Encoding:

pre-Release 6

Release 6

Format:

CLO rd, rs

Purpose:

Count Leading Ones in Word

To count the number of leading ones in a word.

Description:

 GPR[rd] = count_leading_ones GPR[rs]

Bits 31..0 of GPR rs are scanned from most significant to least significant bit. The number of leading ones is counted and the result is written to GPR rd. If all of bits 31..0 were set in GPR rs, the result written to GPR rd is 32.

Restrictions:

Pre-Release 6: To be compliant with the MIPS32 and MIPS64 Architecture, software must place the same GPR number in both the rt and rd fields of the instruction. The operation of the instruction is UNPREDICTABLE if the rt and

rd fields of the instruction contain different values. Release 6's new instruction encoding does not contain an rt field.

If GPR rs does not contain a sign-extended 32-bit value (bits 63..31 equal), then the results of the operation are

UNPREDICTABLE.

Availability and Compatibility:

This instruction has been recoded for Release 6.

Operation:

if NotWordValue(GPR[rs]) then
   UNPREDICTABLE
endif
temp = 32
for i in 31 .. 0
   if GPR[rs]i = 0 then
      temp = 31 - i
      break
   endif
endfor
GPR[rd] = temp

Exceptions:

None

Programming Notes:

As shown in the instruction drawing above, the Release 6 architecture sets the 'rt' field to a value of 00000.