1. A method of dynamically changing the formation of addresses in a memory of a data processing system having a segment descriptor register adapted to hold segment descriptor words for prescribing segments containing instruction words and data stored in the memory and having memory addresses, an address register for designating addresses for data stored in the memory, and an instruction counter for designating addresses of the instruction words stored in the memory, said method comprising a first step for storing said segment descriptor words in said segment descriptor register, a second step for controlling the number of effective bits of said address register and instruction counter in accordance with information in said segment descriptor register, a third step for designating an address of an instruction word in the memory to be executed by adding the content of the segment descriptor register and the content of the effective bits of said instruction counter, and a fourth step for designating an address of a data in the memory by adding the content of said segment descriptor register, the content of the effective bits of said address register and said address of said instruction word.