1. In a computer system having virtual memory partitioned into pages, an addressable main storage for storing an input/output (I/O) event stack and in which virtual pages may reside, a central processing unit (CPU) for retrieving I/O events from the I/O event stack and for processing said I/O events, a plurality of I/O adapters connected to control I/O devices and a channel connecting the plurality of I/O adapters to the CPU and main storage, the improvement comprising: page request means under the control of the I/O adapters for requesting that a particular virtual page be made available in addressable main storage, said page request means specifying channel function information and virtual address information for the requested page; means responsive to the page request means for forming a page request function event incorporating the channel function information and the virtual address information; and means for storing the formed page request function event on the I/O event stack.