clzw

Count leading zero bits in word

This instruction is defined by:

Encoding

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.

Access

Always

Decode Variables

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

Execution

Pruned, XLEN == 64
Original

if ((%%LINK%csr_field;misa.B;CSR[misa].B%% == 1'b0)) {
  %%LINK%func;raise;raise%%(ExceptionCode::IllegalInstruction, %%LINK%func;mode;mode%%(), $encoding);
}
X[rd] = 31 - $signed(%%LINK%func;highest_set_bit;highest_set_bit%%(X[rs1][31:0]));

if (%%LINK%func;implemented?;implemented?%%(ExtensionName::B) && (%%LINK%csr_field;misa.B;CSR[misa].B%% == 1'b0)) {
  %%LINK%func;raise;raise%%(ExceptionCode::IllegalInstruction, %%LINK%func;mode;mode%%(), $encoding);
}
X[rd] = 31 - $signed(%%LINK%func;highest_set_bit;highest_set_bit%%(X[rs1][31:0]));

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction