sext.b

Sign-extend byte

This instruction is defined by:

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

This instruction is included in the following profiles:

RVA22S64 (Mandatory)
RVA22U64 (Mandatory)

Encoding

Assembly format

sext.b rd, rs1

Synopsis

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.

Access

M	HS	U	VS	VU
Always	Always	Always	Always	Always

Always

Decode Variables

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);
}
if (xlen() == 32) {
  X[rd] = {{24{X[rs1][7]}}, X[rs1][7:0]};
} else if (xlen() == 64) {
  X[rd] = {{56{X[rs1][7]}}, X[rs1][7:0]};
}

{
  let rs1_val = X(rs1);
  let result : xlenbits = match op {
    RISCV_SEXTB => sign_extend(rs1_val[7..0]),
    RISCV_SEXTH => sign_extend(rs1_val[15..0]),
    RISCV_ZEXTH => zero_extend(rs1_val[15..0])
  };
  X(rd) = result;
  RETIRE_SUCCESS
}

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction