A printer has a queue for queuing a queued execution command, an immediate execution
agent for executing a write command, and a queued execution agent for executing
a read command. The immediate execution agent immediately executes the received
write command, and writes data in a host. The queued execution agent picks up a
read command from the queue, and reads out data from the host. The host appends
a data transfer request from the printer to a queue, issues a write command to
the printer on the basis of that data transfer request, and issues a read command
to the printer on the basis of a print data transmission request or the like from
an application. Independent full-duplex channels can be provided in two directions.
Also, a write command can be immediately processed.
