Memory Object

Internally, a memory object consists of two objects, cache and pager. Cache holds a set of page frames. All the frames in a memory object is recorded in the cache object.

Pager implements a mechanism to read and write pages to and from memory and external I/O device. Pager is expected to be subtyped. Currently, two types of pagers are provided, Default default pager(aka bogopager) and Real default pager(aka default pager).

• Bogopager always provides a zero filled page on page fault. It doesn't provide the page out operation. Bogopager comes with the vmcore extension.
• Default pager provides a zero filled page on first fault. It supports page-out on generic file system. When loaded into memory, the default pager replaces the public procedures to call bogopager. Thus, the bogopager and the default pager are used in the same way.

The reason the bogopager is there is that the default pager depends on the file system, and file system depends on vmcore. Therefore, we can't put the default pager in vmcore, or we have circular dependency.

Pager Interface

init(): T;

destroy()

pageIn(offset: PageNumber; VAR data : ARRAY OF CHAR) : ResultCode;

Read the contents of the page offset from the external device. The ResultCode is an enumerated type whose values include the following.

Success

This is the normal return code.

AlreadyPagedIn

This indicates that the requested page to should be in memory already. When this code is returned, the memory object rechecks the cache object.

AlreadyPagedOut

This code should be returned only by pageOut method described below. This code indicates that the requested page is already paged out. When the memory object gets this code, it interprets as Success.

DeviceFull

NotImplemented

These are catastrophic event. The memory object is immediately destroyed. (not done now)

pageOut(offset : PageNumber; READONLY data : ARRAY OF CHAR; dirty: BOOLEAN) : ResultCode;

Write the page out to the external device.