*** Vector usage uses the notation v''n''.''s'' where ''n'' is the register number and ''s'' is the shape of the lanes, expressed as the number of lanes and a letter indicating the width of the lanes: q for quad-word (128 bits), d for double-word (64 bits), s for single-word (32 bits), h for half-word (16 bits), and b for byte (8 bits). Therefore, <code>v0.16b</code> is vector register 0 used as 16 lanes of 8 bits (1 byte) each, while <code>v8.4s</code> is vector register 8 used as 4 lanes of 32 bits each. Most instructions permit either 64 or 128 bits of the register to be used.

*** Scalar usage uses the lane width letter followed by the vector register number. Therefore, <code>q3</code> refers to vector register 3 used as a single 128-bit value, and <code>s3</code> refers to the same register used as a single 32-bit register. Note that these are the same register referred to as v3 for vector usage. When using less than 128 bits, the remaining bits are either zero-filled (unsigned usage) or sign-extended (signed usage: the upper bits are filled with the sign bit, i.e., the same value as the high bit of the active part of the register).

** A small number of SIMD operations work across lanes, e.g., to find the lowest or highest value in all of the lanes, to add the lanes together, or to duplicate a single value into all of the lanes of a register. These are usually used to set up or summarize the results of SIMD operations -- for example, a value of 0 might be duplicated into all of the lanes of a result register, then a loop applied to sum array data into the results register, and then a lane-summing operation performed to merge the results from all of the lanes.

* SIMD capabilities can be used in a program in one of three different ways: