1. A data processing unit including a main memory for storing instructions and data, and an instruction processing unit for executing a load instruction for reading out main memory data from said main memory and storing the main memory data to a register numbered in said instruction, a store instruction for storing data from a register numbered in said instruction to said main memory, and an arithmetic instruction for executing an arithmetic operation and storing the result in a register numbered in said instruction, by the use of said main memory data stored in said main memory, said instruction processing unit comprising: a) a greater number of physical registers than number of registers addressable by an instruction; b) a window start pointer register for storing a number indicating the start of a range of physical registers included in a window to be accessed by said instruction, said window start pointer register includes a plurality of bits; c) means for setting values to said window start pointer register upon receiving a window pointer set instruction for setting the value of said window start pointer register; d) a window start pointer valid register for storing a value representing whether or not a register number is to be converted into a physical register number; e) conversion means for determining a conversion process for converting said register number in said instruction to said physical register number based on the number stored in said window start pointer register and a value of window stride included in said instruction, said window stride representing displacement of said window from the value of said window start pointer register, and converting said register number in said instruction to said physical register number in accordance with said conversion process determined by said conversion means when the value of said window start pointer valid register represents that said physical register number is valid; and f) an instruction controller for decoding said instruction, said instruction being one of the following: i) a register pre-load instruction having therein said register number and said window stride, for converting said register number in said register pre-load instruction to said physical register number after said conversion is process determined based on the number stored in said window start pointer register and the value of said window stride by said conversion means, and instructing storage of main memory data to said physical register indicated by said physical register number, ii) a register post-store instruction having therein said register number and said window stride, for converting said register number in said post-store instruction to said physical register number after said conversion process is determined based on the number stored in said window start pointer register and the value of said window stride by said conversion means, and instructing storage of data of a physical register indicated by said physical register number into said main memory, and iii) said load instruction, said store instruction and said arithmetic instruction each having therein said register number, for converting said register number in said instruction to said physical register number after said conversion process is determined based on the number stored in said window start pointer register by said conversion means, and instructing one of load, store and arithmetic operations to said physical register number.