Cache device for supplying a fixed word length of a variable instruction

Abstract

An instruction fetching device includes one or both of a cache device and a branch history table. The cache device stores a plurality of pairs, each pair including an instruction string divided into minimum unit instructions and an address of the instruction string. At the time of reading an instruction, an instruction string is selected and output by every minimum unit instruction from at least two pairs. The branch history table stores a plurality of pairs, each pair including a branch destination address and a set of an address of a branch instruction and a value obtained by subtracting a given value from the address. At the time of reading an instruction, first, a pair having an address of a branch instruction which address is the nearest to a head address of an instruction string to be read out is selected from a plurality of pairs, each pair including an address of a branch instruction which address is in a predetermined address range including the instruction string to be read out, or each pair of the plurality of pairs including a value obtained by subtracting a given value from the address of the branch instruction. Then, a branch destination address is selected and output from the pair which is selected first from the plurality of pairs.