PAGE_READONLY May Be Used as Discardable Memory

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.1, 3.5, and 3.51
      - Microsoft Windows 95 version 4.0
  

Virtual memory pages marked as PAGE_READONLY under Win32 may be used the way discardable segments of memory are used in Windows 3.1. These virtual memory pages are by default not "dirty," so the system may use them (zeroing them first if necessary) without having to first write their contents to disk.

From a system resource perspective, PAGE_READONLY is treated similar to discardable memory under Windows 3.1 when the system needs to free up resources. From a programming standpoint, the system automatically re-reads the memory when the data is next accessed (for example, we attempt to access our page when it has been "discarded," a page fault is generated, and the system reads it back in). Memory-mapped files are convenient for setting up this type of behavior.

If a PAGE_READONLY memory page becomes dirty [by changing the protection via VirtualProtect() to PAGE_READWRITE, changing the data, and restoring PAGE_READONLY], the page will be written to disk before the system uses it.