A data processing system comprising a coherency protocol that indicates and updates
a coherency state of all data lines within said memory subsystem, and responsive
to a naked write operation to a memory location containing a modified copy of data,
changes the coherency state of the memory location from modified (M) to invalid
(I) without initiating a push of the data to a corresponding address location of
main memory. Included within the coherency protocol are specific group of responses
for dealing with a naked write request that is received at a memory controller.
These responses include: (1) issuing a retry response when the memory controller
is unable to allocate a buffer for the write data operation and data at said memory
location is not in an M state; (2) issuing a Null response when the memory controller
is able to allocate the buffer; and (3) issuing a combined Ack_Resend response
when the data at the memory location transitions from an M state to an I state.
The Ack_Resend response indicates that the coherency state transition has occurred
but the memory controller is not yet able to allocate a buffer to the write operation.
The protection of the memory location is transferred to the memory cloner when
a combined response is received at the memory cloner for a particular naked write operation.
