1. An information processing system comprising: a processor capable of executing native instruction codes; a program memory for storing a program at least part of which comprises compressed instruction codes; an instruction code conversion unit connected to said processor and said program memory for receiving an address of a native instruction code as outputted from said processor, converting the address of said native instruction code to the address of a corresponding native instruction code in said program memory, reading out said compressed instruction codes from the address of said compressed instruction code as converted as converted, converting said compressed instruction code to said native instruction code as converted, and transferring said native instruction to said processor, wherein said instruction code conversion unit performs conversion of the address of said native instruction code to the address of the corresponding compressed instruction code in said program memory by shifting the address of said native instruction code as outputted from said processor to the right by n bit.