CODEWITHSUNDEEP
virtual-memorymemory-mapping
sjsam
sjsam

Reputation: 21955

Memory mapping in Virtual Address Space(VAS)

This [wiki article] about Virtual memory says:

The process then starts executing bytes in the exe file. However, the only way the process can use or set '-' values in its VAS is to ask the OS to map them to bytes from a file. A common way to use VAS memory in this way is to map it to the page file.

A diagram follows :

           0                                            4GB
VAS        |---vvvvvvv----vvvvvv---vvvv----vv---v----vvv--|
mapping        |||||||    ||||||   ||||    ||   |    |||
file bytes     app.exe    kernel   user   system_page_file

I didn't understand the part values in its VAS is to ask the OS to map them to bytes from a file.

What is the system page file here?

Upvotes: 0

Views: 357

Answers (1)

JavaMan
JavaMan

Reputation: 5034

First off, I can't imagine such a badly written article to exist in Wikipedia. One has to be an expert already familiar with the topic before being able to understand what was described.

Assuming you understand the rest of the article, the '-' part represents unallocated virtual address within the 4GB address space available to a process. So the sentence "the only way the process can use or set '-' values in its VAS is to ask the OS to map them to bytes from a file" means to allocate virtual memory address e.g. in a Windows native program calling VirtualAlloc(), or a C program calling malloc() to allocate some memory to store program data while those memory were not already existing in the current process's virtual address space.

When Windows allocates memory to a process address space, it normally associate those memory with the paging file in the hard disk. The c:\pagefile.sys is this paging file which is the system_page_file mentioned in the article. Memory page is swapped out to that file when there is not enough physical page to accommodate the demand.

Hope that clarifies

Upvotes: 1

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