Synchronizing mechanism for page replacement control

An apparatus provides synchronization for page replacement control in a paged, virtual memory environment in which either the CPU or the I/O devices may pin and unpin pages to control their replacement by the paging supervisor. Pinning and unpinning of pages by the I/O devices occurs independently of pinning and unpinning performed by the CPU. Synchronization is achieved by means of a virtual address translation mechanism which is common to the CPU and the I/O devices. The virtual address translation mechanism includes a primary directory having entries for each page in main storage, with each entry containing a field in which the pinning and unpinning operations by the CPU and the I/O devices are registered. In particular, this field is a counter which is incremented when a page is pinned by either the CPU or an I/O device and decremented when a page is unpinned. Each page directory entry also includes a field for indicating that references to the page corresponding to a given page directory entry are invalid. When it is determined that a particular page is no longer in use and is eligible for replacement, the page directory entry is set to indicate that further references to the page are invalid. When an I/O device gives up a buffer page, that page is unpinned. Buffer pages newly acquired by the I/O device are pinned when they are acquired by the device and have become available in main storage.