Transaction queue in a graphics controller chip

Abstract

A transaction queue for transferring data between a host bus and an internal system bus within a graphics controller is disclosed. The queue comprises a First In First Out (FIFO) memory having independent clocks for reading and writing. The queue accommodates address information, data, command information, byte enable information, decode information, and a tag. The tag is a 2 bit field used to identify the queued entries as address, last data, and burst data. In a preferred embodiment, single transactions are written to the queue with no wait states. In the case of an address entry, the address associated with the transaction is stored in the queue along with the command and decode information. A tag is also included that identifies the entry as an address. In the case of a final data entry, the data is stored in the next entry along with the byte enable information and a tag to indicate it is the last data of the transaction. In the case of a burst, the address entry is followed by multiple data entries that are each tagged as burst data. The final data entry of the burst will be tagged as the last data of the transaction in the same way as the data of a single transaction.