Applicability of this chapter to configurations:

Configuration

Implementation

CV32A60AX

Implemented extension

CV32A60X

Implemented extension

CV64A6_MMU

Implemented extension

RVZbb: Basic bit-manipulation

The following instructions comprise the Zbb extension:

Logical with negate

The Logical with Negate instructions can be implemented by inverting the rs2 inputs to the base-required AND, OR, and XOR logic instructions.

RV32

RV64

Mnemonic

✔

✔

andn rd, rs1, rs2

✔

✔

orn rd, rs1, rs2

✔

✔

xnor rd, rs1, rs2

RV32 and RV64 Instructions

  • ANDN: AND with inverted operand

    Format: andn rd, rs1, rs2

    Description: This instruction performs the bitwise logical AND operation between rs1 and the bitwise inversion of rs2.

    Pseudocode: X(rd) = X(rs1) & ~X(rs2)

    Invalid values: NONE

    Exception raised: NONE

  • ORN: OR with inverted operand

    Format: orn rd, rs1, rs2

    Description: This instruction performs the bitwise logical AND operation between rs1 and the bitwise inversion of rs2.

    Pseudocode: X(rd) = X(rs1) | ~X(rs2)

    Invalid values: NONE

    Exception raised: NONE

  • XNOR: Exclusive NOR

    Format: xnor rd, rs1, rs2

    Description: This instruction performs the bit-wise exclusive-NOR operation on rs1 and rs2.

    Pseudocode: X(rd) = ~(X(rs1) ^ X(rs2))

    Invalid values: NONE

    Exception raised: NONE

Count leading/trailing zero bits

These instructions are used to count the leading/trailing zero bits.

RV32

RV64

Mnemonic

✔

✔

clz rd, rs

✔

clzw rd, rs

✔

✔

ctz rd, rs

✔

ctzw rd, rs

RV32 and RV64 Instructions

  • CLZ: Count leading zero bits

    Format: clz rd, rs

    Description: This instruction counts the number of 0’s before the first 1, starting at the most-significant bit (i.e., XLEN-1) and progressing to bit 0. Accordingly, if the input is 0, the output is XLEN, and if the most-significant bit of the input is a 1, the output is 0

    Pseudocode: if [x[i]] == 1 then return(i) else return -1

    Invalid values: NONE

    Exception raised: NONE

  • CTZ: Count trailing zeros

    Format: ctz rd, rs

    Description: This instruction counts the number of 0’s before the first 1, starting at the least-significant bit (i.e., 0) and progressing to the most-significant bit (i.e., XLEN-1). Accordingly, if the input is 0, the output is XLEN, and if the least-significant bit of the input is a 1, the output is 0.

    Pseudocode: if [x[i]] == 1 then return(i) else return xlen;

    Invalid values: NONE

    Exception raised: NONE

RV64 specific instructions

  • CLZW: Count leading zero bits in word

    Format: clzw rd, rs

    Description: This instruction counts the number of 0’s before the first 1 starting at bit 31 and progressing to bit 0. Accordingly, if the least-significant word is 0, the output is 32, and if the most-significant bit of the word (i.e., bit 31) is a 1, the output is 0.

    Pseudocode: if [x[i]] == 1 then return(i) else return -1

    Invalid values: NONE

    Exception raised: NONE

  • CTZW: Count trailing zero bits in word

    Format: ctzw rd, rs

    Description: This instruction counts the number of 0’s before the first 1, starting at the least-significant bit (i.e., 0) and progressing to the most-significant bit of the least-significant word (i.e., 31). Accordingly, if the least-significant word is 0, the output is 32, and if the least-significant bit of the input is a 1, the output is 0.

    Pseudocode: if [x[i]] == 1 then return(i) else return 32;

    Invalid values: NONE

    Exception raised: NONE

Count population

These instructions count the number of set bits (1-bits). This is also commonly referred to as population count.

RV32

RV64

Mnemonic

✔

✔

cpop rd, rs

✔

cpopw rd, rs

RV32 and RV64 Instructions

  • CPOP: Count set bits

    Format: cpop rd, rs

    Description: This instructions counts the number of 1’s (i.e., set bits) in the source register.

