 retaining data which has not been accessed for the longest period of time, characterized in that the cash memory device is composed of a cash memory (corresponding to a primary cash memory 11 in the first embodiment which will be mentioned hereinafter) containing two ways each retaining bits taking a value 0 or 1 together with data and an address, respectively; an old way determining means (corresponding to a primary old way determining circuit 13 in the first embodiment which will be mentioned hereinafter) for determining which way of the above described two ways is the above described old way on the basis of patterns of two bits retained in the aforesaid two ways; and a control means (corresponding to a primary cash control 14 in the first embodiment which will be mentioned hereinafter) for inverting only the bits retained in the aforesaid way in the case when the above described replacement has been conducted.
According to the present invention, an old way is determined from two ways based on patterns of two bits in 2-bit composition (combinations of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d) in the old way determining means. When replacement is made with respect to an old way, only the bits retained in the old way are inverted. Therefore, bits in the other way are not inverted. Namely, in this case, the control means accesses to only either way of two ways one time.
As described above, according to the third aspect of the invention, it is arranged in such that when an old way is replaced, only the bits retained in the old way are inverted, so that it is sufficient for only one time access in case of updating bits, whereby a processing speed can be elevated in comparison with two times access in the prior art.
Furthermore, a forth aspect of the invention relates to a cash memory device as described in any one of the first to third aspects of the invention wherein a plurality of the above described cash memories are prepared so as to correspond to their rank levels.
According to the present invention, it is 