Register restrictions for A64 instructions
In A64 instructions, the general-purpose integer registers are W0-W30 for 32-bit registers and 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.