vcntmbb

Vector Count Mask Bits Byte

vcntmbb RT,VRB,MP

Counts the number of true (or false) mask bits in a VSR and places the count in the leftmost byte of a GPR.

Encoding

Binary Layout

0:5

6:10

11:14

VRB

1602

16:31

Format VX-form

Opcode 0x10180642

Extension VMX (AltiVec)

Operands

RT
Target General Purpose Register
VRB
Source Vector Register
MP
Mask Bit (0 or 1)

Example

vcntmbb r3, v3, 0

Reference

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

Description

Counts the number of byte elements in VRB that have bit 0 set to the value specified by MP (0 or 1), and places the count in the leftmost byte of the target GPR RT. This instruction is part of the VMX category and does not affect condition registers or status fields.

Operation

count ← 0
for i in 0 to 15:
  if (VRB.byte[i][0] == MP) then
    count ← count + 1
RT[56:63] ← count
RT[0:55] ← 0

Programming Note

This instruction is useful for counting the number of bytes in a vector register where the least significant bit matches a specified mask value (MP). Ensure that the Vector Facility is enabled by checking and setting the VEC bit in the Machine State Register (MSR) before using this instruction. The result is left-shifted 56 bits, so only the lower byte of GPR[RT] contains the count. This instruction operates at the problem state privilege level.