    Pseudocode: if rs[i] == 1 then bitcount = bitcount + 1 else ()

    Invalid values: NONE

    Exception raised: NONE

RV64 specific instructions

  • CPOPW: Count set bits in word

    Format: cpopw rd, rs

    Description: This instructions counts the number of 1’s (i.e., set bits) in the least-significant word of the source register.

    Pseudocode: if rs[i] == 0b1 then bitcount = bitcount + 1 else ()

    Invalid values: NONE

    Exception raised: NONE

Integer minumum/maximum

The integer minimum/maximum instructions are arithmetic R-type instructions that return the smaller/larger of two operands.

RV32

RV64

Mnemonic

✔

✔

max rd, rs1, rs2

✔

✔

maxu rd, rs1, rs2

✔

✔

min rd, rs1, rs2

✔

✔

minu rd, rs1, rs2

RV32 and RV64 Instructions

  • MAX: Maximum

    Format: max rd, rs1, rs2

    Description: This instruction returns the larger of two signed integers.

    Pseudocode: if rs1_val <_s rs2_val then rs2_val else rs1_val

    Invalid values: NONE

    Exception raised: NONE

  • MAXU: Unsigned maximum

    Format: maxu rd, rs1, rs2

    Description: This instruction returns the larger of two unsigned integers.

    Pseudocode: if rs1_val <_u rs2_val then rs2_val else rs1_val

    Invalid values: NONE

    Exception raised: NONE

  • MIN: Minimum

    Format: min rd, rs1, rs2

    Description: This instruction returns the smaller of two signed integers.

    Pseudocode: if rs1_val <_s rs2_val then rs1_val else rs2_val

    Invalid values: NONE

    Exception raised: NONE

  • MINU: Unsigned minimum

    Format: minu rd, rs1, rs2

    Description: This instruction returns the smaller of two unsigned integers.

    Pseudocode: if rs1_val <_u rs2_val then rs1_val else rs2_val

    Invalid values: NONE

    Exception raised: NONE

Sign and zero-extension

These instructions perform the sign-extension or zero-extension of the least significant 8 bits, 16 bits or 32 bits of the source register.

These instructions replace the generalized idioms slli rD,rS,(XLEN-<size>) + srli (for zero-extension) or slli + srai (for sign-extension) for the sign-extension of 8-bit and 16-bit quantities, and for the zero-extension of 16-bit and 32-bit quantities.

RV32

RV64

Mnemonic

✔

✔

sext.b rd, rs

✔

✔

sext.h rd, rs

✔

✔

zext.h rd, rs

RV32 and RV64 Instructions

  • SEXT.B: Sign-extend byte

    Format: sext.b rd, rs

    Description: This instruction sign-extends the least-significant byte in the source to XLEN by copying the most-significant bit in the byte (i.e., bit 7) to all of the more-significant bits.

    Pseudocode: X(rd) = EXTS(X(rs)[7..0])

    Invalid values: NONE

    Exception raised: NONE

  • SEXT.H: Sign-extend halfword

    Format: sext.h rd, rs

    Description: This instruction sign-extends the least-significant halfword in rs to XLEN by copying the most-significant bit in the halfword (i.e., bit 15) to all of the more-significant bits.

    Pseudocode: X(rd) = EXTS(X(rs)[15..0])

    Invalid values: NONE

    Exception raised: NONE

  • ZEXT.H: Zero-extend halfword

    Format: zext.h rd, rs

    Description: This instruction zero-extends the least-significant halfword of the source to XLEN by inserting 0’s into all of the bits more significant than 15.

    Pseudocode: X(rd) = EXTZ(X(rs)[15..0])

    Invalid values: NONE

    Exception raised: NONE

Bitwise Rotation

Bitwise rotation instructions are similar to the shift-logical operations from the base spec. However, where the shift-logical instructions shift in zeros, the rotate instructions shift in the bits that were shifted out of the other side of the value. Such operations are also referred to as ‘circular shifts’.

