General-purpose registers
Most A64 instructions operate on registers. The architecture provides 31 general purpose registers. Each register can be used as a 64-bit X register (X0..X30), or as a 32-bit W register (W0..W30). These are two separate ways of looking at the same register. For example, this register diagram shows that W0 is the bottom 32 bits of X0, and W1 is the bottom 32 bits of X1:
For data processing instructions, the choice of X or W determines the size of the operation. Using X registers will result in 64-bit calculations, and using W registers will result in 32-bit calculations. This example performs a 32-bit integer addition:
ADD W0, W1, W2
This example performs a 64-bit integer addition:
ADD X0, X1, X2
When a W register is written, as seen in the example above, the top 32 bits of the 64-bit register are zeroed.
There is a separate set of 32 registers used for floating point and vector operations. These registers are 128-bit, but like the general-purpose registers, can be accessed in several ways. Bx is 8 bits, Hx is 16 bits and so on to Qx which is 128 bits.
The name you use for the register determines the size of the calculation. This example performs a 32-bit floating point addition:
FADD S0, S1, S2
This example performs a 64-bit floating point addition:
FADD D0, D1, D2
These registers can also be referred to as V registers. When the V form is used, the register is treated as being a vector. This means that it is treated as though it contains multiple independent values, instead of a single value. This example performs vector floating point addition:
FADD V0.2D, V1.2D, V2.2D
This example performs vector integer addition:
ADD V0.2D, V1.2D, V2.2D
We will look at vector instructions in more detail later in this guide.