How does Windows give 4GB address space each to multiple processes when the total memory it can access is also limited to 4GB.
The solution of above question i found in Windows Memory Management (Written by: Pankaj Garg)
Solution:
To achieve this Windows uses a feature of x86 processor (386 and above) known as paging. Paging allows the software to use a different memory address (known as logical address) than the physical memory address. The Processor’ paging unit translates this logical address to the physicals address transparently. This allows every process in the system to have its own 4GB logical address space.
Can anyone help me to understand it in simpler form?
The basic idea is that you have limited physical RAM. Once it fills up, you start storing stuff on the hard disk instead. When a process requests data that is currently on disk, or asks for new memory, you kick out a page from RAM by transferring it to the disk, and then page in the data you actually need.
The OS maintains a data structure called a page table to keep track of which logical addresses correspond to the data currently in physical memory and where stuff is on the disk.
Each process has its own virtual address space, and operates using logical addresses within this space. The OS is responsible for translating requests for a given process and logical address into a physical address/location on disk. It is also responsible for preventing processes from accessing memory that belongs to other processes.
When a process asks for data that is not currently in physical memory, a page fault is triggered. When this occurs, the OS selects a page to move to disk (if physical memory is full). There are several page replacement algorithms for selecting the page to kick out.
The wrong original assumption is "when the total memory it can access is also limited to 4GB". It is untrue, the total memory OS can access is not that limited.
There is a limit on 32-bit addresses that 32-bit code can access. It is (1 << 32) which is 4 GB. However this is the amount to access simultaneously only. Imagine OS has cards A, B, ..., F and applications can access only four at a time. App1 might be seeing ABCD, App2 - ABEF, App3 - ABCF. The apps see 4, but OS manages 6.
The limit on 32-bit flat memory model does not imply that the entire OS is subject to the same limit.
Windows uses a technique called virtual memory. Each process has its own memory. One of the reasons this is done, is due to security reasons, to forbid accessing the memory of other processes.
As you've pointed out, the assigned virtual memory can be bigger than the actual physical memory. This is where the process of paging comes into places. My knowledge of memory management and microarchitecture is a bit rusty, so I don't want to post anything wrong, but I 'd recommend reading http://en.wikipedia.org/wiki/Virtual_memory
If you are interested in more literature, I'd recommend reading 'Structured Computer Organization – Tannenbaum'
Virtual address space is not RAM. It's an address space. Each page (the size of a page depends on the system) can be unmapped (the page is nowhere and not accessible. it does not exist), mapped to a file (the page is not directly accessible, its content is stored on disk), mapped to RAM (that's the pages that you can actually access).
Pages mapped to RAM can be swappable or pinned. Pinned pages will never be swapped out to disk. Swappable pages are associated to an area on disc and may be written to that area to free up the RAM they are using.
Pages mapped to RAM can also be read only, write only, read write. If they are writable they may be directly writable or copy-on-write.
Multiple pages (both within the same address space and across separate address spaces) may be mapped identically. This i how two separate processes may access the same data in memory (which may happen at different addresses in each process).
In a modern operating system each process has it's own address space. On 32 bit operating systems each process has 4GiB of address space. On 64 bit operating systems 32 bit processes still only have 4GiB (4 gigabinary bytes) of address space but 64 bit processes may have more. Generally they have 18 EiB (18 exabinary bytes, that is 18,874,368 TiB).
The size of the address space is totally independent of both the amount of RAM memory and the amount of actually allocated space. You can have 100 processes each with 18 EiB of address space on a machine with one gigabyte of RAM. In fact windows has been giving 4GiB of address space to each process since the time when the typical machine had just a few megabytes or RAM.
Assuming the context is 32-bit system:
In addition to http://en.wikipedia.org/wiki/Virtual_memory , However the memory abstraction given by the kernel to each process is 4GB, A process can actually use a far lesser than 4GB, because in each process the kernel is also mapped in most of the pages of the process. In general in NT system out of 4GB, 2GB is used by kernel and in *nix system 1 GB is used by kernel.
I read this a long time ago during my OS course with Windows as case study. The numbers I give may not be accurate but they can give you a decent idea of what happens behind the scenes. From what I can recall:
In windows The memory model used is Demand Paging. On Intel a page size is 4k. Initially when you run a program, only 4 pages each of 4K is loaded from your program. which means a total of 16k of memory is allocated. Programs may be bigger but there is no need to load the whole program at once into memory. Some of these pages are data pages i.e. read/writeable where your variables and data structures are. while the other are code pages which contain the executable code i.e. the code segment. The IP is set to the first instruction of the code segment and the program starts its execution under the impression that 4GB is allocated.
When further pages are needed that is you request more memory (data segment) or your program executes further and need other executable instructions (code segment) Windows check if there is sufficient amount of memory available. If yes then these pages are loaded and mapped into the process's address space. if not much memory is available, then windows checks which pages have not been used for quite some time (this is run for all the processes not just the calling process). when it finds such pages, it moves them to the Paging file to free the space in memory and loads the requested pages.
if sometimes your program calls code from some dll that is already loaded windows simply maps those pages into your process's address space. there is no need to load these pages again as they are already availble in the memory. thus it avoids duplication as well as saves space.
So theoretically the processes are using more memory than available and they can use 4GB of memory but in reality only the portion of the process is loaded at one time.
(Do mark my answer if you find it useful)
来源：https://stackoverflow.com/questions/12445369/how-does-windows-give-4gb-address-space-each-to-multiple-processes-when-the-tota