ctz
Count trailing zero bits
This instruction is defined by:
-
anyOf:
-
B, version >= 0
-
Zbb, version >= 0
-
This instruction is included in the following profiles:
-
RVA22S64 (Mandatory)
-
RVA22U64 (Mandatory)
Synopsis
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.
Execution
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IDL
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Sail
if (implemented?(ExtensionName::B) && (CSR[misa].B == 1'b0)) {
raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
X[rd] = (xlen() - 1) - $signed(lowest_set_bit(X[rs1]));{
let rs1_val = X(rs1);
result : nat = 0;
done : bool = false;
foreach (i from 0 to (sizeof(xlen) - 1))
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
}