Memory system for permitting simultaneous processor access to a cache line and sub-cache line sectors fill and writeback to a system memory

In a cache memory system, a mechanism enabling two logical cache lines to coexist within the same physical cache line, during line fill and replacement, thus minimizing the likelihood of stalling accesses to the cache while the line is being filled or replaced. A control mechanism governs access to the cache line and tracks which sub-cache line units contain old or new data, or are empty during the fill/replacement procedure. The control mechanism thus maintains a sub-cache line state for the purpose of permitting a processor to gain access to a portion of the cache line before it is completely filled or replaced.