Byte-wise tracking on write allocate

A cache line allocation apparatus within a pipeline microprocessor, for allocating a selected cache line upon a write miss, the cache line allocation apparatus comprising:

• a data cache, for storing a plurality of cache lines retrieved from external memory, wherein each of said plurality of cache lines comprises 32 bytes;
• and
• cache control logic, coupled to said data cache, configured to store data corresponding to the write miss within the selected cache line, and to update the selected cache line from said external memory, wherein said data is stored before said selected cache line is updated, and wherein selected bytes within the selected cache line are not updated, said selected bytes being those within which said data are stored, wherein said cache control logic comprises:
• a plurality of byte mask registers, each of said plurality of byte mask registers corresponding to each of a plurality of cache ways within said data cache, wherein contents of one of said plurality of byte mask registers denote said selected bytes.

An apparatus for performing write allocation in a data cache when a write miss occurs, the apparatus comprising:

• write allocate logic, configured to update a cache line within the data cache with data bytes corresponding to the write miss and with data from external memory, wherein said data bytes are updated prior to update of said data from said external memory, and wherein byte positions within said cache line corresponding to said data bytes are masked during update of said data, thereby preserving said data bytes within said cache line, said write allocate logic comprising:
• snoop/backoff control logic, configured to detect events that interrupt update of said cache line;
• and
• a write buffer, coupled to said write allocate logic, for storing a speculative write command, said speculative write command directing that said data bytes be stored within said external memory.

A method for allocating a cache line within a pipeline microprocessor, the method comprising:

• a) storing data bytes corresponding to a write miss to an allocated cache line within the data cache;
• b) queuing a speculative write command to external memory corresponding to the write miss;
• c) following said storing, updating remaining bytes within the allocated cache line from external memory;
• and
• d) if said updating is interrupted, issuing the speculative write command, thereby storing the data bytes to the external memory.