bexti

Single-Bit extract (Immediate)

This instruction is defined by:

anyOf:
- B, version >= B@1.0.0
- Zbs, version >= Zbs@1.0.0

This instruction is included in the following profiles:

RVA22U64 (Mandatory)
RVA23U64 (Mandatory)
RVB23U64 (Mandatory)

Encoding

This instruction has different encodings in RV32 and RV64.

RV32
RV64

Assembly format

bexti rd, rs1, shamt

Synopsis

This instruction returns a single bit extracted from rs1 at the index specified in rs2. The index is read from the lower log2(XLEN) bits of shamt. For RV32, the encodings corresponding to shamt[5]=1 are reserved.

Access

M	HS	U	VS	VU
Always	Always	Always	Always	Always

Always

Decode Variables

RV32
RV64

Bits<5> shamt = $encoding[24:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

idl

Bits<6> shamt = $encoding[25:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

idl

Execution

IDL
Sail

if (implemented?(ExtensionName::B) && (CSR[misa].B == 1'b0)) {
  raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
XReg index = shamt & (xlen() - 1);
X[rd] = (X[rs1] >> index) & 1;

idl

{
  let rs1_val = X(rs1);
  let mask : xlenbits = if sizeof(xlen) == 32
                        then zero_extend(0b1) << shamt[4..0]
                        else zero_extend(0b1) << shamt;
  let result : xlenbits = match op {
    RISCV_BCLRI => rs1_val & ~(mask),
    RISCV_BEXTI => zero_extend(bool_to_bits((rs1_val & mask) != zeros())),
    RISCV_BINVI => rs1_val ^ mask,
    RISCV_BSETI => rs1_val | mask
  };
  X(rd) = result;
  RETIRE_SUCCESS
}

sail

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction