CODEWITHSUNDEEP
clinuxunixoperating-system
Utkarsh Srivastav
Utkarsh Srivastav

Reputation: 3227

System Wide Page Table

I have a doubt when each process has its own separate page table then why is a system wide page table required? Also if a page table is such that it maps virtual address to a physical address then I think two process may map to same physical address because all process have same virtual address space. Is this true?

Upvotes: 1

Views: 371

Answers (2)

cjs
cjs

Reputation: 27191

A system-wide page table would be used by the kernel, which in most systems is always mapped in to memory. (Typically a 32-bit system will allocate the lower 2-3 GB of virtual address space to the user process and the upper 1-2 GB to the kernel.) Making the kernel mappings common across all processes means that you don't have to worry about making sure kernel code you're about to run is mapped when you enter a system call from userland.

Upvotes: 0

MetallicPriest
MetallicPriest

Reputation: 30735

About the second part, which is mapping virtual address to the same physical address, for library code and different instances of an application code, this is indeed what is done. The code is given read only access and the same virtual address is mapped to the same physical address. In this way, there is no need to have multiple copies of the same code in the physical memory, all this assuming ASLR is not enabled.

Now concerning the data part, Modern OSs like Linux, use demand paging, that is a page is only brought to the physical memory when it is accessed (read or write). At that point, the kernel can make sure to assign a unique physical address for that page. I don't know what is the purpose of system wide page table though.

Upvotes: 1

Related Questions