cntlzdm

Count Leading Zeros Doubleword under Mask

cntlzdm RA, RS, RB

Counts leading zeros in RS, but only considering bits set in mask RB.

Encoding

Binary Layout

31

0:5

RS

6:10

RA

11:15

RB

16:20

59

21:30

/

31

 

Format X-form

Opcode 0x7C000076

Extension Base

Operands

• RA
  Target
• RS
  Source
• RB
  Mask

Example

cntlzdm r4, r3, r5

Related

More in Base

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

Counts the number of leading zeros in RS considering only the bit positions set to 1 in the mask RB (Power10 Scalar). The count is placed in RA. No CR, XER, or FPSCR fields are affected.

Operation

mask ← RB; masked_value ← RS ∧ mask; if (masked_value = 0) then RA ← 64; else count ← 0; for i in 0 to 63 do; if (masked_value[i] = 1) then break; count ← count + 1; RA ← count;

Programming Note

The cntlzdm instruction is useful for counting leading zeros in a doubleword while considering only the bits that are set to 1 in a mask. Ensure that both the source and mask registers are correctly aligned and that the mask register has at least one bit set to 1 to avoid undefined behavior. This instruction operates at user privilege level and does not generate exceptions under normal conditions, but it may incur performance penalties on processors without hardware support for this operation.