Tham khảo tài liệu 'formal models of operating system kernels phần 9', công nghệ thông tin, hệ điều hành phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Virtual Storage 267 V msg GTPG1 p sg lpno A retrievePageFromDisk p p sg sg lpno lpno pg pg A 3 rpmsg MSG I rpmsg ISPG p sg Ipno pg msgmgr .SendMessage fhandler dest pdsk src rpmsg m V msg DELPROCPG p A removeProcessFromPagingDisk p p Proposition 152. OnPageRequest does what it should. Proof. The proof is by cases on message type. It is assumed that all domains are correct so the error cases ignored in the schema need not be introduced here. Case 1. m STOPG a request to store a page on disk. The predicate of storePageOnDisk p p sg sg lpno lpno pg pg states pagemap pagemap p sg lpno pg so pg pagemap p sg lpno . Case 2. m GTPG a request to retrieve a page. This follows from the fact that retrievePageOnDisk is a function pg pagemap p sg lpno . Case 3. m DELPROCPG. By the last proposition. Placement Demand Paging and LRU There are significant issues to be resolved in the design of the virtual storage mechanism. This section deals with the general area of placement. Placement is concerned with where pages are to be removed and included in main store. When a process refers to a page that is not currently in main store it generates a page fault an asynchronous signal that interrupts the process and leads to the satisfaction of the reference. To do this the support software has to identify the page that is being referenced and then identify a page in physical store that can be swapped out to the paging disk thus making space for the referenced page to be copied into main store. The issue is slightly complicated by the fact that the system might have some free physical pages in main store indeed it might be a policy to keep a block of such pages in reserve . For present purposes it is assumed that all physical pages are allocated and that there is no pool of free pages kept in reserve. The placement algorithm just outlined is demand paging. This is the most common approach. It is documented like many other possible approaches in most textbooks on operating systems