RV32

RV64

Mnemonic

✔

✔

rol rd, rs1, rs2

✔

rolw rd, rs1, rs2

✔

✔

ror rd, rs1, rs2

✔

✔

rori rd, rs1, shamt

✔

roriw rd, rs1, shamt

✔

rorw rd, rs1, rs2

RV32 and RV64 Instructions

  • ROL: Rotate Left (Register)

    Format: rol rd, rs1, rs2

    Description: This instruction performs a rotate left of rs1 by the amount in least-significant log2(XLEN) bits of rs2.

    Pseudocode: (X(rs1) << log2(XLEN)) | (X(rs1) >> (xlen - log2(XLEN)))

    Invalid values: NONE

    Exception raised: NONE

  • ROR: Rotate Right

    Format: ror rd, rs1, rs2

    Description: This instruction performs a rotate right of rs1 by the amount in least-significant log2(XLEN) bits of rs2.

    Pseudocode: (X(rs1) >> log2(XLEN)) | (X(rs1) << (xlen - log2(XLEN)))

    Invalid values: NONE

    Exception raised: NONE

  • RORI: Rotate Right (Immediate)

    Format: rori rd, rs1, shamt

    Description: This instruction performs a rotate right of rs1 by the amount in the least-significant log2(XLEN) bits of shamt. For RV32, the encodings corresponding to shamt[5]=1 are reserved.

    Pseudocode: (X(rs1) >> log2(XLEN)) | (X(rs1) << (xlen - log2(XLEN)));

    Invalid values: NONE

    Exception raised: NONE

RV64 specific instructions

  • ROLW: Rotate Left Word (Register)

    Format: rolw rd, rs1, rs2

    Description: This instruction performs a rotate left on the least-significant word of rs1 by the amount in least-significant 5 bits of rs2. The resulting word value is sign-extended by copying bit 31 to all of the more-significant bits.

    Pseudocode: EXTS((rs1 << X(rs2)[4..0];) | (rs1 >> (32 - X(rs2)[4..0];)))

    Invalid values: NONE

    Exception raised: NONE

  • RORIW: Rotate Right Word by Immediate

    Format: roriw rd, rs1, shamt

    Description: This instruction performs a rotate right on the least-significant word of rs1 by the amount in the least-significant log2(XLEN) bits of shamt. The resulting word value is sign-extended by copying bit 31 to all of the more-significant bits.

    Pseudocode: (rs1_data >> shamt[4..0]) | (rs1_data << (32 - shamt[4..0]))

    Invalid values: NONE

    Exception raised: NONE

  • RORW: Rotate Right Word (Register)

    Format: rorw rd, rs1, rs2

    Description: This instruction performs a rotate right on the least-significant word of rs1 by the amount in least-significant 5 bits of rs2. The resultant word is sign-extended by copying bit 31 to all of the more-significant bits.

    Pseudocode: (rs1 >> X(rs2)[4..0]) | (rs1 << (32 - X(rs2)[4..0]))

    Invalid values: NONE

    Exception raised: NONE

OR Combine

orc.b sets the bits of each byte in the result rd to all zeros if no bit within the respective byte of rs is set, or to all ones if any bit within the respective byte of rs is set.

One use-case is string-processing functions, such as strlen and strcpy, which can use orc.b to test for the terminating zero byte by counting the set bits in leading non-zero bytes in a word.

RV32

RV64

Mnemonic

✔

✔

orc.b rd, rs

RV32 and RV64 Instructions

  • ORC.B: Bitwise OR-Combine, byte granule

    Format: orc.b rd, rs

    Description: Combines the bits within each byte using bitwise logical OR. This sets the bits of each byte in the result rd to all zeros if no bit within the respective byte of rs is set, or to all ones if any bit within the respective byte of rs is set.

    Pseudocode: if { input[(i + 7)..i] == 0 then 0b00000000 else 0b11111111

    Invalid values: NONE

    Exception raised: NONE

Byte-reverse

rev8 reverses the byte-ordering of rs.

RV32

RV64

Mnemonic

✔

✔

rev8 rd, rs

RV32 and RV64 Instructions

  • REV8: Byte-reverse register

    Format: rev8 rd, rs

    Description: This instruction reverses the order of the bytes in rs.

    Pseudocode: output[i..(i + 7)] = input[(j - 7)..j]

    Invalid values: NONE

    Exception raised: NONE