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Assembler Language Reference

cntlzd (Count Leading Zeros Double Word) Instruction

Purpose

Count the number of consecutive zero bits in the contents of a general purpose register, beginning with the high-order bit.

This instruction should only be used on 64-bit PowerPC processors running a 64-bit application.

Syntax

PowerPC64
cntlzd rA, rS (Rc=0)
cntlzd. rA, rS(Rc=1)

Description

A count of the number of consecutive zero bits, starting at bit 0 (the high-order bit) of register GPR RS is placed into GPR RA. This number ranges from 0 to 64, inclusive.

This instruction is defined only for 64-bit implementations. Using it on a 32-bit implementation will cause the system illegal instruction error handler to be invoked.

Other registers altered:

Condition Register (CR0 field):

Affected: LT, GT, EQ, SO (if Rc = 1)

Note: If Rc = 1, then LT is cleard in the CR0 field.

Parameters

RA Specifies the target general purpose register for the results of the instruction.
RS Specifies the source general purpose register containing the double-word to examine.

Implementation

This instruction is defined only for 64-bit implementations. Using it on a 32-bit implementation will cause the system illegal instruction error handler to be invoked.


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