the changes in icbm 302 and oldest 402 after other methods within cache control unit 102 request for a second time that a cache buffer 204 be made available for overwriting the system of the present invention responds to this second request as described above for the first request as shown in fig8 b oldest 402 points at trcl entry number trcl entry number 11 indicates that a previous reference to cache buffer number n is the oldest previous reference in trcl 406 testing the bits 300 labeled n in each icbm 302 304 306 and 308 to reveals that cache buffer number n has been referenced in at least one of the recent time periods and is therefor not currently available for overwriting the system of the present invention next increments oldest 402 to point to trcl entry number 12 trcl entry number 12 indicates that a previous reference to cache buffer number 3 is now the oldest previous reference in trcl 406 the bits 300 labeled 3 in each icbm 302 304 306 and 308 are now tested to determine if cache buffer number 3 has been recently referenced as shown in the icbms of fig8 a 8b and 8c cache buffer number 3 has not been referenced in any of the.