In A64 instructions, the range of general-purpose integer registers is as follows:
W0-W30 for 32-bit registers.
X0-X30 for 64-bit registers.
You cannot refer to register 31 by number. In a few instructions, you can refer to it using one of the following names:
the current stack pointer in a 32-bit context.
the current stack pointer in a 64-bit context.
the zero register in a 32-bit context.
the zero register in a 64-bit context.
You can only use one of these names if it is mentioned in the Syntax section for the instruction.
You cannot refer to the Program Counter (PC) explicitly by name or by number.