1. An execution system for executing instructions in an information processing apparatus, comprising: a plurality of operand registers; input means for inputting a long instruction word which has a fixed length and includes a type code including a field designation and at least one instruction word of variable length the position of which is indicated by the field designation, each variable length instruction word including at least an instruction code and an operand specifying part for specifying a single instruction, the type code specifying a structure of the at least one instruction word, said variable length instruction word having a format selected from one of a first format in which the instruction code instructs an operation between contents of first and second registers and a storage of an operation result of the operation in a third register, a second format in which the instruction code instructs an operation between the content of the first register and an immediate data and a storage of an operation result of the operation in the third register, and a third format in which the instruction code instructs a storage of an immediate data in to the third register; judging means coupled to said operand registers for judging from the type code a combination and a number of instruction words and for providing a judgement result, said judging means having means for specifying one of said operand registers depending on the operand specifying part of each variable length instruction word; a plurality of operation means each for independently executing operations specified by different ones of the instruction words within the long instruction word; and assigning means for assigning one of the operations to one of said operation means depending on the judgement result.