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The Advanced SIMD extension register bank is a collection of registers that can be accessed as either 64-bit or 128-bit registers.
Advanced SIMD and floating-point instructions use the same extension register bank, and is distinct from the ARM register bank.
The following figure shows the views of the extension register bank, and the
overlap between the different size registers. For example, the 128-bit register
Q0 is an alias for two consecutive 64-bit registers
D0 and D1. The 128-bit register
Q8 is an alias for 2 consecutive 64-bit registers
The aliased views enable half-precision, single-precision, and double-precision values, and Advanced SIMD vectors to coexist in different non-overlapped registers at the same time.
You can also use the same overlapped registers to store half-precision, single-precision, and double-precision values, and Advanced SIMD vectors at different times.
Do not attempt to use overlapped 64-bit and 128-bit registers at the same time because it creates meaningless results.
The mapping between the registers is as follows:
D<2n>maps to the least significant half of
D<2n+1>maps to the most significant half of
For example, you can access the least significant half of
the elements of a vector in
Q6 by referring to
and the most significant half of the elements by referring to