6.10 System Interface Buffers

Incoming Buffer

The incoming buffer eliminates the need for the processor to flow-control the external agent that is providing the external data responses. Regardless of the cache bandwidth or internal resource availability, the external agent may supply external data response data for all outstanding read and upgrade requests at the maximum System interface data rate.

The external agent may issue any number of external data responses for a particular request number before issuing a corresponding external completion response. An external data response remains in the incoming buffer until a corresponding external completion response is received. A former buffered external data response for a particular request number is over-written by a subsequent external data response for the same request number.

An external ACK completion response frees buffered data to be forwarded to the caches and other internal resources while an external NACK or ERR completion response purges any corresponding buffered data. For minimum latency, the external agent should issue an external ACK completion response coincident with the first doubleword of an external data response.

External coherency requests that target blocks residing in the incoming buffer are stalled until the incoming buffer data is forwarded to the secondary cache, and the instruction that caused the secondary miss is satisfied.

Each doubleword of the incoming buffer has an Uncorrectable Error flag. When an external data response provides a doubleword, the processor asserts the corresponding incoming buffer Uncorrectable Error flag if the data quality indicator, SysCmd[5], is asserted, or if an uncorrectable ECC error is encountered on the system address/data bus and the ECC check indication on SysCmd[0] is asserted.

When the processor forwards block data from an incoming buffer entry after receiving an external ACK completion response, the associated incoming buffer Uncorrectable Error flags are checked, and if any are asserted, a single Cache Error exception is posted. When the processor forwards double/single/partial-word data from an incoming buffer entry after receiving an external ACK completion response, the associated incoming buffer Uncorrectable Error flag is checked, and if asserted, a Bus Error exception is posted.