1. An information processing system including a plurality of instruction strings containing a plurality of instructions and progressing process by executing instructions in one of said instruction strings in order, comprising: token storing means for storing control tokens, each being set with a start address for each instruction string; arithmetic processing means for executing instructions in said instruction string from said start address in said control token fed from said token storing means; a register group comprising a plurality of data registers for storing data to be accessed by said arithmetic processing means; load controlling means for receiving loaded data and writing in a data register in said register group during execution of an instruction for loading data from an external memory; instruction string controlling means for feeding an instruction next to instruction for loading data in said instruction string to said arithmetic processing means without waiting for completion of a load operation during execution of the instruction by said arithmetic processing means for loading data from the external memory to said data register in said register group, interrupting execution of the currently executed instruction string and feeding the control token of another instruction string to said arithmetic processing means from said token storing means and storing a control token including a resume address of the interrupted instruction string in said token storing means when data to be used for an arithmetic operation is not loaded in said data register in execution of the instruction for performing said arithmetic operation using the data in said data register; token generating means for generating a demand token designating loading of data with respect to an access means for said external memory in response to a command of said instruction string controlling means in execution of the instruction for loading data from said external memory, and generating said control token including said resume address of the interrupted instruction signal in response to a command from said instruction string controlling means upon interruption of execution of the instruction string; a flag indicating whether data loading is completed for a plurality of data registers in said register group, and said load controlling means sets a load completion in said flag of the data register to which the loaded data is written by the load instruction of data, and said instruction string controlling means interrupts execution of the currently executed instruction string when the flag indicates incompleteness of loading in said data register upon execution of the instruction for performing said arithmetic operation with respect to the data in the data register.