Some NEON instructions act on scalars in combination with vectors. NEON scalars can be 8-bit, 16-bit, 32-bit, or 64-bit.
The instruction syntax refers to the scalars using an index, x, into a doubleword vector, so that Dm[x] is the xth element in vector Dm . Other than multiply instructions, instructions that access scalars can access any element in the register bank.
Multiply instructions only allow 16-bit or 32-bit scalars, and can only access the first 32 scalars in the register bank. That is, in multiply instructions:
16-bit scalars are restricted to registers D0-D7, with x in the range 0-3.
32-bit scalars are restricted to registers D0-D15, with x either 0 or 1.