Scalable schedules for serial communications controller in data processing systems

Claim 1

In a programmable communications controller connected between a host data processing system and a plurality of communications lines, a method for controlling data transfer between the communications lines, through a plurality of communications ports contained in the communications controller, and the host data processing system, the method comprising the steps executed by the programmable communications controller of: providing in an intermediate memory a plurality of tasks for performing portions of a data transfer operation between the communications ports and the intermediate memory, and between the intermediate memory and the host data processing system; generating a priority table in the intermediate memory having a highest priority level for requests for data block transfers between the intermediate memory and a communications port, wherein the highest priority level contains only a single entry location for the level, and a plurality of lower priority levels for the tasks, each lower priority level having an entry location for each communications port; inserting entries into the lower priority entry locations identifying which of the tasks are to be performed in response to events which occur at the communications ports and within the host data processing system; generating a counter for the entry location for the highest priority level for tracking the number of outstanding requests for data block transfers; generating a queue, of outstanding requests for data block transfers, external to the priority table for the data block transfers; responsive to execution of a data block transfer, resetting the counter of requests for data block transfers to zero for the duration of the data block transfer, whereby tasks at lower priority levels can be scheduled during the data block transfer; and selecting the tasks for execution from the priority table in order of priority level among the tasks identified by entries in the priority table.