1. A memory access method for accessing a memory of a computer system according to a function ID instruction, wherein the function ID instruction comprises a first function ID value and a second function ID value, the computer system comprises a south bridge having a first index port, a first data port and a default I/O port, and the method comprises: (a) inputting a default value to the default I/O port according to the function ID instruction under the operation of a call-back function and entering an SMM mode; and (b) in the SMM mode, inputting an address value corresponding to the first function ID value to the first index port, meanwhile performing a memory access corresponding to the second function ID value on the first data port according to the address value.