bext

Single-Bit extract (Register)

This instruction is defined by:

anyOf:
- B, version >= 0
- Zbs, version >= 0

This instruction is included in the following profiles:

RVA22S64 (Mandatory)
RVA22U64 (Mandatory)

Encoding

Assembly format

bext rd, rs1, rs2

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 rs2.

Access

M	HS	U	VS	VU
Always	Always	Always	Always	Always

Always

Decode Variables

Bits<5> rs2 = $encoding[24:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

IDL
Sail

if (implemented?(ExtensionName::B) && (CSR[misa].B == 1'b0)) {
  raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
XReg index = X[rs2] & (xlen() - 1);
X[rd] = (X[rs1] >> index) & 1;

{
  let rs1_val = X(rs1);
  let rs2_val = X(rs2);
  let mask : xlenbits = if sizeof(xlen) == 32
                        then zero_extend(0b1) << rs2_val[4..0]
                        else zero_extend(0b1) << rs2_val[5..0];
  let result : xlenbits = match op {
    RISCV_BCLR => rs1_val & ~(mask),
    RISCV_BEXT => zero_extend(bool_to_bits((rs1_val & mask) != zeros())),
    RISCV_BINV => rs1_val ^ mask,
    RISCV_BSET => rs1_val | mask
  };
  X(rd) = result;
  RETIRE_SUCCESS
}

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction