clzw
Count leading zero bits in word
This instruction is defined by:
-
anyOf:
-
B, version >= 0
-
Zbb, version >= 0
-
This instruction is included in the following profiles:
-
RVA22U64 (Mandatory)
Synopsis
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.
Execution
-
IDL
-
Sail
if (implemented?(ExtensionName::B) && (CSR[misa].B == 1'b0)) {
raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
X[rd] = 31 - $signed(highest_set_bit(X[rs1][31:0]));
{
let rs1_val = X(rs1);
result : nat = 0;
done : bool = false;
foreach (i from 31 downto 0)
if not(done) then if rs1_val[i] == bitzero
then result = result + 1
else done = true;
X(rd) = to_bits(sizeof(xlen), result);
RETIRE_SUCCESS
}