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Count Leading Zeros

lesbattersbie lesbattersbie over 12 years ago

Note: This was originally posted on 6th April 2009 at http://forums.arm.com

Using the ARM9 CLZ instruction - I am wondering, generally, how often would it be required to use this function and does it justify its inclusion?

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Jacob Bramley over 12 years ago +1 verified

Note: This was originally posted on 6th April 2009 at http://forums.arm.com It's surprisingly useful when hand-optimizing assembler. I think it has benefits for some cryptographic routines, and it speeds...

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0 Peter Harris over 12 years ago

Note: This was originally posted on 7th April 2009 at http://forums.arm.com

But how many clock cycles does CLZ require to execute???

For an ARM9, assuming you are not targeting the PC as the destination register (because that would be a bit crazy).

One cycle if not using the shifter to shift the second input operand, or using a constant shift.
Two cycles if using the shifter to shift the second operation with a register value input to the shifter.

This is the same as any other ARM9 data processing instruction (ADD, ORR, etc).
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0 Peter Harris over 12 years ago

Note: This was originally posted on 7th April 2009 at http://forums.arm.com

But how many clock cycles does CLZ require to execute???

For an ARM9, assuming you are not targeting the PC as the destination register (because that would be a bit crazy).

One cycle if not using the shifter to shift the second input operand, or using a constant shift.
Two cycles if using the shifter to shift the second operation with a register value input to the shifter.

This is the same as any other ARM9 data processing instruction (ADD, ORR, etc).
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