In a computer system, an I/O adapter comprises at least one I/O request
mailbox for receiving I/O requests and data from a CPU. The mailbox is
connected to at least one I/O request queue storage for storing a
plurality of I/O requests and related data or main memory addresses
received by the mailbox. A credit register is provided for storing the
filling state of the queue storage. A connection network is designed and
operated to transfer the contents of the credit register to the CPU for
checking the filling state of the queue storage, to transfer an I/O
request and related data to a queue of a selected request mailbox, and to
activate a doorbell apparatus for signalling to the I/O adapter to
process the I/O request including an access to the main memory by the I/O
adapter if required.