RISC-V Bit-Manipulation ISA-extensions

In this article, we introduce the RISC-V Bit-Manipulation extension based on “RISC-V Bit-Manipulation ISA-extensions“, which was ratified in November 2021.

If you search the Internet for the RISC-V Bit-Manipulation (bitmanip) extension, you will find many articles with old content. If the article contains the following description, the content may be outdated.

Referenced spec is not v1.0.0
Describes extensions other than Zba, Zbb, Zbc and Zbs
Described as B extension (or RVB)

BitManip Zba, Zbb, Zbc and Zbs extensions

In v1.0.0 of the ratified bitmanip specification, the bitmanip extension collection consists of four Zb* extensions:

Zba: Address generation instructions
Zbb: Basic bit-manipulation
Zbc: Carry-less multiplication
Zbs: Single-bit instructions

The bitmanip specification v0.93 describes 10 Zb* extensions, so v1.0.0 keeps things simple.

Relationship between misa.B and Zb* extensions

In the latest “The RISC-V Instruction Set Manual Volume II: Privileged Architecture”, misa.B is “Reserved” and is not related to bitmanip Zb* extensions.

Specifically, the January 26, 2022 commit c1c77c4, misa.B is changed from “Tentatively reserved for Bit-Manipulation extension” to “Reserved”.

Also, v0.93 of the bitmanip specification states that 8 types of Zb* extensions correspond to the B extension, but v1.0.0 does not state that they correspond to the B extension.

QEMU

The QEMU ChangeLog/6.2 has the following description of the bitmanip extension for RISC-V.

RISC-V

Add Zb[abcs] instruction support

Remove RVB support

The featured image and console output below show the result of running cat /proc/cpuinfo in QEMU 7.2 with four Zb* extensions enabled.

/ # cat /proc/cpuinfo
processor : 0
hart : 0
isa : rv64imafdch_zicsr_zifencei_zihintpause_zba_zbb_zbc_zbs_sstc
mmu : sv48

Looking at the isa output, we can see zba_zbb_zbc_zbs corresponding to the four Zb* extensions, but there is no b corresponding to misa.B.

Rocket Chip

Also, misa.B is not used in Rocket Chip, a RISC-V implementation supporting the ratified Zb* extensions since commit 4ecc497.

Summary

In this article, we introduced the RISC-V bitmanip extension. If you search the Internet for bitmanip extension, you will find many articles with old content, so we recommend checking the ratified specification.