RV32E and RV64E base integer instruction sets, designed for microcontrollers in embedded systems. RV32E and RV64E are reduced versions of RV32I and RV64I, respectively: the only change is to reduce the number of integer registers to 16. This chapter only outlines the differences between RV32E/RV64E and RV32I/RV64I, and so should be read after Chapters [rv32] and [rv64].

RV32E was designed to provide an even smaller base core for embedded microcontrollers. There is also interest in RV64E for microcontrollers within large SoC designs, and to reduce context state for highly threaded 64-bit processors.

Unless otherwise stated, standard extensions compatible with RV32I and RV64I are also compatible with RV32E and RV64E, respectively.

5.1 RV32E and RV64E Programmers’ Model

RV32E and RV64E reduce the integer register count to 16 general-purpose registers, (x0x15), where x0 is a dedicated zero register.

We have found that in the small RV32I core implementations, the upper 16 registers consume around one quarter of the total area of the core excluding memories, thus their removal saves around 25% core area with a corresponding core power reduction.

5.2 RV32E and RV64E Instruction Set Encoding

RV32E and RV64E use the same instruction-set encoding as RV32I and RV64I respectively, except that only registers x0x15 are provided. All encodings specifying the other registers x16 x31 are reserved.

The previous draft of this chapter made all encodings using the x16x31 registers available as custom. This version takes a more conservative approach, making these reserved so that they can be allocated between custom space or new standard encodings at a later date.