非易失性寄存器（Non-volatile register）是它的内容必须通过子程序调用被保存的一个寄存器。如果一个程序改变了一个非易失性寄存器的值，它必须保存在改变这个寄存器之前堆栈中保存旧的值和在返回之前恢复那个值。
Nonvolatile Register
Nonvolatile register is a register whose contents must be preserved across subroutine calls. If a routine changes the value of a nonvolatile register, it must save the old value on the stack before changing the register and restore that value before returning.